diff --git a/3rdparty b/3rdparty
index 5d09a7f56e2d0..e73f85f09109f 160000
--- a/3rdparty
+++ b/3rdparty
@@ -1 +1 @@
-Subproject commit 5d09a7f56e2d01b5f4083e65db77c4f7aa775252
+Subproject commit e73f85f09109f77c033b643c68f5acbed6ce1244
diff --git a/apps/settings/lib/SetupChecks/PhpModules.php b/apps/settings/lib/SetupChecks/PhpModules.php
index 64fa6e15c0a85..d4c3d2c5c2a59 100644
--- a/apps/settings/lib/SetupChecks/PhpModules.php
+++ b/apps/settings/lib/SetupChecks/PhpModules.php
@@ -24,6 +24,7 @@ class PhpModules implements ISetupCheck {
 		'openssl',
 		'posix',
 		'session',
+		'sodium',
 		'xml',
 		'xmlreader',
 		'xmlwriter',
@@ -35,7 +36,6 @@ class PhpModules implements ISetupCheck {
 		'exif',
 		'gmp',
 		'intl',
-		'sodium',
 		'sysvsem',
 	];
 
@@ -58,7 +58,6 @@ public function getCategory(): string {
 	protected function getRecommendedModuleDescription(string $module): string {
 		return match($module) {
 			'intl' => $this->l10n->t('increases language translation performance and fixes sorting of non-ASCII characters'),
-			'sodium' => $this->l10n->t('for Argon2 for password hashing'),
 			'gmp' => $this->l10n->t('required for SFTP storage and recommended for WebAuthn performance'),
 			'exif' => $this->l10n->t('for picture rotation in server and metadata extraction in the Photos app'),
 			default => '',
diff --git a/build/stubs/openssl.php b/build/stubs/openssl.php
new file mode 100644
index 0000000000000..5bf410a677445
--- /dev/null
+++ b/build/stubs/openssl.php
@@ -0,0 +1,12 @@
+<?php
+
+/**
+ * SPDX-FileCopyrightText: 2026 Nextcloud GmbH and Nextcloud contributors
+ * SPDX-License-Identifier: AGPL-3.0-or-later
+ */
+
+// ext-openssl padding mode constants for psalm. PSS omitted: PHP 8.5+ only.
+const OPENSSL_PKCS1_PADDING = 1;
+const OPENSSL_SSLV23_PADDING = 2;
+const OPENSSL_NO_PADDING = 3;
+const OPENSSL_PKCS1_OAEP_PADDING = 4;
diff --git a/build/stubs/sodium.php b/build/stubs/sodium.php
new file mode 100644
index 0000000000000..4bded9687dfbb
--- /dev/null
+++ b/build/stubs/sodium.php
@@ -0,0 +1,13 @@
+<?php
+
+/**
+ * SPDX-FileCopyrightText: 2026 Nextcloud GmbH and Nextcloud contributors
+ * SPDX-License-Identifier: AGPL-3.0-or-later
+ */
+
+// ext-sodium Ed25519 size constants for psalm.
+const SODIUM_CRYPTO_SIGN_BYTES = 64;
+const SODIUM_CRYPTO_SIGN_SEEDBYTES = 32;
+const SODIUM_CRYPTO_SIGN_PUBLICKEYBYTES = 32;
+const SODIUM_CRYPTO_SIGN_SECRETKEYBYTES = 64;
+const SODIUM_CRYPTO_SIGN_KEYPAIRBYTES = 96;
diff --git a/composer.json b/composer.json
index 028a493b6f564..849c052ab1f80 100644
--- a/composer.json
+++ b/composer.json
@@ -41,6 +41,7 @@
 		"ext-posix": "*",
 		"ext-session": "*",
 		"ext-simplexml": "*",
+		"ext-sodium": "*",
 		"ext-xml": "*",
 		"ext-xmlreader": "*",
 		"ext-xmlwriter": "*",
diff --git a/core/AppInfo/Application.php b/core/AppInfo/Application.php
index 15cf42c4a5505..cd655ac386f98 100644
--- a/core/AppInfo/Application.php
+++ b/core/AppInfo/Application.php
@@ -23,6 +23,7 @@
 use OC\Core\Listener\PasswordUpdatedListener;
 use OC\Core\Notification\CoreNotifier;
 use OC\OCM\OCMDiscoveryHandler;
+use OC\OCM\OCMJwksHandler;
 use OC\TagManager;
 use OCP\AppFramework\App;
 use OCP\AppFramework\Bootstrap\IBootContext;
@@ -88,6 +89,7 @@ public function register(IRegistrationContext $context): void {
 		$context->registerConfigLexicon(ConfigLexicon::class);
 
 		$context->registerWellKnownHandler(OCMDiscoveryHandler::class);
+		$context->registerWellKnownHandler(OCMJwksHandler::class);
 		$context->registerCapability(Capabilities::class);
 	}
 
diff --git a/core/Command/OCM/ActivateKey.php b/core/Command/OCM/ActivateKey.php
new file mode 100644
index 0000000000000..090538e00248d
--- /dev/null
+++ b/core/Command/OCM/ActivateKey.php
@@ -0,0 +1,42 @@
+<?php
+
+declare(strict_types=1);
+
+/**
+ * SPDX-FileCopyrightText: 2026 Nextcloud GmbH and Nextcloud contributors
+ * SPDX-License-Identifier: AGPL-3.0-or-later
+ */
+namespace OC\Core\Command\OCM;
+
+use OC\Core\Command\Base;
+use OC\OCM\OCMSignatoryManager;
+use Symfony\Component\Console\Input\InputInterface;
+use Symfony\Component\Console\Output\OutputInterface;
+
+class ActivateKey extends Base {
+	public function __construct(
+		private readonly OCMSignatoryManager $signatoryManager,
+	) {
+		parent::__construct();
+	}
+
+	#[\Override]
+	protected function configure(): void {
+		$this
+			->setName('ocm:keys:activate')
+			->setDescription('promote the staged Ed25519 key to active; the previous active key moves to retiring');
+	}
+
+	#[\Override]
+	protected function execute(InputInterface $input, OutputInterface $output): int {
+		try {
+			$this->signatoryManager->activateStagedEd25519Key();
+		} catch (\RuntimeException $e) {
+			$output->writeln('<error>' . $e->getMessage() . '</error>');
+			return 1;
+		}
+		$output->writeln('<info>Staged key promoted to active.</info>');
+		$output->writeln('Run <info>occ ocm:keys:retire</info> once any in-flight signatures using the previous key have been verified.');
+		return 0;
+	}
+}
diff --git a/core/Command/OCM/ListKeys.php b/core/Command/OCM/ListKeys.php
new file mode 100644
index 0000000000000..f73a476311134
--- /dev/null
+++ b/core/Command/OCM/ListKeys.php
@@ -0,0 +1,54 @@
+<?php
+
+declare(strict_types=1);
+
+/**
+ * SPDX-FileCopyrightText: 2026 Nextcloud GmbH and Nextcloud contributors
+ * SPDX-License-Identifier: AGPL-3.0-or-later
+ */
+namespace OC\Core\Command\OCM;
+
+use OC\Core\Command\Base;
+use OC\OCM\OCMSignatoryManager;
+use Symfony\Component\Console\Helper\Table;
+use Symfony\Component\Console\Input\InputInterface;
+use Symfony\Component\Console\Output\OutputInterface;
+
+class ListKeys extends Base {
+	public function __construct(
+		private readonly OCMSignatoryManager $signatoryManager,
+	) {
+		parent::__construct();
+	}
+
+	#[\Override]
+	protected function configure(): void {
+		$this
+			->setName('ocm:keys:list')
+			->setDescription('list Ed25519 keys used by OCM RFC 9421 HTTP Message Signatures');
+		parent::configure();
+	}
+
+	#[\Override]
+	protected function execute(InputInterface $input, OutputInterface $output): int {
+		$keys = $this->signatoryManager->listEd25519Keys();
+		$format = $input->getOption('output');
+		if ($format === self::OUTPUT_FORMAT_JSON || $format === self::OUTPUT_FORMAT_JSON_PRETTY) {
+			$output->writeln(json_encode($keys, $format === self::OUTPUT_FORMAT_JSON_PRETTY ? JSON_PRETTY_PRINT : 0));
+			return 0;
+		}
+
+		if ($keys === []) {
+			$output->writeln('<comment>No Ed25519 keys yet; one will be generated on first OCM request.</comment>');
+			return 0;
+		}
+
+		$table = new Table($output);
+		$table->setHeaders(['Pool', 'Slot', 'Key ID']);
+		foreach ($keys as $key) {
+			$table->addRow([$key['poolId'], $key['slot'] ?? '-', $key['kid']]);
+		}
+		$table->render();
+		return 0;
+	}
+}
diff --git a/core/Command/OCM/RetireKey.php b/core/Command/OCM/RetireKey.php
new file mode 100644
index 0000000000000..58db976077c5f
--- /dev/null
+++ b/core/Command/OCM/RetireKey.php
@@ -0,0 +1,41 @@
+<?php
+
+declare(strict_types=1);
+
+/**
+ * SPDX-FileCopyrightText: 2026 Nextcloud GmbH and Nextcloud contributors
+ * SPDX-License-Identifier: AGPL-3.0-or-later
+ */
+namespace OC\Core\Command\OCM;
+
+use OC\Core\Command\Base;
+use OC\OCM\OCMSignatoryManager;
+use Symfony\Component\Console\Input\InputInterface;
+use Symfony\Component\Console\Output\OutputInterface;
+
+class RetireKey extends Base {
+	public function __construct(
+		private readonly OCMSignatoryManager $signatoryManager,
+	) {
+		parent::__construct();
+	}
+
+	#[\Override]
+	protected function configure(): void {
+		$this
+			->setName('ocm:keys:retire')
+			->setDescription('delete the retiring Ed25519 key; signatures that referenced its kid can no longer be verified');
+	}
+
+	#[\Override]
+	protected function execute(InputInterface $input, OutputInterface $output): int {
+		try {
+			$this->signatoryManager->retireEd25519Key();
+		} catch (\RuntimeException $e) {
+			$output->writeln('<error>' . $e->getMessage() . '</error>');
+			return 1;
+		}
+		$output->writeln('<info>Retiring key deleted.</info>');
+		return 0;
+	}
+}
diff --git a/core/Command/OCM/StageKey.php b/core/Command/OCM/StageKey.php
new file mode 100644
index 0000000000000..75437f460bfc2
--- /dev/null
+++ b/core/Command/OCM/StageKey.php
@@ -0,0 +1,42 @@
+<?php
+
+declare(strict_types=1);
+
+/**
+ * SPDX-FileCopyrightText: 2026 Nextcloud GmbH and Nextcloud contributors
+ * SPDX-License-Identifier: AGPL-3.0-or-later
+ */
+namespace OC\Core\Command\OCM;
+
+use OC\Core\Command\Base;
+use OC\OCM\OCMSignatoryManager;
+use Symfony\Component\Console\Input\InputInterface;
+use Symfony\Component\Console\Output\OutputInterface;
+
+class StageKey extends Base {
+	public function __construct(
+		private readonly OCMSignatoryManager $signatoryManager,
+	) {
+		parent::__construct();
+	}
+
+	#[\Override]
+	protected function configure(): void {
+		$this
+			->setName('ocm:keys:stage')
+			->setDescription('generate a new Ed25519 key and advertise it via JWKS without using it for signing yet');
+	}
+
+	#[\Override]
+	protected function execute(InputInterface $input, OutputInterface $output): int {
+		try {
+			$signatory = $this->signatoryManager->stageEd25519Key();
+		} catch (\RuntimeException $e) {
+			$output->writeln('<error>' . $e->getMessage() . '</error>');
+			return 1;
+		}
+		$output->writeln('Staged new Ed25519 key: <info>' . $signatory->getKeyId() . '</info>');
+		$output->writeln('Wait for federated peers to refresh their JWKS cache before activating.');
+		return 0;
+	}
+}
diff --git a/core/register_command.php b/core/register_command.php
index d28c1633c62bb..856894b5c4c77 100644
--- a/core/register_command.php
+++ b/core/register_command.php
@@ -74,6 +74,10 @@
 use OC\Core\Command\Memcache\DistributedGet;
 use OC\Core\Command\Memcache\DistributedSet;
 use OC\Core\Command\Memcache\RedisCommand;
+use OC\Core\Command\OCM\ActivateKey as OCMActivateKey;
+use OC\Core\Command\OCM\ListKeys as OCMListKeys;
+use OC\Core\Command\OCM\RetireKey as OCMRetireKey;
+use OC\Core\Command\OCM\StageKey as OCMStageKey;
 use OC\Core\Command\Preview\Generate;
 use OC\Core\Command\Preview\ResetRenderedTexts;
 use OC\Core\Command\Router\ListRoutes;
@@ -251,6 +255,11 @@
 	$application->add(Server::get(SnowflakeDecodeId::class));
 	$application->add(Server::get(Get::class));
 
+	$application->add(Server::get(OCMListKeys::class));
+	$application->add(Server::get(OCMStageKey::class));
+	$application->add(Server::get(OCMActivateKey::class));
+	$application->add(Server::get(OCMRetireKey::class));
+
 	$application->add(Server::get(GetCommand::class));
 	$application->add(Server::get(EnabledCommand::class));
 	$application->add(Server::get(Command\TaskProcessing\ListCommand::class));
diff --git a/lib/composer/composer/autoload_classmap.php b/lib/composer/composer/autoload_classmap.php
index 28373a5af8597..7ca7f6837ce4c 100644
--- a/lib/composer/composer/autoload_classmap.php
+++ b/lib/composer/composer/autoload_classmap.php
@@ -1402,6 +1402,10 @@
     'OC\\Core\\Command\\Memcache\\DistributedGet' => $baseDir . '/core/Command/Memcache/DistributedGet.php',
     'OC\\Core\\Command\\Memcache\\DistributedSet' => $baseDir . '/core/Command/Memcache/DistributedSet.php',
     'OC\\Core\\Command\\Memcache\\RedisCommand' => $baseDir . '/core/Command/Memcache/RedisCommand.php',
+    'OC\\Core\\Command\\OCM\\ActivateKey' => $baseDir . '/core/Command/OCM/ActivateKey.php',
+    'OC\\Core\\Command\\OCM\\ListKeys' => $baseDir . '/core/Command/OCM/ListKeys.php',
+    'OC\\Core\\Command\\OCM\\RetireKey' => $baseDir . '/core/Command/OCM/RetireKey.php',
+    'OC\\Core\\Command\\OCM\\StageKey' => $baseDir . '/core/Command/OCM/StageKey.php',
     'OC\\Core\\Command\\Preview\\Cleanup' => $baseDir . '/core/Command/Preview/Cleanup.php',
     'OC\\Core\\Command\\Preview\\Generate' => $baseDir . '/core/Command/Preview/Generate.php',
     'OC\\Core\\Command\\Preview\\ResetRenderedTexts' => $baseDir . '/core/Command/Preview/ResetRenderedTexts.php',
@@ -1947,7 +1951,9 @@
     'OC\\OCM\\Model\\OCMResource' => $baseDir . '/lib/private/OCM/Model/OCMResource.php',
     'OC\\OCM\\OCMDiscoveryHandler' => $baseDir . '/lib/private/OCM/OCMDiscoveryHandler.php',
     'OC\\OCM\\OCMDiscoveryService' => $baseDir . '/lib/private/OCM/OCMDiscoveryService.php',
+    'OC\\OCM\\OCMJwksHandler' => $baseDir . '/lib/private/OCM/OCMJwksHandler.php',
     'OC\\OCM\\OCMSignatoryManager' => $baseDir . '/lib/private/OCM/OCMSignatoryManager.php',
+    'OC\\OCM\\Rfc9421SignatoryManager' => $baseDir . '/lib/private/OCM/Rfc9421SignatoryManager.php',
     'OC\\OCS\\ApiHelper' => $baseDir . '/lib/private/OCS/ApiHelper.php',
     'OC\\OCS\\CoreCapabilities' => $baseDir . '/lib/private/OCS/CoreCapabilities.php',
     'OC\\OCS\\DiscoveryService' => $baseDir . '/lib/private/OCS/DiscoveryService.php',
@@ -2151,7 +2157,13 @@
     'OC\\Security\\Signature\\Db\\SignatoryMapper' => $baseDir . '/lib/private/Security/Signature/Db/SignatoryMapper.php',
     'OC\\Security\\Signature\\Model\\IncomingSignedRequest' => $baseDir . '/lib/private/Security/Signature/Model/IncomingSignedRequest.php',
     'OC\\Security\\Signature\\Model\\OutgoingSignedRequest' => $baseDir . '/lib/private/Security/Signature/Model/OutgoingSignedRequest.php',
+    'OC\\Security\\Signature\\Model\\Rfc9421IncomingSignedRequest' => $baseDir . '/lib/private/Security/Signature/Model/Rfc9421IncomingSignedRequest.php',
+    'OC\\Security\\Signature\\Model\\Rfc9421OutgoingSignedRequest' => $baseDir . '/lib/private/Security/Signature/Model/Rfc9421OutgoingSignedRequest.php',
     'OC\\Security\\Signature\\Model\\SignedRequest' => $baseDir . '/lib/private/Security/Signature/Model/SignedRequest.php',
+    'OC\\Security\\Signature\\Rfc9421\\Algorithm' => $baseDir . '/lib/private/Security/Signature/Rfc9421/Algorithm.php',
+    'OC\\Security\\Signature\\Rfc9421\\ContentDigest' => $baseDir . '/lib/private/Security/Signature/Rfc9421/ContentDigest.php',
+    'OC\\Security\\Signature\\Rfc9421\\IJwkResolvingSignatoryManager' => $baseDir . '/lib/private/Security/Signature/Rfc9421/IJwkResolvingSignatoryManager.php',
+    'OC\\Security\\Signature\\Rfc9421\\SignatureBase' => $baseDir . '/lib/private/Security/Signature/Rfc9421/SignatureBase.php',
     'OC\\Security\\Signature\\SignatureManager' => $baseDir . '/lib/private/Security/Signature/SignatureManager.php',
     'OC\\Security\\TrustedDomainHelper' => $baseDir . '/lib/private/Security/TrustedDomainHelper.php',
     'OC\\Security\\VerificationToken\\CleanUpJob' => $baseDir . '/lib/private/Security/VerificationToken/CleanUpJob.php',
diff --git a/lib/composer/composer/autoload_static.php b/lib/composer/composer/autoload_static.php
index e28dc90763681..f50fce4223260 100644
--- a/lib/composer/composer/autoload_static.php
+++ b/lib/composer/composer/autoload_static.php
@@ -1443,6 +1443,10 @@ class ComposerStaticInit749170dad3f5e7f9ca158f5a9f04f6a2
         'OC\\Core\\Command\\Memcache\\DistributedGet' => __DIR__ . '/../../..' . '/core/Command/Memcache/DistributedGet.php',
         'OC\\Core\\Command\\Memcache\\DistributedSet' => __DIR__ . '/../../..' . '/core/Command/Memcache/DistributedSet.php',
         'OC\\Core\\Command\\Memcache\\RedisCommand' => __DIR__ . '/../../..' . '/core/Command/Memcache/RedisCommand.php',
+        'OC\\Core\\Command\\OCM\\ActivateKey' => __DIR__ . '/../../..' . '/core/Command/OCM/ActivateKey.php',
+        'OC\\Core\\Command\\OCM\\ListKeys' => __DIR__ . '/../../..' . '/core/Command/OCM/ListKeys.php',
+        'OC\\Core\\Command\\OCM\\RetireKey' => __DIR__ . '/../../..' . '/core/Command/OCM/RetireKey.php',
+        'OC\\Core\\Command\\OCM\\StageKey' => __DIR__ . '/../../..' . '/core/Command/OCM/StageKey.php',
         'OC\\Core\\Command\\Preview\\Cleanup' => __DIR__ . '/../../..' . '/core/Command/Preview/Cleanup.php',
         'OC\\Core\\Command\\Preview\\Generate' => __DIR__ . '/../../..' . '/core/Command/Preview/Generate.php',
         'OC\\Core\\Command\\Preview\\ResetRenderedTexts' => __DIR__ . '/../../..' . '/core/Command/Preview/ResetRenderedTexts.php',
@@ -1988,7 +1992,9 @@ class ComposerStaticInit749170dad3f5e7f9ca158f5a9f04f6a2
         'OC\\OCM\\Model\\OCMResource' => __DIR__ . '/../../..' . '/lib/private/OCM/Model/OCMResource.php',
         'OC\\OCM\\OCMDiscoveryHandler' => __DIR__ . '/../../..' . '/lib/private/OCM/OCMDiscoveryHandler.php',
         'OC\\OCM\\OCMDiscoveryService' => __DIR__ . '/../../..' . '/lib/private/OCM/OCMDiscoveryService.php',
+        'OC\\OCM\\OCMJwksHandler' => __DIR__ . '/../../..' . '/lib/private/OCM/OCMJwksHandler.php',
         'OC\\OCM\\OCMSignatoryManager' => __DIR__ . '/../../..' . '/lib/private/OCM/OCMSignatoryManager.php',
+        'OC\\OCM\\Rfc9421SignatoryManager' => __DIR__ . '/../../..' . '/lib/private/OCM/Rfc9421SignatoryManager.php',
         'OC\\OCS\\ApiHelper' => __DIR__ . '/../../..' . '/lib/private/OCS/ApiHelper.php',
         'OC\\OCS\\CoreCapabilities' => __DIR__ . '/../../..' . '/lib/private/OCS/CoreCapabilities.php',
         'OC\\OCS\\DiscoveryService' => __DIR__ . '/../../..' . '/lib/private/OCS/DiscoveryService.php',
@@ -2192,7 +2198,13 @@ class ComposerStaticInit749170dad3f5e7f9ca158f5a9f04f6a2
         'OC\\Security\\Signature\\Db\\SignatoryMapper' => __DIR__ . '/../../..' . '/lib/private/Security/Signature/Db/SignatoryMapper.php',
         'OC\\Security\\Signature\\Model\\IncomingSignedRequest' => __DIR__ . '/../../..' . '/lib/private/Security/Signature/Model/IncomingSignedRequest.php',
         'OC\\Security\\Signature\\Model\\OutgoingSignedRequest' => __DIR__ . '/../../..' . '/lib/private/Security/Signature/Model/OutgoingSignedRequest.php',
+        'OC\\Security\\Signature\\Model\\Rfc9421IncomingSignedRequest' => __DIR__ . '/../../..' . '/lib/private/Security/Signature/Model/Rfc9421IncomingSignedRequest.php',
+        'OC\\Security\\Signature\\Model\\Rfc9421OutgoingSignedRequest' => __DIR__ . '/../../..' . '/lib/private/Security/Signature/Model/Rfc9421OutgoingSignedRequest.php',
         'OC\\Security\\Signature\\Model\\SignedRequest' => __DIR__ . '/../../..' . '/lib/private/Security/Signature/Model/SignedRequest.php',
+        'OC\\Security\\Signature\\Rfc9421\\Algorithm' => __DIR__ . '/../../..' . '/lib/private/Security/Signature/Rfc9421/Algorithm.php',
+        'OC\\Security\\Signature\\Rfc9421\\ContentDigest' => __DIR__ . '/../../..' . '/lib/private/Security/Signature/Rfc9421/ContentDigest.php',
+        'OC\\Security\\Signature\\Rfc9421\\IJwkResolvingSignatoryManager' => __DIR__ . '/../../..' . '/lib/private/Security/Signature/Rfc9421/IJwkResolvingSignatoryManager.php',
+        'OC\\Security\\Signature\\Rfc9421\\SignatureBase' => __DIR__ . '/../../..' . '/lib/private/Security/Signature/Rfc9421/SignatureBase.php',
         'OC\\Security\\Signature\\SignatureManager' => __DIR__ . '/../../..' . '/lib/private/Security/Signature/SignatureManager.php',
         'OC\\Security\\TrustedDomainHelper' => __DIR__ . '/../../..' . '/lib/private/Security/TrustedDomainHelper.php',
         'OC\\Security\\VerificationToken\\CleanUpJob' => __DIR__ . '/../../..' . '/lib/private/Security/VerificationToken/CleanUpJob.php',
diff --git a/lib/private/OCM/OCMDiscoveryService.php b/lib/private/OCM/OCMDiscoveryService.php
index 9459e9a03f043..77b7d63ec0d53 100644
--- a/lib/private/OCM/OCMDiscoveryService.php
+++ b/lib/private/OCM/OCMDiscoveryService.php
@@ -199,10 +199,15 @@ public function getLocalOCMProvider(bool $fullDetails = true): IOCMProvider {
 			return $provider;
 		}
 
+		$signingEnabled = !$this->appConfig->getValueBool('core', OCMSignatoryManager::APPCONFIG_SIGN_DISABLED, lazy: true);
+
 		$provider->setEnabled(true);
 		$provider->setApiVersion(self::API_VERSION);
 		$provider->setEndPoint(substr($url, 0, $pos));
 		$provider->setCapabilities(['invite-accepted', 'notifications', 'shares']);
+		if ($signingEnabled) {
+			$provider->setCapabilities(['http-sig']);
+		}
 
 		// The inviteAcceptDialog is available from the contacts app, if this config value is set
 		$inviteAcceptDialog = $this->appConfig->getValueString('core', ConfigLexicon::OCM_INVITE_ACCEPT_DIALOG);
@@ -217,9 +222,8 @@ public function getLocalOCMProvider(bool $fullDetails = true): IOCMProvider {
 		$provider->addResourceType($resource);
 
 		if ($fullDetails) {
-			// Adding a public key to the ocm discovery
 			try {
-				if (!$this->appConfig->getValueBool('core', OCMSignatoryManager::APPCONFIG_SIGN_DISABLED, lazy: true)) {
+				if ($signingEnabled) {
 					/**
 					 * @experimental 31.0.0
 					 * @psalm-suppress UndefinedInterfaceMethod
@@ -342,10 +346,11 @@ public function requestRemoteOcmEndpoint(
 	}
 
 	/**
-	 * add entries to the payload to auth the whole request
+	 * Sign the outgoing payload using the scheme the remote advertises
+	 * (RFC 9421 if `http-sig`, else cavage if a `publicKey` is present).
+	 * APPCONFIG_SIGN_ENFORCED / APPCONFIG_SIGN_DISABLED still apply.
 	 *
 	 * @throws OCMProviderException
-	 * @return array
 	 */
 	private function prepareOcmPayload(string $uri, string $method, array $options, string $payload, bool $signed): array {
 		$payload = array_merge($this->generateRequestOptions($options), ['body' => $payload]);
@@ -353,20 +358,31 @@ private function prepareOcmPayload(string $uri, string $method, array $options,
 			return $payload;
 		}
 
-		if ($this->appConfig->getValueBool('core', OCMSignatoryManager::APPCONFIG_SIGN_ENFORCED, lazy: true)
-			&& $this->signatoryManager->getRemoteSignatory($this->signatureManager->extractIdentityFromUri($uri)) === null) {
+		$origin = $this->signatureManager->extractIdentityFromUri($uri);
+		$ocmProvider = $this->discover($origin);
+
+		$useRfc9421 = $ocmProvider->hasCapability('http-sig');
+		$hasPublicKey = $this->signatoryManager->getRemoteSignatory($origin) !== null;
+
+		if (!$useRfc9421 && !$hasPublicKey
+			&& $this->appConfig->getValueBool('core', OCMSignatoryManager::APPCONFIG_SIGN_ENFORCED, lazy: true)) {
 			throw new OCMProviderException('remote endpoint does not support signed request');
 		}
 
-		if (!$this->appConfig->getValueBool('core', OCMSignatoryManager::APPCONFIG_SIGN_DISABLED, lazy: true)) {
-			$signedPayload = $this->signatureManager->signOutgoingRequestIClientPayload(
-				$this->signatoryManager,
-				$payload,
-				$method, $uri
-			);
+		if ($this->appConfig->getValueBool('core', OCMSignatoryManager::APPCONFIG_SIGN_DISABLED, lazy: true)) {
+			return $payload;
 		}
 
-		return $signedPayload ?? $payload;
+		$signatoryManager = $useRfc9421
+			? new Rfc9421SignatoryManager($this->signatoryManager)
+			: $this->signatoryManager;
+
+		return $this->signatureManager->signOutgoingRequestIClientPayload(
+			$signatoryManager,
+			$payload,
+			$method,
+			$uri,
+		);
 	}
 
 	private function generateRequestOptions(array $options): array {
diff --git a/lib/private/OCM/OCMJwksHandler.php b/lib/private/OCM/OCMJwksHandler.php
new file mode 100644
index 0000000000000..281c3eaab2d88
--- /dev/null
+++ b/lib/private/OCM/OCMJwksHandler.php
@@ -0,0 +1,49 @@
+<?php
+
+declare(strict_types=1);
+
+/**
+ * SPDX-FileCopyrightText: 2026 Nextcloud GmbH and Nextcloud contributors
+ * SPDX-License-Identifier: AGPL-3.0-or-later
+ */
+
+namespace OC\OCM;
+
+use OCP\AppFramework\Http\JSONResponse;
+use OCP\Http\WellKnown\GenericResponse;
+use OCP\Http\WellKnown\IHandler;
+use OCP\Http\WellKnown\IRequestContext;
+use OCP\Http\WellKnown\IResponse;
+use OCP\IAppConfig;
+use Psr\Log\LoggerInterface;
+use Throwable;
+
+/** Serves `/.well-known/jwks.json` (RFC 7517) for the RFC 9421 keys. */
+class OCMJwksHandler implements IHandler {
+	public function __construct(
+		private readonly IAppConfig $appConfig,
+		private readonly OCMSignatoryManager $signatoryManager,
+		private readonly LoggerInterface $logger,
+	) {
+	}
+
+	#[\Override]
+	public function handle(string $service, IRequestContext $context, ?IResponse $previousResponse): ?IResponse {
+		if ($service !== 'jwks.json') {
+			return $previousResponse;
+		}
+
+		$keys = [];
+		if (!$this->appConfig->getValueBool('core', OCMSignatoryManager::APPCONFIG_SIGN_DISABLED, lazy: true)) {
+			try {
+				foreach ($this->signatoryManager->getLocalEd25519Jwks() as $jwk) {
+					$keys[] = $jwk;
+				}
+			} catch (Throwable $e) {
+				$this->logger->warning('failed to build local Ed25519 JWKs', ['exception' => $e]);
+			}
+		}
+
+		return new GenericResponse(new JSONResponse(['keys' => $keys]));
+	}
+}
diff --git a/lib/private/OCM/OCMSignatoryManager.php b/lib/private/OCM/OCMSignatoryManager.php
index b239a4d1bceca..d60dc845e4ab7 100644
--- a/lib/private/OCM/OCMSignatoryManager.php
+++ b/lib/private/OCM/OCMSignatoryManager.php
@@ -9,21 +9,31 @@
 
 namespace OC\OCM;
 
+use Firebase\JWT\JWK;
+use Firebase\JWT\JWT;
+use Firebase\JWT\Key;
+use JsonException;
 use OC\Security\IdentityProof\Manager;
+use OC\Security\Signature\Rfc9421\Algorithm;
+use OC\Security\Signature\Rfc9421\IJwkResolvingSignatoryManager;
+use OCP\Http\Client\IClientService;
 use OCP\IAppConfig;
+use OCP\ICache;
+use OCP\ICacheFactory;
+use OCP\IConfig;
 use OCP\IURLGenerator;
 use OCP\OCM\Exceptions\OCMProviderException;
 use OCP\Security\Signature\Enum\DigestAlgorithm;
 use OCP\Security\Signature\Enum\SignatoryType;
 use OCP\Security\Signature\Enum\SignatureAlgorithm;
 use OCP\Security\Signature\Exceptions\IdentityNotFoundException;
-use OCP\Security\Signature\ISignatoryManager;
 use OCP\Security\Signature\ISignatureManager;
 use OCP\Security\Signature\Model\Signatory;
 use OCP\Server;
 use Psr\Container\ContainerExceptionInterface;
 use Psr\Container\NotFoundExceptionInterface;
 use Psr\Log\LoggerInterface;
+use Throwable;
 
 /**
  * @inheritDoc
@@ -33,19 +43,41 @@
  *
  * @since 31.0.0
  */
-class OCMSignatoryManager implements ISignatoryManager {
+class OCMSignatoryManager implements IJwkResolvingSignatoryManager {
 	public const PROVIDER_ID = 'ocm';
 	public const APPCONFIG_SIGN_IDENTITY_EXTERNAL = 'ocm_signed_request_identity_external';
 	public const APPCONFIG_SIGN_DISABLED = 'ocm_signed_request_disabled';
 	public const APPCONFIG_SIGN_ENFORCED = 'ocm_signed_request_enforced';
+	private const APPKEY_CAVAGE = 'ocm_external';
+	private const KEYID_FRAGMENT_CAVAGE = 'signature';
+	private const KEYID_FRAGMENT_ED25519 = 'ed25519';
+	/** Ed25519 keypairs live in numbered pool appkeys; slots point to them by id. */
+	private const APPKEY_ED25519_POOL_PREFIX = 'ocm_ed25519_pool_';
+	private const APPCONFIG_ED25519_POOL_COUNTER = 'ocm_ed25519_pool_counter';
+	private const APPCONFIG_ED25519_POOL_KID_PREFIX = 'ocm_ed25519_pool_kid_';
+	/** Stable kid identity portion, reused across rotations so kids stay on one hostname. */
+	private const APPCONFIG_ED25519_KID_BASE = 'ocm_ed25519_kid_base';
+	public const SLOT_ACTIVE = 'active';
+	public const SLOT_PENDING = 'pending';
+	public const SLOT_RETIRING = 'retiring';
+	/** All slots in advertise order. */
+	public const ED25519_SLOTS = [self::SLOT_ACTIVE, self::SLOT_PENDING, self::SLOT_RETIRING];
+	/** Remote JWKS cache TTL (seconds). */
+	private const JWKS_CACHE_TTL = 3600;
+
+	private readonly ICache $jwksCache;
 
 	public function __construct(
 		private readonly IAppConfig $appConfig,
 		private readonly ISignatureManager $signatureManager,
 		private readonly IURLGenerator $urlGenerator,
 		private readonly Manager $identityProofManager,
+		private readonly IClientService $clientService,
+		private readonly IConfig $config,
+		ICacheFactory $cacheFactory,
 		private readonly LoggerInterface $logger,
 	) {
+		$this->jwksCache = $cacheFactory->createDistributed('ocm-jwks');
 	}
 
 	/**
@@ -91,21 +123,16 @@ public function getLocalSignatory(): Signatory {
 		 * TODO: manage multiple identity (external, internal, ...) to allow a limitation
 		 * based on the requested interface (ie. only accept shares from globalscale)
 		 */
-		if ($this->appConfig->hasKey('core', self::APPCONFIG_SIGN_IDENTITY_EXTERNAL, true)) {
-			$identity = $this->appConfig->getValueString('core', self::APPCONFIG_SIGN_IDENTITY_EXTERNAL, lazy: true);
-			$keyId = 'https://' . $identity . '/ocm#signature';
-		} else {
-			$keyId = $this->generateKeyId();
-		}
+		$keyId = $this->buildLocalKeyId(self::KEYID_FRAGMENT_CAVAGE);
 
-		if (!$this->identityProofManager->hasAppKey('core', 'ocm_external')) {
-			$this->identityProofManager->generateAppKey('core', 'ocm_external', [
+		if (!$this->identityProofManager->hasAppKey('core', self::APPKEY_CAVAGE)) {
+			$this->identityProofManager->generateAppKey('core', self::APPKEY_CAVAGE, [
 				'algorithm' => 'rsa',
 				'private_key_bits' => 2048,
 				'private_key_type' => OPENSSL_KEYTYPE_RSA,
 			]);
 		}
-		$keyPair = $this->identityProofManager->getAppKey('core', 'ocm_external');
+		$keyPair = $this->identityProofManager->getAppKey('core', self::APPKEY_CAVAGE);
 
 		$signatory = new Signatory(true);
 		$signatory->setKeyId($keyId);
@@ -115,28 +142,263 @@ public function getLocalSignatory(): Signatory {
 
 	}
 
+	/** Active Ed25519 signing key, lazily provisioned. */
+	public function getLocalEd25519Signatory(): ?Signatory {
+		$poolId = $this->getSlotPool(self::SLOT_ACTIVE);
+		if ($poolId === null) {
+			$poolId = $this->generatePool($this->nextEd25519PoolKid());
+			$this->setSlotPool(self::SLOT_ACTIVE, $poolId);
+		}
+		return $this->signatoryFromPool($poolId);
+	}
+
+	/**
+	 * JWKs for the active/pending/retiring slots, in advertise order. The
+	 * active slot is provisioned if missing so first-hit returns a key.
+	 *
+	 * @return list<array<string, string>>
+	 */
+	public function getLocalEd25519Jwks(): array {
+		if ($this->getSlotPool(self::SLOT_ACTIVE) === null) {
+			$this->getLocalEd25519Signatory();
+		}
+
+		$jwks = [];
+		foreach (self::ED25519_SLOTS as $slot) {
+			$poolId = $this->getSlotPool($slot);
+			if ($poolId === null) {
+				continue;
+			}
+			$signatory = $this->signatoryFromPool($poolId);
+			if ($signatory !== null) {
+				$jwks[] = self::buildEd25519JwkArray($signatory->getPublicKey(), $signatory->getKeyId());
+			}
+		}
+		return $jwks;
+	}
+
+	/**
+	 * Generate a pending Ed25519 keypair (advertised in JWKS, not yet used
+	 * for outbound signing).
+	 *
+	 * @throws \RuntimeException if pending is already populated
+	 */
+	public function stageEd25519Key(): Signatory {
+		if ($this->getSlotPool(self::SLOT_PENDING) !== null) {
+			throw new \RuntimeException('a pending Ed25519 key already exists; activate or retire it first');
+		}
+		// Need an active key first; staging a next from nothing makes no sense.
+		if ($this->getSlotPool(self::SLOT_ACTIVE) === null) {
+			$this->getLocalEd25519Signatory();
+		}
+		$poolId = $this->generatePool($this->nextEd25519PoolKid());
+		$this->setSlotPool(self::SLOT_PENDING, $poolId);
+		$signatory = $this->signatoryFromPool($poolId);
+		if ($signatory === null) {
+			throw new \RuntimeException('failed to materialise newly staged Ed25519 key');
+		}
+		return $signatory;
+	}
+
+	/**
+	 * pending -> active, previous active -> retiring. The retiring slot
+	 * stays in JWKS until {@see retireEd25519Key} is run.
+	 *
+	 * @throws \RuntimeException if no pending key is staged, or retiring is occupied
+	 */
+	public function activateStagedEd25519Key(): void {
+		$pending = $this->getSlotPool(self::SLOT_PENDING);
+		if ($pending === null) {
+			throw new \RuntimeException('no pending Ed25519 key to activate; run `ocm:keys:stage` first');
+		}
+		if ($this->getSlotPool(self::SLOT_RETIRING) !== null) {
+			throw new \RuntimeException('a retiring Ed25519 key still exists; retire it before activating a new one');
+		}
+		$active = $this->getSlotPool(self::SLOT_ACTIVE);
+
+		$this->setSlotPool(self::SLOT_ACTIVE, $pending);
+		$this->clearSlot(self::SLOT_PENDING);
+		if ($active !== null) {
+			$this->setSlotPool(self::SLOT_RETIRING, $active);
+		}
+	}
+
+	/**
+	 * Delete the retiring key. In-flight signatures referencing its kid
+	 * stop verifying after this returns.
+	 *
+	 * @throws \RuntimeException if retiring is empty
+	 */
+	public function retireEd25519Key(): void {
+		$poolId = $this->getSlotPool(self::SLOT_RETIRING);
+		if ($poolId === null) {
+			throw new \RuntimeException('no retiring Ed25519 key to remove');
+		}
+		$this->identityProofManager->deleteAppKey('core', self::APPKEY_ED25519_POOL_PREFIX . $poolId);
+		$this->appConfig->deleteKey('core', self::APPCONFIG_ED25519_POOL_KID_PREFIX . $poolId);
+		$this->clearSlot(self::SLOT_RETIRING);
+	}
+
+	/**
+	 * Diagnostics snapshot. `slot` is null for orphaned pools.
+	 *
+	 * @return list<array{poolId: int, kid: string, slot: ?string}>
+	 */
+	public function listEd25519Keys(): array {
+		$bySlot = [];
+		foreach (self::ED25519_SLOTS as $slot) {
+			$id = $this->getSlotPool($slot);
+			if ($id !== null) {
+				$bySlot[$id] = $slot;
+			}
+		}
+
+		$max = $this->appConfig->getValueInt('core', self::APPCONFIG_ED25519_POOL_COUNTER, 0);
+		$entries = [];
+		for ($id = 1; $id <= $max; $id++) {
+			if (!$this->identityProofManager->hasAppKey('core', self::APPKEY_ED25519_POOL_PREFIX . $id)) {
+				continue;
+			}
+			$entries[] = [
+				'poolId' => $id,
+				'kid' => $this->canonicalKid(
+					$this->appConfig->getValueString('core', self::APPCONFIG_ED25519_POOL_KID_PREFIX . $id, ''),
+				),
+				'slot' => $bySlot[$id] ?? null,
+			];
+		}
+		return $entries;
+	}
+
+	/**
+	 * Generate keypair into a new pool. Kid is canonicalised through
+	 * {@see Signatory::setKeyId} so admin output and wire form agree.
+	 */
+	private function generatePool(string $kid): int {
+		$poolId = $this->appConfig->getValueInt('core', self::APPCONFIG_ED25519_POOL_COUNTER, 0) + 1;
+		$this->appConfig->setValueInt('core', self::APPCONFIG_ED25519_POOL_COUNTER, $poolId);
+
+		$this->identityProofManager->generateEd25519AppKey('core', self::APPKEY_ED25519_POOL_PREFIX . $poolId);
+		$this->appConfig->setValueString('core', self::APPCONFIG_ED25519_POOL_KID_PREFIX . $poolId, $this->canonicalKid($kid));
+		return $poolId;
+	}
+
+	/** Canonical wire-form via a transient {@see Signatory::setKeyId} round-trip. */
+	private function canonicalKid(string $kid): string {
+		$probe = new Signatory(true);
+		$probe->setKeyId($kid);
+		return $probe->getKeyId();
+	}
+
 	/**
-	 * - tries to generate a keyId using global configuration (from signature manager) if available
-	 * - generate a keyId using the current route to ocm shares
+	 * Build the next kid. Identity portion is derived once and persisted so
+	 * CLI-triggered rotations stay on the same hostname.
 	 *
+	 * @throws \RuntimeException if no instance identity can be derived
+	 */
+	private function nextEd25519PoolKid(): string {
+		$base = $this->resolveEd25519KidBase();
+		$next = $this->appConfig->getValueInt('core', self::APPCONFIG_ED25519_POOL_COUNTER, 0) + 1;
+		return $base . '-' . $next;
+	}
+
+	/**
+	 * Stable identity portion (before the `-N` suffix). Resolution order:
+	 * stored APPCONFIG_ED25519_KID_BASE > active pool's kid sans suffix >
+	 * fresh from {@see buildLocalKeyId}. Persisted so CLI rotations stay
+	 * on one hostname.
+	 *
+	 * @throws \RuntimeException if no instance identity can be derived
+	 */
+	private function resolveEd25519KidBase(): string {
+		$base = $this->appConfig->getValueString('core', self::APPCONFIG_ED25519_KID_BASE, '');
+		if ($base !== '') {
+			return $base;
+		}
+
+		$activePool = $this->getSlotPool(self::SLOT_ACTIVE);
+		if ($activePool !== null) {
+			$kid = $this->canonicalKid(
+				$this->appConfig->getValueString('core', self::APPCONFIG_ED25519_POOL_KID_PREFIX . $activePool, ''),
+			);
+			$pos = strrpos($kid, '-');
+			if ($pos !== false) {
+				$base = substr($kid, 0, $pos);
+			}
+		}
+
+		if ($base === '') {
+			try {
+				$base = $this->canonicalKid($this->buildLocalKeyId(self::KEYID_FRAGMENT_ED25519));
+			} catch (IdentityNotFoundException $e) {
+				throw new \RuntimeException('cannot derive instance identity for Ed25519 kid', 0, $e);
+			}
+		}
+
+		$this->appConfig->setValueString('core', self::APPCONFIG_ED25519_KID_BASE, $base);
+		return $base;
+	}
+
+	private function getSlotPool(string $slot): ?int {
+		$key = 'ocm_ed25519_slot_' . $slot;
+		if (!$this->appConfig->hasKey('core', $key)) {
+			return null;
+		}
+		$value = $this->appConfig->getValueInt('core', $key, 0);
+		return $value > 0 ? $value : null;
+	}
+
+	private function setSlotPool(string $slot, int $poolId): void {
+		$this->appConfig->setValueInt('core', 'ocm_ed25519_slot_' . $slot, $poolId);
+	}
+
+	private function clearSlot(string $slot): void {
+		$this->appConfig->deleteKey('core', 'ocm_ed25519_slot_' . $slot);
+	}
+
+	/** Returns null if the underlying appkey was manually deleted. */
+	private function signatoryFromPool(int $poolId): ?Signatory {
+		$appKey = self::APPKEY_ED25519_POOL_PREFIX . $poolId;
+		if (!$this->identityProofManager->hasAppKey('core', $appKey)) {
+			return null;
+		}
+		$kid = $this->appConfig->getValueString('core', self::APPCONFIG_ED25519_POOL_KID_PREFIX . $poolId, '');
+		if ($kid === '') {
+			return null;
+		}
+		$keyPair = $this->identityProofManager->getAppKey('core', $appKey);
+		$signatory = new Signatory(true);
+		$signatory->setKeyId($kid);
+		$signatory->setPublicKey($keyPair->getPublic());
+		$signatory->setPrivateKey($keyPair->getPrivate());
+		return $signatory;
+	}
+
+	/**
+	 * @param string $fragment URL fragment (e.g. 'signature', 'ed25519')
 	 * @return string
 	 * @throws IdentityNotFoundException
 	 */
-	private function generateKeyId(): string {
+	private function buildLocalKeyId(string $fragment): string {
+		if ($this->appConfig->hasKey('core', self::APPCONFIG_SIGN_IDENTITY_EXTERNAL, true)) {
+			$identity = $this->appConfig->getValueString('core', self::APPCONFIG_SIGN_IDENTITY_EXTERNAL, lazy: true);
+			return 'https://' . $identity . '/ocm#' . $fragment;
+		}
+
 		try {
-			return $this->signatureManager->generateKeyIdFromConfig('/ocm#signature');
+			return $this->signatureManager->generateKeyIdFromConfig('/ocm#' . $fragment);
 		} catch (IdentityNotFoundException) {
 		}
 
 		$url = $this->urlGenerator->linkToRouteAbsolute('cloud_federation_api.requesthandlercontroller.addShare');
 		$identity = $this->signatureManager->extractIdentityFromUri($url);
 
-		// catching possible subfolder to create a keyId like 'https://hostname/subfolder/ocm#signature
+		// catching possible subfolder to create a keyId like 'https://hostname/subfolder/ocm#<fragment>'
 		$path = parse_url($url, PHP_URL_PATH);
 		$pos = strpos($path, '/ocm/shares');
 		$sub = ($pos) ? substr($path, 0, $pos) : '';
 
-		return 'https://' . $identity . $sub . '/ocm#signature';
+		return 'https://' . $identity . $sub . '/ocm#' . $fragment;
 	}
 
 	/**
@@ -163,4 +425,122 @@ public function getRemoteSignatory(string $remote): ?Signatory {
 			return null;
 		}
 	}
+
+	/**
+	 * Resolve a peer's JWK by kid. Cached per-origin for {@see JWKS_CACHE_TTL}s
+	 * with a single refetch on cache-hit-but-kid-missing so rotations propagate.
+	 */
+	#[\Override]
+	public function getRemoteKey(string $origin, string $keyId): ?Key {
+		$keys = $this->readCachedJwks($origin);
+		$fromCache = $keys !== null;
+		if (!$fromCache) {
+			$keys = $this->fetchJwks($origin);
+			if ($keys !== null) {
+				$this->jwksCache->set($origin, json_encode($keys), self::JWKS_CACHE_TTL);
+			}
+		}
+
+		$key = $this->findKid($keys, $keyId);
+		if ($key !== null) {
+			return $key;
+		}
+		// Only refetch when the miss came from cache; fresh is authoritative.
+		if (!$fromCache) {
+			return null;
+		}
+
+		$keys = $this->fetchJwks($origin);
+		if ($keys === null) {
+			return null;
+		}
+		$this->jwksCache->set($origin, json_encode($keys), self::JWKS_CACHE_TTL);
+		return $this->findKid($keys, $keyId);
+	}
+
+	/** @return list<array<string, mixed>>|null null on cold/corrupt cache */
+	private function readCachedJwks(string $origin): ?array {
+		$cached = $this->jwksCache->get($origin);
+		if (!is_string($cached)) {
+			return null;
+		}
+		try {
+			$decoded = json_decode($cached, true, 8, JSON_THROW_ON_ERROR);
+		} catch (JsonException) {
+			return null;
+		}
+		if (!is_array($decoded)) {
+			return null;
+		}
+		/** @var list<array<string, mixed>> $decoded */
+		return array_values(array_filter($decoded, 'is_array'));
+	}
+
+	/**
+	 * @return list<array<string, mixed>>|null
+	 */
+	private function fetchJwks(string $origin): ?array {
+		$url = 'https://' . $origin . '/.well-known/jwks.json';
+		$options = [
+			'timeout' => 10,
+			'connect_timeout' => 10,
+		];
+		if ($this->config->getSystemValueBool('sharing.federation.allowSelfSignedCertificates') === true) {
+			$options['verify'] = false;
+		}
+
+		try {
+			$response = $this->clientService->newClient()->get($url, $options);
+		} catch (Throwable $e) {
+			$this->logger->warning('failed to fetch remote JWKS', ['exception' => $e, 'url' => $url]);
+			return null;
+		}
+
+		try {
+			$decoded = json_decode((string)$response->getBody(), true, 8, JSON_THROW_ON_ERROR);
+		} catch (JsonException $e) {
+			$this->logger->warning('remote JWKS is not valid JSON', ['exception' => $e, 'url' => $url]);
+			return null;
+		}
+
+		if (!is_array($decoded) || !is_array($decoded['keys'] ?? null)) {
+			return null;
+		}
+		return array_values(array_filter($decoded['keys'], 'is_array'));
+	}
+
+	/**
+	 * @param list<array<string, mixed>>|null $keys
+	 */
+	private function findKid(?array $keys, string $keyId): ?Key {
+		if ($keys === null) {
+			return null;
+		}
+		foreach ($keys as $entry) {
+			if (($entry['kid'] ?? null) !== $keyId) {
+				continue;
+			}
+			try {
+				return JWK::parseKey($entry, Algorithm::deriveJoseAlgFromJwk($entry));
+			} catch (Throwable $e) {
+				$this->logger->warning('failed to parse remote JWK', ['exception' => $e, 'kid' => $keyId]);
+				return null;
+			}
+		}
+		return null;
+	}
+
+	/**
+	 * @return array<string, string>
+	 */
+	private static function buildEd25519JwkArray(string $rawPublicKey, string $kid): array {
+		return [
+			'kty' => 'OKP',
+			'crv' => 'Ed25519',
+			'kid' => $kid,
+			'alg' => 'EdDSA',
+			'use' => 'sig',
+			'x' => JWT::urlsafeB64Encode($rawPublicKey),
+		];
+	}
 }
diff --git a/lib/private/OCM/Rfc9421SignatoryManager.php b/lib/private/OCM/Rfc9421SignatoryManager.php
new file mode 100644
index 0000000000000..f0756d9ca6eff
--- /dev/null
+++ b/lib/private/OCM/Rfc9421SignatoryManager.php
@@ -0,0 +1,56 @@
+<?php
+
+declare(strict_types=1);
+
+/**
+ * SPDX-FileCopyrightText: 2026 Nextcloud GmbH and Nextcloud contributors
+ * SPDX-License-Identifier: AGPL-3.0-or-later
+ */
+
+namespace OC\OCM;
+
+use Firebase\JWT\Key;
+use OC\Security\Signature\Rfc9421\IJwkResolvingSignatoryManager;
+use OCP\Security\Signature\Exceptions\IdentityNotFoundException;
+use OCP\Security\Signature\Model\Signatory;
+
+/**
+ * Per-call wrapper around {@see OCMSignatoryManager} that swaps in the
+ * Ed25519 signatory and sets `rfc9421.format`. Wrapping (vs mutating) keeps
+ * the underlying DI-managed instance stateless across requests.
+ */
+final class Rfc9421SignatoryManager implements IJwkResolvingSignatoryManager {
+	public function __construct(
+		private readonly OCMSignatoryManager $delegate,
+	) {
+	}
+
+	#[\Override]
+	public function getProviderId(): string {
+		return $this->delegate->getProviderId();
+	}
+
+	#[\Override]
+	public function getOptions(): array {
+		return array_merge($this->delegate->getOptions(), ['rfc9421.format' => true]);
+	}
+
+	#[\Override]
+	public function getLocalSignatory(): Signatory {
+		$signatory = $this->delegate->getLocalEd25519Signatory();
+		if ($signatory === null) {
+			throw new IdentityNotFoundException('no Ed25519 signatory available');
+		}
+		return $signatory;
+	}
+
+	#[\Override]
+	public function getRemoteSignatory(string $remote): ?Signatory {
+		return $this->delegate->getRemoteSignatory($remote);
+	}
+
+	#[\Override]
+	public function getRemoteKey(string $origin, string $keyId): ?Key {
+		return $this->delegate->getRemoteKey($origin, $keyId);
+	}
+}
diff --git a/lib/private/Security/IdentityProof/Manager.php b/lib/private/Security/IdentityProof/Manager.php
index ef0faeb6ad632..d6ebe3813b21e 100644
--- a/lib/private/Security/IdentityProof/Manager.php
+++ b/lib/private/Security/IdentityProof/Manager.php
@@ -178,6 +178,30 @@ public function generateAppKey(string $app, string $name, array $options = []):
 		return $this->generateKey($this->generateAppKeyId($app, $name), $options);
 	}
 
+	/**
+	 * Generate an Ed25519 keypair via libsodium. Returns raw 32-byte public
+	 * + 64-byte secret (sodium seed||publickey), no PEM. Overwrites if
+	 * already present.
+	 */
+	public function generateEd25519AppKey(string $app, string $name): Key {
+		$keyPair = sodium_crypto_sign_keypair();
+		$publicKey = sodium_crypto_sign_publickey($keyPair);
+		$privateKey = sodium_crypto_sign_secretkey($keyPair);
+
+		$id = $this->generateAppKeyId($app, $name);
+		try {
+			$this->appData->newFolder($id);
+		} catch (\Exception) {
+		}
+		$folder = $this->appData->getFolder($id);
+		$folder->newFile('private')
+			->putContent($this->crypto->encrypt($privateKey));
+		$folder->newFile('public')
+			->putContent($publicKey);
+
+		return new Key($publicKey, $privateKey);
+	}
+
 	public function deleteAppKey(string $app, string $name): bool {
 		try {
 			$folder = $this->appData->getFolder($this->generateAppKeyId($app, $name));
diff --git a/lib/private/Security/Signature/Model/Rfc9421IncomingSignedRequest.php b/lib/private/Security/Signature/Model/Rfc9421IncomingSignedRequest.php
new file mode 100644
index 0000000000000..3af3f7c0f8d7a
--- /dev/null
+++ b/lib/private/Security/Signature/Model/Rfc9421IncomingSignedRequest.php
@@ -0,0 +1,465 @@
+<?php
+
+declare(strict_types=1);
+
+/**
+ * SPDX-FileCopyrightText: 2026 Nextcloud GmbH and Nextcloud contributors
+ * SPDX-License-Identifier: AGPL-3.0-or-later
+ */
+
+namespace OC\Security\Signature\Model;
+
+use Firebase\JWT\Key;
+use gapple\StructuredFields\Bytes;
+use gapple\StructuredFields\InnerList;
+use gapple\StructuredFields\Item;
+use gapple\StructuredFields\Parameters;
+use gapple\StructuredFields\ParseException;
+use gapple\StructuredFields\Parser;
+use gapple\StructuredFields\Token;
+use JsonSerializable;
+use OC\Security\Signature\Rfc9421\Algorithm;
+use OC\Security\Signature\Rfc9421\ContentDigest;
+use OC\Security\Signature\Rfc9421\SignatureBase;
+use OC\Security\Signature\SignatureManager;
+use OCP\IRequest;
+use OCP\Security\Signature\Exceptions\IdentityNotFoundException;
+use OCP\Security\Signature\Exceptions\IncomingRequestException;
+use OCP\Security\Signature\Exceptions\InvalidSignatureException;
+use OCP\Security\Signature\Exceptions\SignatoryNotFoundException;
+use OCP\Security\Signature\Exceptions\SignatureException;
+use OCP\Security\Signature\Exceptions\SignatureNotFoundException;
+use OCP\Security\Signature\IIncomingSignedRequest;
+use OCP\Security\Signature\Model\Signatory;
+
+/**
+ * RFC 9421 implementation of {@see IIncomingSignedRequest}. Parses the
+ * inbound Signature-Input / Signature dictionaries, picks the OCM-labeled
+ * entry (RFC 9421 §3.2 lets verifiers scope by policy), and rebuilds the
+ * signature base per RFC 9421 §2.5. Crypto is deferred to {@see verify()},
+ * which needs a {@see Key} attached via {@see setKey()}. Body integrity
+ * (RFC 9530 content-digest) is checked before verify() if covered.
+ */
+class Rfc9421IncomingSignedRequest extends SignedRequest implements
+	IIncomingSignedRequest,
+	JsonSerializable {
+	/** Baseline cover for OCM. Override via `rfc9421.requiredComponents`. */
+	private const DEFAULT_REQUIRED_COMPONENTS = [
+		'@method',
+		'@target-uri',
+		'content-digest',
+		'content-length',
+		'date',
+	];
+
+	/** Max clock skew (seconds) for `created`. Override via `rfc9421.maxClockSkew`. */
+	private const DEFAULT_MAX_FUTURE_SKEW = 60;
+
+	private string $origin = '';
+	/** @var list<string> */
+	private array $components;
+	/** @var array<string, scalar> */
+	private array $signatureParams;
+	private string $signatureBaseString;
+	private string $rawSignature;
+	private ?Key $key = null;
+
+	/**
+	 * @throws IncomingRequestException if anything looks wrong with the request structure
+	 * @throws SignatureNotFoundException if the request is not signed
+	 * @throws SignatureException if signature metadata is malformed or covered components reference missing fields
+	 */
+	public function __construct(
+		string $body,
+		private readonly IRequest $request,
+		private readonly array $options = [],
+	) {
+		parent::__construct($body);
+
+		$signatureInputHeader = $request->getHeader('Signature-Input');
+		$signatureHeader = $request->getHeader('Signature');
+		if ($signatureInputHeader === '') {
+			throw new SignatureNotFoundException('missing Signature-Input header');
+		}
+		if ($signatureHeader === '') {
+			throw new SignatureNotFoundException('missing Signature header');
+		}
+
+		$inputs = self::parseSignatureInput($signatureInputHeader);
+		$signatures = self::parseSignature($signatureHeader);
+
+		// OCM policy (stricter than RFC 8941 §4.2 last-wins): a duplicate
+		// `ocm` entry is ambiguous; the entire request MUST be rejected.
+		if (self::countLabel($signatureInputHeader, 'ocm') > 1
+			|| self::countLabel($signatureHeader, 'ocm') > 1) {
+			throw new IncomingRequestException(
+				'multiple "' . 'ocm' . '" entries in signature headers'
+			);
+		}
+
+		if (!isset($inputs['ocm'])) {
+			throw new SignatureNotFoundException('missing "' . 'ocm' . '" entry in Signature-Input');
+		}
+		if (!isset($signatures['ocm'])) {
+			throw new SignatureNotFoundException('missing "' . 'ocm' . '" entry in Signature');
+		}
+
+		$entry = $inputs['ocm'];
+		$this->components = $entry['components'];
+		$this->signatureParams = $entry['params'];
+		$this->rawSignature = $signatures['ocm'];
+
+		$this->verifyRequiredComponents();
+		$this->verifyTimestamps();
+		$this->verifyContentDigestIfCovered($body);
+		$this->verifyContentLengthIfCovered($body);
+
+		$keyId = $this->signatureParams['keyid'] ?? null;
+		if (!is_string($keyId) || $keyId === '') {
+			throw new IncomingRequestException('missing keyid in Signature-Input');
+		}
+		try {
+			$this->origin = Signatory::extractIdentityFromUri($keyId);
+		} catch (IdentityNotFoundException) {
+			// keyid may follow the OCM convention `<fqdn>#<id>`; the OCM layer
+			// derives origin from the message body in that case.
+			$this->origin = '';
+		}
+
+		$paramsLine = SignatureBase::serializeSignatureParams($this->components, $this->signatureParams);
+		$this->signatureBaseString = SignatureBase::build(
+			$request->getMethod(),
+			$this->reconstructTargetUri(),
+			$this->collectHeaders(),
+			$this->components,
+			$paramsLine,
+		);
+
+		$this->setSigningElements([
+			'label' => 'ocm',
+			'keyId' => $keyId,
+			'algorithm' => isset($this->signatureParams['alg']) ? (string)$this->signatureParams['alg'] : '',
+			'created' => isset($this->signatureParams['created']) ? (string)$this->signatureParams['created'] : '',
+			'components' => implode(' ', $this->components),
+			'params' => $paramsLine,
+			'signature' => base64_encode($this->rawSignature),
+		]);
+		$this->setSignature(base64_encode($this->rawSignature));
+		$this->setSignatureData([$this->signatureBaseString]);
+	}
+
+	#[\Override]
+	public function getRequest(): IRequest {
+		return $this->request;
+	}
+
+	#[\Override]
+	public function getOrigin(): string {
+		if ($this->origin === '') {
+			throw new IncomingRequestException('empty origin');
+		}
+		return $this->origin;
+	}
+
+	#[\Override]
+	public function getKeyId(): string {
+		return $this->getSigningElement('keyId');
+	}
+
+	/** Required before {@see verify()} is called. */
+	public function setKey(Key $key): self {
+		$this->key = $key;
+		return $this;
+	}
+
+	public function getKey(): ?Key {
+		return $this->key;
+	}
+
+	/** Signature-Input `alg` if present, else null (RFC 9421 §3.3.7 omitted-alg path). */
+	public function getAlgorithm(): ?string {
+		return isset($this->signatureParams['alg']) ? (string)$this->signatureParams['alg'] : null;
+	}
+
+	/**
+	 * @return array<string, scalar>
+	 */
+	public function getSignatureParams(): array {
+		return $this->signatureParams;
+	}
+
+	/**
+	 * @return list<string>
+	 */
+	public function getCoveredComponents(): array {
+		return $this->components;
+	}
+
+	public function getSignatureBaseString(): string {
+		return $this->signatureBaseString;
+	}
+
+	#[\Override]
+	public function verify(): void {
+		if ($this->key === null) {
+			throw new SignatoryNotFoundException('no JWK set for verification');
+		}
+		try {
+			$ok = Algorithm::verify(
+				$this->signatureBaseString,
+				$this->rawSignature,
+				$this->key,
+				$this->getAlgorithm(),
+			);
+		} catch (SignatureException $e) {
+			throw new InvalidSignatureException($e->getMessage(), 0, $e);
+		}
+		if (!$ok) {
+			throw new InvalidSignatureException('signature verification failed');
+		}
+	}
+
+	/** @throws IncomingRequestException if the signature doesn't cover the OCM-required components */
+	private function verifyRequiredComponents(): void {
+		/** @var list<string> $required */
+		$required = $this->options['rfc9421.requiredComponents'] ?? self::DEFAULT_REQUIRED_COMPONENTS;
+		$missing = array_values(array_diff($required, $this->components));
+		if ($missing !== []) {
+			throw new IncomingRequestException(
+				'signature does not cover required components: ' . implode(', ', $missing)
+			);
+		}
+	}
+
+	/** @throws IncomingRequestException on stale, future-dated, or missing `created` */
+	private function verifyTimestamps(): void {
+		$ttl = (int)($this->options['ttl'] ?? SignatureManager::DATE_TTL);
+		$skew = (int)($this->options['rfc9421.maxClockSkew'] ?? self::DEFAULT_MAX_FUTURE_SKEW);
+		$now = time();
+
+		if (!isset($this->signatureParams['created'])) {
+			throw new IncomingRequestException('signature missing required `created` parameter');
+		}
+		$created = (int)$this->signatureParams['created'];
+		if ($created > $now + $skew) {
+			throw new IncomingRequestException('signature `created` is too far in the future');
+		}
+		if ($ttl > 0 && $created < $now - $ttl) {
+			throw new IncomingRequestException('signature is too old');
+		}
+
+		if (isset($this->signatureParams['expires'])) {
+			$expires = (int)$this->signatureParams['expires'];
+			if ($expires < $now) {
+				throw new IncomingRequestException('signature has expired');
+			}
+		}
+	}
+
+	private function verifyContentDigestIfCovered(string $body): void {
+		if (!in_array('content-digest', $this->components, true)) {
+			return;
+		}
+		$header = $this->request->getHeader('Content-Digest');
+		if ($header === '') {
+			throw new IncomingRequestException('content-digest covered but missing from request');
+		}
+		if (!ContentDigest::verify($header, $body)) {
+			throw new IncomingRequestException('content-digest does not match body');
+		}
+	}
+
+	private function verifyContentLengthIfCovered(string $body): void {
+		if (!in_array('content-length', $this->components, true)) {
+			return;
+		}
+		$header = $this->request->getHeader('Content-Length');
+		if ($header === '') {
+			throw new IncomingRequestException('content-length covered but missing from request');
+		}
+		if ((int)$header !== strlen($body)) {
+			throw new IncomingRequestException('content-length does not match body size');
+		}
+	}
+
+	private function reconstructTargetUri(): string {
+		$scheme = $this->request->getServerProtocol();
+		$host = $this->request->getServerHost();
+		$path = $this->request->getRequestUri();
+		return $scheme . '://' . $host . $path;
+	}
+
+	/**
+	 * Collect the HTTP request fields covered by the signature, keyed by their
+	 * lowercased name. Derived components (`@*`) are produced inside
+	 * {@see SignatureBase}; we only collect plain fields here.
+	 *
+	 * @return array<string, string>
+	 */
+	private function collectHeaders(): array {
+		$out = [];
+		foreach ($this->components as $component) {
+			if (str_starts_with($component, '@')) {
+				continue;
+			}
+			$value = $this->request->getHeader($component);
+			if ($value === '' && strtolower($component) === 'host') {
+				$value = $this->request->getServerHost();
+			}
+			$out[strtolower($component)] = $value;
+		}
+		return $out;
+	}
+
+	#[\Override]
+	public function jsonSerialize(): array {
+		return array_merge(
+			parent::jsonSerialize(),
+			[
+				'origin' => $this->origin,
+				'label' => 'ocm',
+				'components' => $this->components,
+				'signatureParams' => $this->signatureParams,
+				'signatureBase' => $this->signatureBaseString,
+			]
+		);
+	}
+
+	/**
+	 * @return array<string, array{components: list<string>, params: array<string, scalar>}>
+	 * @throws SignatureException
+	 */
+	private static function parseSignatureInput(string $header): array {
+		try {
+			$dict = Parser::parseDictionary($header);
+		} catch (ParseException $e) {
+			throw new SignatureException('malformed Signature-Input: ' . $e->getMessage(), 0, $e);
+		}
+
+		$out = [];
+		foreach ($dict as $label => $entry) {
+			if (!$entry instanceof InnerList) {
+				throw new SignatureException('Signature-Input value for ' . $label . ' is not an inner list');
+			}
+			$components = [];
+			foreach ($entry->getValue() as $item) {
+				$value = $item->getValue();
+				if (!is_string($value)) {
+					throw new SignatureException('component identifier in Signature-Input must be a string');
+				}
+				$components[] = $value;
+			}
+			$parameters = $entry->getParameters();
+			if (!$parameters instanceof Parameters) {
+				throw new SignatureException('Signature-Input parameters for ' . $label . ' are not iterable');
+			}
+			$out[$label] = [
+				'components' => $components,
+				'params' => self::normalizeParameters($parameters),
+			];
+		}
+		return $out;
+	}
+
+	/**
+	 * @return array<string, string> raw signature bytes keyed by label
+	 * @throws SignatureException
+	 */
+	private static function parseSignature(string $header): array {
+		try {
+			$dict = Parser::parseDictionary($header);
+		} catch (ParseException $e) {
+			throw new SignatureException('malformed Signature: ' . $e->getMessage(), 0, $e);
+		}
+
+		$out = [];
+		foreach ($dict as $label => $entry) {
+			if (!$entry instanceof Item || !$entry->getValue() instanceof Bytes) {
+				throw new SignatureException('Signature value for ' . $label . ' is not a byte sequence');
+			}
+			$out[$label] = (string)$entry->getValue();
+		}
+		return $out;
+	}
+
+	/**
+	 * @param iterable<string, mixed> $parameters
+	 * @return array<string, scalar>
+	 */
+	private static function normalizeParameters(iterable $parameters): array {
+		$out = [];
+		foreach ($parameters as $name => $value) {
+			$out[(string)$name] = match (true) {
+				is_string($value), is_int($value), is_bool($value) => $value,
+				$value instanceof Token => (string)$value,
+				default => throw new SignatureException('unsupported parameter type for ' . $name),
+			};
+		}
+		return $out;
+	}
+
+	/** Count $label occurrences in a dictionary header (gapple collapses dups per RFC 8941 §4.2). */
+	private static function countLabel(string $header, string $label): int {
+		$count = 0;
+		$len = strlen($header);
+		$i = 0;
+		while ($i < $len) {
+			while ($i < $len && ($header[$i] === ' ' || $header[$i] === "\t")) {
+				$i++;
+			}
+			$start = $i;
+			while ($i < $len) {
+				$c = $header[$i];
+				if (!ctype_lower($c) && !ctype_digit($c) && $c !== '*' && $c !== '_' && $c !== '-' && $c !== '.') {
+					break;
+				}
+				$i++;
+			}
+			if ($i === $start) {
+				break;
+			}
+			if (substr($header, $start, $i - $start) === $label) {
+				$count++;
+			}
+			// Skip to next top-level comma; track strings, byte-sequences, parens.
+			$inString = false;
+			$inByteSeq = false;
+			$depth = 0;
+			while ($i < $len) {
+				$c = $header[$i];
+				if ($inString) {
+					if ($c === '\\' && $i + 1 < $len) {
+						$i += 2;
+						continue;
+					}
+					if ($c === '"') {
+						$inString = false;
+					}
+					$i++;
+					continue;
+				}
+				if ($inByteSeq) {
+					if ($c === ':') {
+						$inByteSeq = false;
+					}
+					$i++;
+					continue;
+				}
+				if ($c === '"') {
+					$inString = true;
+				} elseif ($c === ':') {
+					$inByteSeq = true;
+				} elseif ($c === '(') {
+					$depth++;
+				} elseif ($c === ')') {
+					$depth--;
+				} elseif ($c === ',' && $depth === 0) {
+					$i++;
+					break;
+				}
+				$i++;
+			}
+		}
+		return $count;
+	}
+}
diff --git a/lib/private/Security/Signature/Model/Rfc9421OutgoingSignedRequest.php b/lib/private/Security/Signature/Model/Rfc9421OutgoingSignedRequest.php
new file mode 100644
index 0000000000000..3a44776ef4ad1
--- /dev/null
+++ b/lib/private/Security/Signature/Model/Rfc9421OutgoingSignedRequest.php
@@ -0,0 +1,210 @@
+<?php
+
+declare(strict_types=1);
+
+/**
+ * SPDX-FileCopyrightText: 2026 Nextcloud GmbH and Nextcloud contributors
+ * SPDX-License-Identifier: AGPL-3.0-or-later
+ */
+
+namespace OC\Security\Signature\Model;
+
+use JsonSerializable;
+use OC\Security\Signature\Rfc9421\Algorithm;
+use OC\Security\Signature\Rfc9421\ContentDigest;
+use OC\Security\Signature\Rfc9421\SignatureBase;
+use OC\Security\Signature\SignatureManager;
+use OCP\Security\Signature\Enum\DigestAlgorithm;
+use OCP\Security\Signature\Enum\SignatureAlgorithm;
+use OCP\Security\Signature\Exceptions\SignatoryException;
+use OCP\Security\Signature\Exceptions\SignatoryNotFoundException;
+use OCP\Security\Signature\IOutgoingSignedRequest;
+use OCP\Security\Signature\ISignatoryManager;
+
+/**
+ * RFC 9421 implementation of {@see IOutgoingSignedRequest}, sibling to the
+ * draft-cavage {@see OutgoingSignedRequest}. Default Ed25519 with the `alg`
+ * parameter omitted (RFC 9421 §3.3.7); verifier resolves it from the JWK.
+ *
+ * Options from {@see ISignatoryManager::getOptions()}: `rfc9421.signingAlgorithm`,
+ * `rfc9421.coveredComponents`, `rfc9421.contentDigestAlgorithm`,
+ * `rfc9421.includeAlgParameter`, `dateHeader`.
+ */
+class Rfc9421OutgoingSignedRequest extends SignedRequest implements
+	IOutgoingSignedRequest,
+	JsonSerializable {
+	private const DEFAULT_COMPONENTS = ['@method', '@target-uri', 'content-digest', 'content-length', 'date'];
+
+	private string $host = '';
+	private array $headers = [];
+	/** @var list<string> $headerList */
+	private array $headerList = [];
+	private SignatureAlgorithm $algorithm;
+	private string $signingAlgorithm;
+	/** @var array<string, scalar> */
+	private array $signatureParams;
+	private string $signatureBaseString;
+
+	public function __construct(
+		string $body,
+		ISignatoryManager $signatoryManager,
+		private readonly string $identity,
+		private readonly string $method,
+		private readonly string $uri,
+	) {
+		parent::__construct($body);
+
+		$options = $signatoryManager->getOptions();
+		$this->setHost($identity)
+			->setAlgorithm($options['algorithm'] ?? SignatureAlgorithm::RSA_SHA256)
+			->setSignatory($signatoryManager->getLocalSignatory())
+			->setDigestAlgorithm($options['digestAlgorithm'] ?? DigestAlgorithm::SHA256);
+
+		$this->signingAlgorithm = (string)($options['rfc9421.signingAlgorithm'] ?? 'ed25519');
+		$contentDigestAlgorithm = (string)($options['rfc9421.contentDigestAlgorithm'] ?? ContentDigest::ALGO_SHA256);
+		/** @var list<string> $components */
+		$components = $options['rfc9421.coveredComponents'] ?? self::DEFAULT_COMPONENTS;
+		$includeAlg = (bool)($options['rfc9421.includeAlgParameter'] ?? false);
+		$dateHeaderFormat = (string)($options['dateHeader'] ?? SignatureManager::DATE_HEADER);
+
+		$this->addHeader('Content-Digest', ContentDigest::compute($body, $contentDigestAlgorithm))
+			->addHeader('Content-Length', strlen($body))
+			->addHeader('Date', gmdate($dateHeaderFormat));
+		if (in_array('host', $components, true)) {
+			$this->addHeader('Host', $this->host);
+		}
+
+		$this->setHeaderList($components);
+		$this->signatureParams = [
+			'created' => time(),
+			'keyid' => $this->getSignatory()->getKeyId(),
+		];
+		if ($includeAlg) {
+			// Off by default per RFC 9421 §3.3.7 (verifier resolves alg from JWK).
+			$this->signatureParams['alg'] = $this->signingAlgorithm;
+		}
+
+		$this->signatureBaseString = SignatureBase::build(
+			$this->method,
+			$this->uri,
+			$this->headersByLowercaseName(),
+			$this->headerList,
+			SignatureBase::serializeSignatureParams($this->headerList, $this->signatureParams)
+		);
+		$this->setSignatureData([$this->signatureBaseString]);
+	}
+
+	#[\Override]
+	public function setHost(string $host): self {
+		$this->host = $host;
+		return $this;
+	}
+
+	#[\Override]
+	public function getHost(): string {
+		return $this->host;
+	}
+
+	#[\Override]
+	public function addHeader(string $key, string|int|float $value): self {
+		$this->headers[$key] = $value;
+		return $this;
+	}
+
+	#[\Override]
+	public function getHeaders(): array {
+		return $this->headers;
+	}
+
+	#[\Override]
+	public function setHeaderList(array $list): self {
+		$this->headerList = $list;
+		return $this;
+	}
+
+	#[\Override]
+	public function getHeaderList(): array {
+		return $this->headerList;
+	}
+
+	#[\Override]
+	public function setAlgorithm(SignatureAlgorithm $algorithm): self {
+		$this->algorithm = $algorithm;
+		return $this;
+	}
+
+	#[\Override]
+	public function getAlgorithm(): SignatureAlgorithm {
+		return $this->algorithm;
+	}
+
+	/** RFC 9421 alg name (e.g. `ed25519`). Distinct from cavage's {@see getAlgorithm()}. */
+	public function getSigningAlgorithm(): string {
+		return $this->signingAlgorithm;
+	}
+
+	public function getSignatureBaseString(): string {
+		return $this->signatureBaseString;
+	}
+
+	#[\Override]
+	public function sign(): self {
+		$privateKey = $this->getSignatory()->getPrivateKey();
+		if ($privateKey === '') {
+			throw new SignatoryException('empty private key');
+		}
+
+		$rawSignature = Algorithm::sign(
+			$this->signatureBaseString,
+			$privateKey,
+			$this->signingAlgorithm,
+		);
+		$this->setSignature(base64_encode($rawSignature));
+
+		$paramsLine = SignatureBase::serializeSignatureParams($this->headerList, $this->signatureParams);
+		$this->addHeader('Signature-Input', 'ocm=' . $paramsLine);
+		$this->addHeader('Signature', 'ocm=:' . base64_encode($rawSignature) . ':');
+
+		$this->setSigningElements([
+			'label' => 'ocm',
+			'components' => implode(' ', $this->headerList),
+			'params' => $paramsLine,
+			'signature' => $this->getSignature(),
+		]);
+
+		return $this;
+	}
+
+	/**
+	 * @return array<string, string>
+	 */
+	private function headersByLowercaseName(): array {
+		$out = [];
+		foreach ($this->headers as $name => $value) {
+			$out[strtolower($name)] = (string)$value;
+		}
+		return $out;
+	}
+
+	/**
+	 * @throws SignatoryNotFoundException
+	 */
+	#[\Override]
+	public function jsonSerialize(): array {
+		return array_merge(
+			parent::jsonSerialize(),
+			[
+				'host' => $this->host,
+				'headers' => $this->headers,
+				'algorithm' => $this->algorithm->value,
+				'signingAlgorithm' => $this->signingAlgorithm,
+				'method' => $this->method,
+				'identity' => $this->identity,
+				'uri' => $this->uri,
+				'components' => $this->headerList,
+				'signatureBase' => $this->signatureBaseString,
+				'signatureParams' => $this->signatureParams,
+			]
+		);
+	}
+}
diff --git a/lib/private/Security/Signature/Rfc9421/Algorithm.php b/lib/private/Security/Signature/Rfc9421/Algorithm.php
new file mode 100644
index 0000000000000..155aead60135f
--- /dev/null
+++ b/lib/private/Security/Signature/Rfc9421/Algorithm.php
@@ -0,0 +1,221 @@
+<?php
+
+declare(strict_types=1);
+
+/**
+ * SPDX-FileCopyrightText: 2026 Nextcloud GmbH and Nextcloud contributors
+ * SPDX-License-Identifier: AGPL-3.0-or-later
+ */
+
+namespace OC\Security\Signature\Rfc9421;
+
+use Firebase\JWT\JWT;
+use Firebase\JWT\Key;
+use InvalidArgumentException;
+use OCP\Security\Signature\Exceptions\SignatureException;
+use Throwable;
+
+/**
+ * RFC 9421 §3.3 sign/verify primitives.
+ *
+ * Asymmetric algorithms only: RSA-PKCS1-v1_5 (SHA-256/384/512), ECDSA P-256
+ * SHA-256, ECDSA P-384 SHA-384, Ed25519. JOSE aliases (RFC 7518 / RFC 8037)
+ * accepted per RFC 9421 §3.3.7. RSA-PSS is rejected: OPENSSL_PKCS1_PSS_PADDING
+ * needs PHP 8.5 and we still support 8.2-8.4.
+ *
+ * Sign delegates to {@see JWT::sign}. Verify takes a {@see Key} parsed by
+ * firebase/php-jwt (which has already validated the JWK's kty/crv/alg
+ * consistency) and only enforces the cross-source agreement between the JWK
+ * `alg` and the Signature-Input `alg` parameter (RFC 9421 §3.2 step 6).
+ */
+final class Algorithm {
+	public const NATIVE = [
+		'rsa-v1_5-sha256',
+		'ecdsa-p256-sha256',
+		'ecdsa-p384-sha384',
+		'ed25519',
+	];
+
+	/**
+	 * For Ed25519 $privateKey is the raw 64-byte sodium secret key; otherwise
+	 * a PEM private key. Returns raw signature bytes (R||S for ECDSA).
+	 *
+	 * Ed25519 calls sodium directly: JWT::sign runs the key through
+	 * `validateEdDSAKey` which base64url-decodes it first, which mangles raw
+	 * sodium bytes.
+	 *
+	 * @throws SignatureException
+	 */
+	public static function sign(string $signatureBase, string $privateKey, string $algorithm): string {
+		$normalized = self::normalize($algorithm);
+
+		if ($normalized === 'ed25519') {
+			if (strlen($privateKey) !== SODIUM_CRYPTO_SIGN_SECRETKEYBYTES) {
+				throw new SignatureException('Ed25519 secret key must be ' . SODIUM_CRYPTO_SIGN_SECRETKEYBYTES . ' bytes');
+			}
+			return sodium_crypto_sign_detached($signatureBase, $privateKey);
+		}
+
+		try {
+			return JWT::sign($signatureBase, $privateKey, self::nativeToJose($normalized));
+		} catch (Throwable $e) {
+			throw new SignatureException('signing failed for ' . $normalized . ': ' . $e->getMessage(), 0, $e);
+		}
+	}
+
+	/**
+	 * @param string $signature raw signature bytes (already base64-decoded)
+	 * @param string|null $algorithm algorithm hint from Signature-Input `alg=`
+	 * @throws SignatureException
+	 */
+	public static function verify(string $signatureBase, string $signature, Key $key, ?string $algorithm): bool {
+		$resolved = self::normalize($key->getAlgorithm());
+
+		if ($algorithm !== null && $algorithm !== '') {
+			$hintNative = self::normalize($algorithm);
+			if ($hintNative !== $resolved) {
+				throw new SignatureException(
+					'algorithm sources disagree: Signature-Input alg says ' . $hintNative . ', JWK alg says ' . $resolved
+				);
+			}
+		}
+
+		$material = $key->getKeyMaterial();
+
+		if ($resolved === 'ed25519') {
+			if (strlen($signature) !== SODIUM_CRYPTO_SIGN_BYTES) {
+				return false;
+			}
+			// parseKey hands OKP material as plain base64 of the 32 raw bytes.
+			$rawPublic = base64_decode((string)$material, true);
+			if ($rawPublic === false || strlen($rawPublic) !== SODIUM_CRYPTO_SIGN_PUBLICKEYBYTES) {
+				return false;
+			}
+			return sodium_crypto_sign_verify_detached($signature, $signatureBase, $rawPublic);
+		}
+
+		[$opensslAlgo, $encoding] = self::opensslParametersForAlgorithm($resolved);
+
+		if ($encoding === 'ecdsa') {
+			$signature = self::ecdsaRawToDer($signature, self::ecdsaCoordinateSize($resolved));
+			if ($signature === null) {
+				return false;
+			}
+		}
+
+		return openssl_verify($signatureBase, $signature, $material, $opensslAlgo) === 1;
+	}
+
+	/**
+	 * Map a JOSE alg (RFC 7518/8037) to the RFC 9421 native identifier.
+	 * Pass-through if already native.
+	 *
+	 * @throws SignatureException
+	 */
+	public static function normalize(string $algorithm): string {
+		$lower = strtolower($algorithm);
+		if (in_array($lower, self::NATIVE, true)) {
+			return $lower;
+		}
+		return match ($algorithm) {
+			'EdDSA' => 'ed25519',
+			'ES256' => 'ecdsa-p256-sha256',
+			'ES384' => 'ecdsa-p384-sha384',
+			'RS256' => 'rsa-v1_5-sha256',
+			'RS384' => 'rsa-v1_5-sha384',
+			'RS512' => 'rsa-v1_5-sha512',
+			default => throw new SignatureException('unsupported signature algorithm: ' . $algorithm),
+		};
+	}
+
+	/**
+	 * Default JOSE alg for {@see \Firebase\JWT\JWK::parseKey} when the JWK has
+	 * no `alg` (RFC 7517 leaves it optional). Null if kty/crv don't pin one
+	 * down (e.g. RSA, where the hash isn't determined).
+	 *
+	 * @param array<string, mixed> $jwk
+	 */
+	public static function deriveJoseAlgFromJwk(array $jwk): ?string {
+		return match ($jwk['kty'] ?? '') {
+			'OKP' => match ($jwk['crv'] ?? '') {
+				'Ed25519' => 'EdDSA',
+				default => null,
+			},
+			'EC' => match ($jwk['crv'] ?? '') {
+				'P-256' => 'ES256',
+				'P-384' => 'ES384',
+				default => null,
+			},
+			default => null,
+		};
+	}
+
+	private static function nativeToJose(string $native): string {
+		return match ($native) {
+			'ed25519' => 'EdDSA',
+			'ecdsa-p256-sha256' => 'ES256',
+			'ecdsa-p384-sha384' => 'ES384',
+			'rsa-v1_5-sha256' => 'RS256',
+			'rsa-v1_5-sha384' => 'RS384',
+			'rsa-v1_5-sha512' => 'RS512',
+			default => throw new SignatureException('unsupported signature algorithm: ' . $native),
+		};
+	}
+
+	/**
+	 * @return array{0: int, 1: string} [openssl digest, wire encoding]
+	 */
+	private static function opensslParametersForAlgorithm(string $native): array {
+		return match ($native) {
+			'rsa-v1_5-sha256' => [OPENSSL_ALGO_SHA256, 'raw'],
+			'rsa-v1_5-sha384' => [OPENSSL_ALGO_SHA384, 'raw'],
+			'rsa-v1_5-sha512' => [OPENSSL_ALGO_SHA512, 'raw'],
+			'ecdsa-p256-sha256' => [OPENSSL_ALGO_SHA256, 'ecdsa'],
+			'ecdsa-p384-sha384' => [OPENSSL_ALGO_SHA384, 'ecdsa'],
+			default => throw new SignatureException('unsupported signature algorithm: ' . $native),
+		};
+	}
+
+	private static function ecdsaCoordinateSize(string $native): int {
+		return match ($native) {
+			'ecdsa-p256-sha256' => 32,
+			'ecdsa-p384-sha384' => 48,
+			default => throw new InvalidArgumentException('not an ECDSA algorithm: ' . $native),
+		};
+	}
+
+	/**
+	 * Raw R||S (RFC 9421 §3.3.4 wire form) to DER for openssl_verify.
+	 * firebase/php-jwt has the inverse but keeps it private.
+	 */
+	public static function ecdsaRawToDer(string $raw, int $coordinateSize): ?string {
+		if (strlen($raw) !== $coordinateSize * 2) {
+			return null;
+		}
+		$r = ltrim(substr($raw, 0, $coordinateSize), "\x00");
+		$s = ltrim(substr($raw, $coordinateSize), "\x00");
+		// DER INTEGER must be positive; pad if high bit is set.
+		if ($r === '' || (ord($r[0]) & 0x80) !== 0) {
+			$r = "\x00" . $r;
+		}
+		if ($s === '' || (ord($s[0]) & 0x80) !== 0) {
+			$s = "\x00" . $s;
+		}
+		$rEncoded = "\x02" . self::derLength(strlen($r)) . $r;
+		$sEncoded = "\x02" . self::derLength(strlen($s)) . $s;
+		$body = $rEncoded . $sEncoded;
+		return "\x30" . self::derLength(strlen($body)) . $body;
+	}
+
+	private static function derLength(int $length): string {
+		if ($length < 0x80) {
+			return chr($length);
+		}
+		$bytes = '';
+		while ($length > 0) {
+			$bytes = chr($length & 0xff) . $bytes;
+			$length >>= 8;
+		}
+		return chr(0x80 | strlen($bytes)) . $bytes;
+	}
+}
diff --git a/lib/private/Security/Signature/Rfc9421/ContentDigest.php b/lib/private/Security/Signature/Rfc9421/ContentDigest.php
new file mode 100644
index 0000000000000..7df49f624d22f
--- /dev/null
+++ b/lib/private/Security/Signature/Rfc9421/ContentDigest.php
@@ -0,0 +1,72 @@
+<?php
+
+declare(strict_types=1);
+
+/**
+ * SPDX-FileCopyrightText: 2026 Nextcloud GmbH and Nextcloud contributors
+ * SPDX-License-Identifier: AGPL-3.0-or-later
+ */
+
+namespace OC\Security\Signature\Rfc9421;
+
+use InvalidArgumentException;
+
+/** RFC 9530 `Content-Digest` helpers; covered by RFC 9421 §7.2.5 in OCM signatures. */
+final class ContentDigest {
+	public const ALGO_SHA256 = 'sha-256';
+	public const ALGO_SHA512 = 'sha-512';
+
+	public static function compute(string $body, string $algorithm = self::ALGO_SHA256): string {
+		$hashAlgorithm = self::hashAlgorithmFor($algorithm);
+		return $algorithm . '=:' . base64_encode(hash($hashAlgorithm, $body, true)) . ':';
+	}
+
+	/**
+	 * True iff at least one recognised algorithm matches and none mismatch.
+	 * Stricter than RFC 9530 §2's "any-match"; OCM treats mismatches as an
+	 * attack on the weaker algorithm.
+	 */
+	public static function verify(string $header, string $body): bool {
+		$matched = false;
+		foreach (self::parse($header) as $algorithm => $digest) {
+			try {
+				$hashAlgorithm = self::hashAlgorithmFor($algorithm);
+			} catch (InvalidArgumentException) {
+				continue;
+			}
+			if (!hash_equals(hash($hashAlgorithm, $body, true), $digest)) {
+				return false;
+			}
+			$matched = true;
+		}
+		return $matched;
+	}
+
+	/** @return array<string, string> [algorithm => raw bytes] */
+	public static function parse(string $header): array {
+		$out = [];
+		foreach (explode(',', $header) as $entry) {
+			$entry = trim($entry);
+			if ($entry === '') {
+				continue;
+			}
+			if (!preg_match('#^([a-z0-9-]+)=:([A-Za-z0-9+/=]*):$#', $entry, $m)) {
+				continue;
+			}
+			$decoded = base64_decode($m[2], true);
+			if ($decoded === false) {
+				continue;
+			}
+			$out[strtolower($m[1])] = $decoded;
+		}
+		return $out;
+	}
+
+	private static function hashAlgorithmFor(string $algorithm): string {
+		return match (strtolower($algorithm)) {
+			self::ALGO_SHA256 => 'sha256',
+			self::ALGO_SHA512 => 'sha512',
+			default => throw new InvalidArgumentException('unsupported content-digest algorithm: ' . $algorithm),
+		};
+	}
+}
diff --git a/lib/private/Security/Signature/Rfc9421/IJwkResolvingSignatoryManager.php b/lib/private/Security/Signature/Rfc9421/IJwkResolvingSignatoryManager.php
new file mode 100644
index 0000000000000..5747ccb43d8d2
--- /dev/null
+++ b/lib/private/Security/Signature/Rfc9421/IJwkResolvingSignatoryManager.php
@@ -0,0 +1,29 @@
+<?php
+
+declare(strict_types=1);
+
+/**
+ * SPDX-FileCopyrightText: 2026 Nextcloud GmbH and Nextcloud contributors
+ * SPDX-License-Identifier: AGPL-3.0-or-later
+ */
+
+namespace OC\Security\Signature\Rfc9421;
+
+use Firebase\JWT\Key;
+use OCP\Security\Signature\ISignatoryManager;
+
+/**
+ * Capability bit for {@see ISignatoryManager} implementations that can resolve
+ * a remote JWK for RFC 9421 verification. {@see \OC\Security\Signature\SignatureManager}
+ * checks this via instanceof on the RFC 9421 path; cavage doesn't need it.
+ */
+interface IJwkResolvingSignatoryManager extends ISignatoryManager {
+	/**
+	 * Resolve the JWK identified by $keyId for the remote at $origin and
+	 * return it as a parsed {@see Key}. Null when no matching JWK is found.
+	 *
+	 * @param string $origin host of the remote that signed the request
+	 * @param string $keyId raw `keyid` from Signature-Input; matched against JWK `kid`
+	 */
+	public function getRemoteKey(string $origin, string $keyId): ?Key;
+}
diff --git a/lib/private/Security/Signature/Rfc9421/SignatureBase.php b/lib/private/Security/Signature/Rfc9421/SignatureBase.php
new file mode 100644
index 0000000000000..d3684d0251f20
--- /dev/null
+++ b/lib/private/Security/Signature/Rfc9421/SignatureBase.php
@@ -0,0 +1,124 @@
+<?php
+
+declare(strict_types=1);
+
+/**
+ * SPDX-FileCopyrightText: 2026 Nextcloud GmbH and Nextcloud contributors
+ * SPDX-License-Identifier: AGPL-3.0-or-later
+ */
+
+namespace OC\Security\Signature\Rfc9421;
+
+use InvalidArgumentException;
+use OCP\Security\Signature\Exceptions\SignatureException;
+
+/**
+ * RFC 9421 §2.5 signature base construction. Implements the derived
+ * components OCM uses (`@method`, `@target-uri`, `@authority`, `@scheme`,
+ * `@path`, `@query`, `@request-target`) plus plain HTTP fields.
+ */
+final class SignatureBase {
+	/**
+	 * @param array<string,string> $headers headers keyed by lowercase name
+	 * @param list<string> $components covered component identifiers, in order
+	 * @param string $signatureParamsLine `(...);params...` for `@signature-params`
+	 * @throws SignatureException when a covered field is missing from $headers
+	 */
+	public static function build(
+		string $method,
+		string $uri,
+		array $headers,
+		array $components,
+		string $signatureParamsLine,
+	): string {
+		$lines = [];
+		foreach ($components as $component) {
+			$lines[] = '"' . $component . '": ' . self::componentValue($component, $method, $uri, $headers);
+		}
+		$lines[] = '"@signature-params": ' . $signatureParamsLine;
+		return implode("\n", $lines);
+	}
+
+	/**
+	 * Serialize `(comp...)` + `;k=v` parameters for `@signature-params` and
+	 * Signature-Input dictionary entries.
+	 *
+	 * @param list<string> $components
+	 * @param array<string, scalar> $params
+	 */
+	public static function serializeSignatureParams(array $components, array $params): string {
+		$inner = array_map(static fn (string $c): string => '"' . $c . '"', $components);
+		$out = '(' . implode(' ', $inner) . ')';
+		foreach ($params as $name => $value) {
+			$out .= ';' . $name . '=' . self::serializeBareItem($value);
+		}
+		return $out;
+	}
+
+	/**
+	 * @param scalar $value
+	 */
+	public static function serializeBareItem(mixed $value): string {
+		if (is_string($value)) {
+			return '"' . str_replace(['\\', '"'], ['\\\\', '\\"'], $value) . '"';
+		}
+		if (is_int($value)) {
+			return (string)$value;
+		}
+		if (is_bool($value)) {
+			return $value ? '?1' : '?0';
+		}
+		throw new InvalidArgumentException('unsupported parameter value type');
+	}
+
+	private static function componentValue(string $component, string $method, string $uri, array $headers): string {
+		if (str_starts_with($component, '@')) {
+			return self::derivedValue($component, $method, $uri);
+		}
+		$lower = strtolower($component);
+		if (!array_key_exists($lower, $headers)) {
+			throw new SignatureException('missing field for signature: ' . $component);
+		}
+		return self::normalizeFieldValue($headers[$lower]);
+	}
+
+	private static function derivedValue(string $component, string $method, string $uri): string {
+		$parts = parse_url($uri);
+		if ($parts === false) {
+			throw new SignatureException('cannot parse target URI');
+		}
+		return match ($component) {
+			'@method' => strtoupper($method),
+			'@target-uri' => $uri,
+			'@authority' => self::authority($parts),
+			'@scheme' => strtolower($parts['scheme'] ?? ''),
+			'@path' => $parts['path'] ?? '/',
+			'@query' => isset($parts['query']) ? '?' . $parts['query'] : '',
+			'@request-target' => ($parts['path'] ?? '/') . (isset($parts['query']) ? '?' . $parts['query'] : ''),
+			default => throw new SignatureException('unsupported derived component: ' . $component),
+		};
+	}
+
+	private static function authority(array $parts): string {
+		$host = strtolower((string)($parts['host'] ?? ''));
+		if ($host === '') {
+			return '';
+		}
+		$port = $parts['port'] ?? null;
+		$scheme = strtolower((string)($parts['scheme'] ?? ''));
+		// RFC 9421 §2.2.3: default ports are omitted.
+		if ($port !== null && !self::isDefaultPort($scheme, (int)$port)) {
+			return $host . ':' . $port;
+		}
+		return $host;
+	}
+
+	private static function isDefaultPort(string $scheme, int $port): bool {
+		return ($scheme === 'https' && $port === 443) || ($scheme === 'http' && $port === 80);
+	}
+
+	private static function normalizeFieldValue(string $value): string {
+		// RFC 9421 §2.1: strip OWS, collapse internal whitespace.
+		return preg_replace('/[ \t]+/', ' ', trim($value)) ?? '';
+	}
+}
diff --git a/lib/private/Security/Signature/SignatureManager.php b/lib/private/Security/Signature/SignatureManager.php
index 11aff48438dbf..f094ac5148a37 100644
--- a/lib/private/Security/Signature/SignatureManager.php
+++ b/lib/private/Security/Signature/SignatureManager.php
@@ -11,6 +11,9 @@
 use OC\Security\Signature\Db\SignatoryMapper;
 use OC\Security\Signature\Model\IncomingSignedRequest;
 use OC\Security\Signature\Model\OutgoingSignedRequest;
+use OC\Security\Signature\Model\Rfc9421IncomingSignedRequest;
+use OC\Security\Signature\Model\Rfc9421OutgoingSignedRequest;
+use OC\Security\Signature\Rfc9421\IJwkResolvingSignatoryManager;
 use OCP\DB\Exception as DBException;
 use OCP\IAppConfig;
 use OCP\IRequest;
@@ -101,6 +104,11 @@ public function getIncomingSignedRequest(
 			throw new IncomingRequestException('content of request is too big');
 		}
 
+		// `Signature-Input` is unique to RFC 9421; cavage uses `Signature` only.
+		if ($this->request->getHeader('Signature-Input') !== '') {
+			return $this->getRfc9421IncomingSignedRequest($signatoryManager, $body, $options);
+		}
+
 		// generate IncomingSignedRequest based on body and request
 		$signedRequest = new IncomingSignedRequest($body, $this->request, $options);
 
@@ -121,6 +129,45 @@ public function getIncomingSignedRequest(
 		return $signedRequest;
 	}
 
+	/**
+	 * RFC 9421 inbound path. Requires {@see IJwkResolvingSignatoryManager}.
+	 *
+	 * @throws IncomingRequestException
+	 * @throws SignatureException
+	 * @throws SignatureNotFoundException
+	 */
+	private function getRfc9421IncomingSignedRequest(
+		ISignatoryManager $signatoryManager,
+		string $body,
+		array $options,
+	): IIncomingSignedRequest {
+		if (!($signatoryManager instanceof IJwkResolvingSignatoryManager)) {
+			throw new IncomingRequestException('RFC 9421 inbound is not supported by ' . get_class($signatoryManager));
+		}
+
+		$signedRequest = new Rfc9421IncomingSignedRequest($body, $this->request, $options);
+
+		try {
+			$key = $signatoryManager->getRemoteKey($signedRequest->getOrigin(), $signedRequest->getKeyId());
+			if ($key === null) {
+				throw new SignatoryNotFoundException('no JWK resolved for keyid ' . $signedRequest->getKeyId());
+			}
+			$signedRequest->setKey($key);
+			$signedRequest->verify();
+		} catch (SignatureException $e) {
+			$this->logger->warning(
+				'RFC 9421 signature could not be verified', [
+					'exception' => $e,
+					'signedRequest' => $signedRequest,
+					'signatoryManager' => get_class($signatoryManager),
+				]
+			);
+			throw $e;
+		}
+
+		return $signedRequest;
+	}
+
 	/**
 	 * confirm that the Signature is signed using the correct private key, using
 	 * clear version of the Signature and the public key linked to the keyId
@@ -199,13 +246,22 @@ public function getOutgoingSignedRequest(
 		string $method,
 		string $uri,
 	): IOutgoingSignedRequest {
-		$signedRequest = new OutgoingSignedRequest(
-			$content,
-			$signatoryManager,
-			$this->extractIdentityFromUri($uri),
-			$method,
-			parse_url($uri, PHP_URL_PATH) ?? '/'
-		);
+		$options = $signatoryManager->getOptions();
+		$signedRequest = ($options['rfc9421.format'] ?? false)
+			? new Rfc9421OutgoingSignedRequest(
+				$content,
+				$signatoryManager,
+				$this->extractIdentityFromUri($uri),
+				$method,
+				$uri,
+			)
+			: new OutgoingSignedRequest(
+				$content,
+				$signatoryManager,
+				$this->extractIdentityFromUri($uri),
+				$method,
+				parse_url($uri, PHP_URL_PATH) ?? '/',
+			);
 
 		$signedRequest->sign();
 
diff --git a/psalm.xml b/psalm.xml
index f43487afeb31b..23d063915e05c 100644
--- a/psalm.xml
+++ b/psalm.xml
@@ -116,9 +116,11 @@
 		<file name="build/stubs/SensitiveParameter.phpstub"/>
 		<file name="build/stubs/ldap.php"/>
 		<file name="build/stubs/memcached.php"/>
+		<file name="build/stubs/openssl.php"/>
 		<file name="build/stubs/redis.php"/>
 		<file name="build/stubs/redis_cluster.php"/>
 		<file name="build/stubs/sftp.php"/>
+		<file name="build/stubs/sodium.php"/>
 		<file name="build/stubs/ssh2.php"/>
 		<file name="build/stubs/xsl.php"/>
 		<file name="build/stubs/ftp.php"/>
diff --git a/tests/lib/OCM/DiscoveryServiceTest.php b/tests/lib/OCM/DiscoveryServiceTest.php
index 1cf026a64bc0b..58a22a07bd166 100644
--- a/tests/lib/OCM/DiscoveryServiceTest.php
+++ b/tests/lib/OCM/DiscoveryServiceTest.php
@@ -128,6 +128,13 @@ public function testLocalBaseCapability(): void {
 		$this->assertEmpty(array_diff(['notifications', 'shares'], $local->getCapabilities()));
 	}
 
+	public function testLocalCapabilitiesAdvertiseHttpSigByDefault(): void {
+		// `http-sig` is the OCM-spec flag signalling RFC 9421 support backed
+		// by /.well-known/jwks.json. Advertised whenever signing is not
+		// disabled outright.
+		$local = $this->discoveryService->getLocalOCMProvider();
+		$this->assertTrue($local->hasCapability('http-sig'));
+	}
 
 	public function testLocalAddedCapability(): void {
 		$this->context->for('ocm-capability-app')->registerEventListener(LocalOCMDiscoveryEvent::class, LocalOCMDiscoveryTestEvent::class);
diff --git a/tests/lib/OCM/OCMJwksHandlerTest.php b/tests/lib/OCM/OCMJwksHandlerTest.php
new file mode 100644
index 0000000000000..7040b19f67537
--- /dev/null
+++ b/tests/lib/OCM/OCMJwksHandlerTest.php
@@ -0,0 +1,117 @@
+<?php
+
+declare(strict_types=1);
+
+/**
+ * SPDX-FileCopyrightText: 2026 Nextcloud GmbH and Nextcloud contributors
+ * SPDX-License-Identifier: AGPL-3.0-or-later
+ */
+
+namespace Test\OCM;
+
+use OC\OCM\OCMJwksHandler;
+use OC\OCM\OCMSignatoryManager;
+use OCP\AppFramework\Http\JSONResponse;
+use OCP\Http\WellKnown\GenericResponse;
+use OCP\Http\WellKnown\IRequestContext;
+use OCP\Http\WellKnown\IResponse;
+use OCP\Http\WellKnown\JrdResponse;
+use OCP\IAppConfig;
+use PHPUnit\Framework\MockObject\MockObject;
+use Psr\Log\LoggerInterface;
+use Test\TestCase;
+
+class OCMJwksHandlerTest extends TestCase {
+	private IAppConfig&MockObject $appConfig;
+	private OCMSignatoryManager&MockObject $signatoryManager;
+	private LoggerInterface&MockObject $logger;
+	private IRequestContext&MockObject $context;
+	private OCMJwksHandler $handler;
+
+	#[\Override]
+	protected function setUp(): void {
+		parent::setUp();
+
+		$this->appConfig = $this->createMock(IAppConfig::class);
+		$this->signatoryManager = $this->createMock(OCMSignatoryManager::class);
+		$this->logger = $this->createMock(LoggerInterface::class);
+		$this->context = $this->createMock(IRequestContext::class);
+
+		$this->handler = new OCMJwksHandler(
+			$this->appConfig,
+			$this->signatoryManager,
+			$this->logger,
+		);
+	}
+
+	public function testIgnoresUnrelatedService(): void {
+		$previous = new JrdResponse('foo');
+		$result = $this->handler->handle('webfinger', $this->context, $previous);
+		$this->assertSame($previous, $result);
+	}
+
+	public function testEmptyKeySetWhenSigningDisabled(): void {
+		$this->appConfig->method('getValueBool')
+			->with('core', OCMSignatoryManager::APPCONFIG_SIGN_DISABLED, false, true)
+			->willReturn(true);
+		$this->signatoryManager->expects($this->never())->method('getLocalEd25519Jwks');
+
+		$body = $this->jsonBody($this->handler->handle('jwks.json', $this->context, null));
+		$this->assertSame(['keys' => []], $body);
+	}
+
+	public function testPublishesEd25519JwksWhenAvailable(): void {
+		$this->appConfig->method('getValueBool')->willReturn(false);
+		$jwk = [
+			'kty' => 'OKP',
+			'crv' => 'Ed25519',
+			'kid' => 'https://example.org/ocm#ed25519',
+			'alg' => 'EdDSA',
+			'use' => 'sig',
+			'x' => 'AAAA',
+		];
+		$this->signatoryManager->method('getLocalEd25519Jwks')->willReturn([$jwk]);
+
+		$body = $this->jsonBody($this->handler->handle('jwks.json', $this->context, null));
+		$this->assertSame(['keys' => [$jwk]], $body);
+	}
+
+	public function testPublishesAllSlotsAdvertisedDuringRotation(): void {
+		$this->appConfig->method('getValueBool')->willReturn(false);
+		$active = [
+			'kty' => 'OKP', 'crv' => 'Ed25519', 'kid' => 'kid-1', 'alg' => 'EdDSA', 'use' => 'sig', 'x' => 'AAAA',
+		];
+		$pending = [
+			'kty' => 'OKP', 'crv' => 'Ed25519', 'kid' => 'kid-2', 'alg' => 'EdDSA', 'use' => 'sig', 'x' => 'BBBB',
+		];
+		$this->signatoryManager->method('getLocalEd25519Jwks')->willReturn([$active, $pending]);
+
+		$body = $this->jsonBody($this->handler->handle('jwks.json', $this->context, null));
+		$this->assertSame(['keys' => [$active, $pending]], $body);
+	}
+
+	public function testEmptyKeySetWhenSignatoryUnavailable(): void {
+		$this->appConfig->method('getValueBool')->willReturn(false);
+		$this->signatoryManager->method('getLocalEd25519Jwks')->willReturn([]);
+
+		$body = $this->jsonBody($this->handler->handle('jwks.json', $this->context, null));
+		$this->assertSame(['keys' => []], $body);
+	}
+
+	public function testFailingJwkBuildIsLoggedAndYieldsEmptyKeySet(): void {
+		$this->appConfig->method('getValueBool')->willReturn(false);
+		$this->signatoryManager->method('getLocalEd25519Jwks')
+			->willThrowException(new \RuntimeException('boom'));
+		$this->logger->expects($this->once())->method('warning');
+
+		$body = $this->jsonBody($this->handler->handle('jwks.json', $this->context, null));
+		$this->assertSame(['keys' => []], $body);
+	}
+
+	private function jsonBody(?IResponse $response): array {
+		$this->assertInstanceOf(GenericResponse::class, $response);
+		$http = $response->toHttpResponse();
+		$this->assertInstanceOf(JSONResponse::class, $http);
+		return $http->getData();
+	}
+}
diff --git a/tests/lib/OCM/OCMSignatoryManagerJwksTest.php b/tests/lib/OCM/OCMSignatoryManagerJwksTest.php
new file mode 100644
index 0000000000000..7fcc0818e31fc
--- /dev/null
+++ b/tests/lib/OCM/OCMSignatoryManagerJwksTest.php
@@ -0,0 +1,177 @@
+<?php
+
+declare(strict_types=1);
+
+/**
+ * SPDX-FileCopyrightText: 2026 Nextcloud GmbH and Nextcloud contributors
+ * SPDX-License-Identifier: AGPL-3.0-or-later
+ */
+
+namespace Test\OCM;
+
+use OC\OCM\OCMSignatoryManager;
+use OC\Security\IdentityProof\Manager as IdentityProofManager;
+use OCP\Http\Client\IClient;
+use OCP\Http\Client\IClientService;
+use OCP\Http\Client\IResponse;
+use OCP\IAppConfig;
+use OCP\ICacheFactory;
+use OCP\IConfig;
+use OCP\IURLGenerator;
+use OCP\Security\Signature\ISignatureManager;
+use PHPUnit\Framework\MockObject\MockObject;
+use Psr\Log\LoggerInterface;
+use Test\TestCase;
+
+class OCMSignatoryManagerJwksTest extends TestCase {
+	private IAppConfig&MockObject $appConfig;
+	private ISignatureManager&MockObject $signatureManager;
+	private IURLGenerator&MockObject $urlGenerator;
+	private IdentityProofManager&MockObject $identityProofManager;
+	private IClientService&MockObject $clientService;
+	private IConfig&MockObject $config;
+	private LoggerInterface&MockObject $logger;
+	private IClient&MockObject $client;
+	private OCMSignatoryManager $signatoryManager;
+
+	#[\Override]
+	protected function setUp(): void {
+		parent::setUp();
+
+		$this->appConfig = $this->createMock(IAppConfig::class);
+		$this->signatureManager = $this->createMock(ISignatureManager::class);
+		$this->urlGenerator = $this->createMock(IURLGenerator::class);
+		$this->identityProofManager = $this->createMock(IdentityProofManager::class);
+		$this->clientService = $this->createMock(IClientService::class);
+		$this->config = $this->createMock(IConfig::class);
+		$this->logger = $this->createMock(LoggerInterface::class);
+		$this->client = $this->createMock(IClient::class);
+
+		$this->clientService->method('newClient')->willReturn($this->client);
+
+		$cacheFactory = $this->createMock(ICacheFactory::class);
+		$cacheFactory->method('createDistributed')->willReturn(new \OC\Memcache\ArrayCache(''));
+
+		$this->signatoryManager = new OCMSignatoryManager(
+			$this->appConfig,
+			$this->signatureManager,
+			$this->urlGenerator,
+			$this->identityProofManager,
+			$this->clientService,
+			$this->config,
+			$cacheFactory,
+			$this->logger,
+		);
+	}
+
+	public function testGetRemoteKeyFetchesAndMatchesByKid(): void {
+		$kid = 'sender.example.org#key1';
+		$jwks = [
+			'keys' => [
+				['kty' => 'OKP', 'crv' => 'Ed25519', 'kid' => 'other', 'x' => 'AAAA'],
+				['kty' => 'OKP', 'crv' => 'Ed25519', 'kid' => $kid, 'x' => 'BBBB'],
+			],
+		];
+		$this->respondWith($jwks);
+
+		$key = $this->signatoryManager->getRemoteKey('sender.example.org', $kid);
+		$this->assertNotNull($key);
+		$this->assertSame('EdDSA', $key->getAlgorithm());
+		// Key stores OKP material as plain base64 of the raw bytes.
+		$this->assertSame('BBBB', $key->getKeyMaterial());
+	}
+
+	public function testGetRemoteKeyReturnsNullWhenKidMissing(): void {
+		$this->respondWith(['keys' => [['kty' => 'OKP', 'crv' => 'Ed25519', 'kid' => 'unrelated', 'x' => 'AAAA']]]);
+		$this->assertNull($this->signatoryManager->getRemoteKey('sender.example.org', 'other-kid'));
+	}
+
+	public function testGetRemoteKeyReturnsNullOnHttpError(): void {
+		$this->client->method('get')->willThrowException(new \RuntimeException('boom'));
+		$this->logger->expects($this->once())->method('warning');
+		$this->assertNull($this->signatoryManager->getRemoteKey('sender.example.org', 'kid'));
+	}
+
+	public function testGetRemoteKeyReturnsNullOnInvalidJson(): void {
+		$response = $this->createMock(IResponse::class);
+		$response->method('getBody')->willReturn('not json');
+		$this->client->method('get')->willReturn($response);
+		$this->logger->expects($this->once())->method('warning');
+		$this->assertNull($this->signatoryManager->getRemoteKey('sender.example.org', 'kid'));
+	}
+
+	public function testGetRemoteKeyReturnsNullWhenKeysMissing(): void {
+		$this->respondWith(['no-keys-here' => []]);
+		$this->assertNull($this->signatoryManager->getRemoteKey('sender.example.org', 'kid'));
+	}
+
+	public function testGetRemoteKeyReturnsNullOnUnparseableJwk(): void {
+		// JWK with kty=OKP but no crv: parseKey rejects.
+		$this->respondWith(['keys' => [['kty' => 'OKP', 'kid' => 'kid', 'x' => 'AAAA']]]);
+		$this->logger->expects($this->once())->method('warning');
+		$this->assertNull($this->signatoryManager->getRemoteKey('sender.example.org', 'kid'));
+	}
+
+	public function testGetRemoteKeyUsesWellKnownPath(): void {
+		$this->client->expects($this->once())
+			->method('get')
+			->with(
+				$this->equalTo('https://sender.example.org/.well-known/jwks.json'),
+				$this->isType('array'),
+			)
+			->willReturn($this->jsonResponse(['keys' => []]));
+
+		$this->signatoryManager->getRemoteKey('sender.example.org', 'kid');
+	}
+
+	public function testGetRemoteKeyPassesSelfSignedFlagThrough(): void {
+		$this->config->method('getSystemValueBool')
+			->with('sharing.federation.allowSelfSignedCertificates')
+			->willReturn(true);
+
+		$this->client->expects($this->once())
+			->method('get')
+			->with(
+				$this->anything(),
+				$this->callback(static fn (array $opts): bool => ($opts['verify'] ?? null) === false),
+			)
+			->willReturn($this->jsonResponse(['keys' => []]));
+
+		$this->signatoryManager->getRemoteKey('sender.example.org', 'kid');
+	}
+
+	public function testJwksCachedAcrossCallsToTheSameOrigin(): void {
+		$kid = 'sender.example.org#key1';
+		$jwks = ['keys' => [['kty' => 'OKP', 'crv' => 'Ed25519', 'kid' => $kid, 'x' => 'AAAA']]];
+		$this->client->expects($this->once())
+			->method('get')
+			->willReturn($this->jsonResponse($jwks));
+
+		$this->assertNotNull($this->signatoryManager->getRemoteKey('sender.example.org', $kid));
+		$this->assertNotNull($this->signatoryManager->getRemoteKey('sender.example.org', $kid));
+	}
+
+	public function testCacheMissOnNewKidTriggersRefetchOnce(): void {
+		$first = ['keys' => [['kty' => 'OKP', 'crv' => 'Ed25519', 'kid' => 'old', 'x' => 'AAAA']]];
+		$second = ['keys' => [['kty' => 'OKP', 'crv' => 'Ed25519', 'kid' => 'new', 'x' => 'BBBB']]];
+		$this->client->expects($this->exactly(2))
+			->method('get')
+			->willReturnOnConsecutiveCalls(
+				$this->jsonResponse($first),
+				$this->jsonResponse($second),
+			);
+
+		$this->assertNotNull($this->signatoryManager->getRemoteKey('sender.example.org', 'old'));
+		$this->assertNotNull($this->signatoryManager->getRemoteKey('sender.example.org', 'new'));
+	}
+
+	private function respondWith(array $body): void {
+		$this->client->method('get')->willReturn($this->jsonResponse($body));
+	}
+
+	private function jsonResponse(array $body): IResponse {
+		$response = $this->createMock(IResponse::class);
+		$response->method('getBody')->willReturn(json_encode($body, JSON_THROW_ON_ERROR));
+		return $response;
+	}
+}
diff --git a/tests/lib/OCM/OCMSignatoryManagerRotationTest.php b/tests/lib/OCM/OCMSignatoryManagerRotationTest.php
new file mode 100644
index 0000000000000..9b52d88c61f7b
--- /dev/null
+++ b/tests/lib/OCM/OCMSignatoryManagerRotationTest.php
@@ -0,0 +1,273 @@
+<?php
+
+declare(strict_types=1);
+
+/**
+ * SPDX-FileCopyrightText: 2026 Nextcloud GmbH and Nextcloud contributors
+ * SPDX-License-Identifier: AGPL-3.0-or-later
+ */
+
+namespace Test\OCM;
+
+use OC\OCM\OCMSignatoryManager;
+use OC\Security\IdentityProof\Key;
+use OC\Security\IdentityProof\Manager as IdentityProofManager;
+use OCP\Http\Client\IClient;
+use OCP\Http\Client\IClientService;
+use OCP\IAppConfig;
+use OCP\ICacheFactory;
+use OCP\IConfig;
+use OCP\IURLGenerator;
+use OCP\Security\Signature\Exceptions\IdentityNotFoundException;
+use OCP\Security\Signature\ISignatureManager;
+use PHPUnit\Framework\MockObject\MockObject;
+use Psr\Log\LoggerInterface;
+use Test\TestCase;
+
+/** Ed25519 stage / activate / retire lifecycle, with stateful IAppConfig + IdentityProofManager fakes. */
+class OCMSignatoryManagerRotationTest extends TestCase {
+	private IAppConfig&MockObject $appConfig;
+	private IdentityProofManager&MockObject $identityProofManager;
+	private OCMSignatoryManager $signatoryManager;
+
+	/** @var array<string, string> in-memory backing store for IAppConfig core/* */
+	private array $appConfigStore = [];
+	/** @var array<string, Key> in-memory backing store for IdentityProofManager appkeys */
+	private array $appKeyStore = [];
+
+	#[\Override]
+	protected function setUp(): void {
+		parent::setUp();
+
+		$this->appConfig = $this->createMock(IAppConfig::class);
+		$this->identityProofManager = $this->createMock(IdentityProofManager::class);
+
+		$this->wireAppConfig();
+		$this->wireIdentityProofManager();
+
+		$signatureManager = $this->createMock(ISignatureManager::class);
+		$signatureManager->method('generateKeyIdFromConfig')
+			->willReturnCallback(static fn (string $suffix): string => 'https://alice.example/' . ltrim($suffix, '/'));
+
+		$cacheFactory = $this->createMock(ICacheFactory::class);
+		$cacheFactory->method('createDistributed')->willReturn(new \OC\Memcache\ArrayCache(''));
+
+		$this->signatoryManager = new OCMSignatoryManager(
+			$this->appConfig,
+			$signatureManager,
+			$this->createMock(IURLGenerator::class),
+			$this->identityProofManager,
+			$this->stubClientService(),
+			$this->createMock(IConfig::class),
+			$cacheFactory,
+			$this->createMock(LoggerInterface::class),
+		);
+	}
+
+	public function testJwksBootstrapsActiveKeyOnFirstFetch(): void {
+		// Fresh instance: first JWKS hit must provision the active key.
+		$jwks = $this->signatoryManager->getLocalEd25519Jwks();
+		$this->assertCount(1, $jwks);
+		$this->assertSame('https://alice.example/ocm#ed25519-1', $jwks[0]['kid']);
+
+		// And the bootstrapped key is the active one for outbound signing.
+		$signatory = $this->signatoryManager->getLocalEd25519Signatory();
+		$this->assertSame($jwks[0]['kid'], $signatory->getKeyId());
+	}
+
+	public function testFirstCallProvisionsActiveKey(): void {
+		$signatory = $this->signatoryManager->getLocalEd25519Signatory();
+		$this->assertNotNull($signatory);
+		$this->assertSame('https://alice.example/ocm#ed25519-1', $signatory->getKeyId());
+
+		$jwks = $this->signatoryManager->getLocalEd25519Jwks();
+		$this->assertCount(1, $jwks);
+		$this->assertSame($signatory->getKeyId(), $jwks[0]['kid']);
+
+		$listed = $this->signatoryManager->listEd25519Keys();
+		$this->assertSame([['poolId' => 1, 'kid' => $signatory->getKeyId(), 'slot' => 'active']], $listed);
+	}
+
+	public function testStageDoesNotChangeActiveSignerButPublishesNewJwk(): void {
+		$initial = $this->signatoryManager->getLocalEd25519Signatory();
+		$staged = $this->signatoryManager->stageEd25519Key();
+		$this->assertNotSame($initial->getKeyId(), $staged->getKeyId());
+
+		// Active signer is unchanged.
+		$this->assertSame($initial->getKeyId(), $this->signatoryManager->getLocalEd25519Signatory()->getKeyId());
+
+		// JWKS now advertises both kids, active first then pending.
+		$jwks = $this->signatoryManager->getLocalEd25519Jwks();
+		$this->assertSame([$initial->getKeyId(), $staged->getKeyId()], array_column($jwks, 'kid'));
+	}
+
+	public function testStageRefusesIfPendingAlreadyExists(): void {
+		$this->signatoryManager->stageEd25519Key();
+		$this->expectException(\RuntimeException::class);
+		$this->expectExceptionMessageMatches('/pending Ed25519 key already exists/');
+		$this->signatoryManager->stageEd25519Key();
+	}
+
+	public function testActivatePromotesPendingAndDemotesActive(): void {
+		$first = $this->signatoryManager->getLocalEd25519Signatory();
+		$staged = $this->signatoryManager->stageEd25519Key();
+		$this->signatoryManager->activateStagedEd25519Key();
+
+		// New signer is the formerly-staged key.
+		$this->assertSame($staged->getKeyId(), $this->signatoryManager->getLocalEd25519Signatory()->getKeyId());
+
+		// JWKS still advertises the former active key as retiring so peers
+		// verifying in-flight signatures with its kid don't fail.
+		$kids = array_column($this->signatoryManager->getLocalEd25519Jwks(), 'kid');
+		$this->assertContains($first->getKeyId(), $kids);
+		$this->assertContains($staged->getKeyId(), $kids);
+	}
+
+	public function testActivateRefusesIfRetiringStillPopulated(): void {
+		$this->signatoryManager->getLocalEd25519Signatory();
+		$this->signatoryManager->stageEd25519Key();
+		$this->signatoryManager->activateStagedEd25519Key();
+		// Retiring slot is now populated; staging again is allowed but
+		// activating must refuse until the admin explicitly retires the old
+		// key.
+		$this->signatoryManager->stageEd25519Key();
+		$this->expectException(\RuntimeException::class);
+		$this->expectExceptionMessageMatches('/retiring Ed25519 key still exists/');
+		$this->signatoryManager->activateStagedEd25519Key();
+	}
+
+	public function testActivateRefusesWithoutPendingKey(): void {
+		$this->signatoryManager->getLocalEd25519Signatory();
+		$this->expectException(\RuntimeException::class);
+		$this->expectExceptionMessageMatches('/no pending Ed25519 key/');
+		$this->signatoryManager->activateStagedEd25519Key();
+	}
+
+	public function testRetireRemovesRetiringKeyFromJwks(): void {
+		$first = $this->signatoryManager->getLocalEd25519Signatory();
+		$staged = $this->signatoryManager->stageEd25519Key();
+		$this->signatoryManager->activateStagedEd25519Key();
+		$this->signatoryManager->retireEd25519Key();
+
+		$kids = array_column($this->signatoryManager->getLocalEd25519Jwks(), 'kid');
+		$this->assertSame([$staged->getKeyId()], $kids);
+		// listEd25519Keys also drops the retired pool.
+		$listed = $this->signatoryManager->listEd25519Keys();
+		$this->assertCount(1, $listed);
+		$this->assertSame($staged->getKeyId(), $listed[0]['kid']);
+		$this->assertNotContains($first->getKeyId(), array_column($listed, 'kid'));
+	}
+
+	public function testRetireRefusesWhenNothingToRetire(): void {
+		$this->signatoryManager->getLocalEd25519Signatory();
+		$this->expectException(\RuntimeException::class);
+		$this->expectExceptionMessageMatches('/no retiring Ed25519 key/');
+		$this->signatoryManager->retireEd25519Key();
+	}
+
+	public function testKidStaysStableThroughLifecycle(): void {
+		$first = $this->signatoryManager->getLocalEd25519Signatory();
+		$staged = $this->signatoryManager->stageEd25519Key();
+		// kid for the staged key must stay the same once it is activated;
+		// peers that cached it during the stage window must still resolve it.
+		$this->signatoryManager->activateStagedEd25519Key();
+		$this->assertSame($staged->getKeyId(), $this->signatoryManager->getLocalEd25519Signatory()->getKeyId());
+
+		$this->signatoryManager->retireEd25519Key();
+		$this->signatoryManager->stageEd25519Key();
+		// And every newly minted kid must differ from prior ones, no pool
+		// counter rewinding.
+		$kids = array_column($this->signatoryManager->listEd25519Keys(), 'kid');
+		$this->assertNotContains($first->getKeyId(), $kids);
+		$this->assertSame($kids, array_unique($kids));
+	}
+
+	public function testSignerReturnsNullWhenIdentityCannotBeDerived(): void {
+		// Replace the signature manager with one that cannot derive an
+		// identity at all; provisioning the first key should fail loudly so
+		// the admin gets a clear message instead of a corrupt half-state.
+		$signatureManager = $this->createMock(ISignatureManager::class);
+		$signatureManager->method('generateKeyIdFromConfig')
+			->willThrowException(new IdentityNotFoundException('no identity'));
+		$urlGenerator = $this->createMock(IURLGenerator::class);
+		$urlGenerator->method('linkToRouteAbsolute')
+			->willThrowException(new IdentityNotFoundException('no url either'));
+
+		$cacheFactory = $this->createMock(ICacheFactory::class);
+		$cacheFactory->method('createDistributed')->willReturn(new \OC\Memcache\ArrayCache(''));
+
+		$manager = new OCMSignatoryManager(
+			$this->appConfig,
+			$signatureManager,
+			$urlGenerator,
+			$this->identityProofManager,
+			$this->stubClientService(),
+			$this->createMock(IConfig::class),
+			$cacheFactory,
+			$this->createMock(LoggerInterface::class),
+		);
+
+		$this->expectException(\RuntimeException::class);
+		$manager->getLocalEd25519Signatory();
+	}
+
+	private function wireAppConfig(): void {
+		$this->appConfig->method('hasKey')->willReturnCallback(
+			fn (string $app, string $key): bool => $app === 'core' && array_key_exists($key, $this->appConfigStore)
+		);
+		$this->appConfig->method('getValueInt')->willReturnCallback(
+			fn (string $app, string $key, int $default = 0): int => (int)($this->appConfigStore[$key] ?? $default)
+		);
+		$this->appConfig->method('setValueInt')->willReturnCallback(
+			function (string $app, string $key, int $value): bool {
+				$this->appConfigStore[$key] = (string)$value;
+				return true;
+			}
+		);
+		$this->appConfig->method('getValueString')->willReturnCallback(
+			fn (string $app, string $key, string $default = '') => $this->appConfigStore[$key] ?? $default
+		);
+		$this->appConfig->method('setValueString')->willReturnCallback(
+			function (string $app, string $key, string $value): bool {
+				$this->appConfigStore[$key] = $value;
+				return true;
+			}
+		);
+		$this->appConfig->method('getValueBool')->willReturn(false);
+		$this->appConfig->method('deleteKey')->willReturnCallback(
+			function (string $app, string $key): void {
+				unset($this->appConfigStore[$key]);
+			}
+		);
+	}
+
+	private function wireIdentityProofManager(): void {
+		$this->identityProofManager->method('hasAppKey')->willReturnCallback(
+			fn (string $app, string $name): bool => isset($this->appKeyStore[$app . '/' . $name])
+		);
+		$this->identityProofManager->method('generateEd25519AppKey')->willReturnCallback(
+			function (string $app, string $name): Key {
+				$keyPair = sodium_crypto_sign_keypair();
+				$key = new Key(sodium_crypto_sign_publickey($keyPair), sodium_crypto_sign_secretkey($keyPair));
+				$this->appKeyStore[$app . '/' . $name] = $key;
+				return $key;
+			}
+		);
+		$this->identityProofManager->method('getAppKey')->willReturnCallback(
+			fn (string $app, string $name): Key => $this->appKeyStore[$app . '/' . $name]
+		);
+		$this->identityProofManager->method('deleteAppKey')->willReturnCallback(
+			function (string $app, string $name): bool {
+				$existed = isset($this->appKeyStore[$app . '/' . $name]);
+				unset($this->appKeyStore[$app . '/' . $name]);
+				return $existed;
+			}
+		);
+	}
+
+	private function stubClientService(): IClientService&MockObject {
+		$service = $this->createMock(IClientService::class);
+		$service->method('newClient')->willReturn($this->createMock(IClient::class));
+		return $service;
+	}
+}
diff --git a/tests/lib/OCM/Rfc9421SignatoryManagerTest.php b/tests/lib/OCM/Rfc9421SignatoryManagerTest.php
new file mode 100644
index 0000000000000..f186986cf81a2
--- /dev/null
+++ b/tests/lib/OCM/Rfc9421SignatoryManagerTest.php
@@ -0,0 +1,78 @@
+<?php
+
+declare(strict_types=1);
+
+/**
+ * SPDX-FileCopyrightText: 2026 Nextcloud GmbH and Nextcloud contributors
+ * SPDX-License-Identifier: AGPL-3.0-or-later
+ */
+
+namespace Test\OCM;
+
+use Firebase\JWT\Key;
+use OC\OCM\OCMSignatoryManager;
+use OC\OCM\Rfc9421SignatoryManager;
+use OCP\Security\Signature\Exceptions\IdentityNotFoundException;
+use OCP\Security\Signature\Model\Signatory;
+use PHPUnit\Framework\MockObject\MockObject;
+use Test\TestCase;
+
+class Rfc9421SignatoryManagerTest extends TestCase {
+	private OCMSignatoryManager&MockObject $delegate;
+	private Rfc9421SignatoryManager $wrapper;
+
+	#[\Override]
+	protected function setUp(): void {
+		parent::setUp();
+		$this->delegate = $this->createMock(OCMSignatoryManager::class);
+		$this->wrapper = new Rfc9421SignatoryManager($this->delegate);
+	}
+
+	public function testGetOptionsForcesRfc9421Format(): void {
+		$this->delegate->method('getOptions')->willReturn([
+			'algorithm' => 'rsa-sha512',
+			'rfc9421.format' => false,
+		]);
+
+		$options = $this->wrapper->getOptions();
+		$this->assertTrue($options['rfc9421.format']);
+		$this->assertSame('rsa-sha512', $options['algorithm']);
+	}
+
+	public function testGetLocalSignatoryReturnsEd25519Key(): void {
+		$signatory = $this->createMock(Signatory::class);
+		$this->delegate->method('getLocalEd25519Signatory')->willReturn($signatory);
+
+		$this->assertSame($signatory, $this->wrapper->getLocalSignatory());
+	}
+
+	public function testGetLocalSignatoryThrowsWhenEd25519Unavailable(): void {
+		$this->delegate->method('getLocalEd25519Signatory')->willReturn(null);
+
+		$this->expectException(IdentityNotFoundException::class);
+		$this->wrapper->getLocalSignatory();
+	}
+
+	public function testProviderIdDelegated(): void {
+		$this->delegate->method('getProviderId')->willReturn('ocm');
+		$this->assertSame('ocm', $this->wrapper->getProviderId());
+	}
+
+	public function testRemoteSignatoryDelegated(): void {
+		$signatory = $this->createMock(Signatory::class);
+		$this->delegate->expects($this->once())
+			->method('getRemoteSignatory')
+			->with('sender.example.org')
+			->willReturn($signatory);
+		$this->assertSame($signatory, $this->wrapper->getRemoteSignatory('sender.example.org'));
+	}
+
+	public function testRemoteKeyDelegated(): void {
+		$key = $this->createMock(Key::class);
+		$this->delegate->expects($this->once())
+			->method('getRemoteKey')
+			->with('sender.example.org', 'kid-1')
+			->willReturn($key);
+		$this->assertSame($key, $this->wrapper->getRemoteKey('sender.example.org', 'kid-1'));
+	}
+}
diff --git a/tests/lib/Security/Signature/Model/Rfc9421RoundTripTest.php b/tests/lib/Security/Signature/Model/Rfc9421RoundTripTest.php
new file mode 100644
index 0000000000000..5f4285f14ccde
--- /dev/null
+++ b/tests/lib/Security/Signature/Model/Rfc9421RoundTripTest.php
@@ -0,0 +1,316 @@
+<?php
+
+declare(strict_types=1);
+
+/**
+ * SPDX-FileCopyrightText: 2026 Nextcloud GmbH and Nextcloud contributors
+ * SPDX-License-Identifier: AGPL-3.0-or-later
+ */
+
+namespace Test\Security\Signature\Model;
+
+use Firebase\JWT\JWK;
+use OC\Security\Signature\Model\Rfc9421IncomingSignedRequest;
+use OC\Security\Signature\Model\Rfc9421OutgoingSignedRequest;
+use OCP\IRequest;
+use OCP\Security\Signature\Enum\DigestAlgorithm;
+use OCP\Security\Signature\Enum\SignatureAlgorithm;
+use OCP\Security\Signature\Exceptions\IncomingRequestException;
+use OCP\Security\Signature\Exceptions\InvalidSignatureException;
+use OCP\Security\Signature\Exceptions\SignatureNotFoundException;
+use OCP\Security\Signature\ISignatoryManager;
+use OCP\Security\Signature\Model\Signatory;
+use Test\TestCase;
+
+class Rfc9421RoundTripTest extends TestCase {
+	public function testEd25519RoundTripVerifies(): void {
+		[$signatory, $jwk] = $this->ed25519Material('https://sender.example.org/ocm#ed25519');
+		$signatoryManager = $this->makeSignatoryManager($signatory);
+
+		$body = '{"hello":"world"}';
+		$method = 'POST';
+		$uri = 'https://receiver.example.org/ocm/shares';
+
+		$out = new Rfc9421OutgoingSignedRequest($body, $signatoryManager, 'receiver.example.org', $method, $uri);
+		$out->sign();
+
+		$req = $this->mockRequestFromOutgoing($out, $method, '/ocm/shares', 'receiver.example.org');
+		$in = new Rfc9421IncomingSignedRequest($body, $req);
+		$in->setKey($jwk);
+
+		$this->assertSame($out->getSignatureBaseString(), $in->getSignatureBaseString());
+		$in->verify(); // throws on failure
+		$this->addToAssertionCount(1);
+	}
+
+	public function testTamperedBodyRejected(): void {
+		[$signatory, $jwk] = $this->ed25519Material('https://sender.example.org/ocm#ed25519');
+		$signatoryManager = $this->makeSignatoryManager($signatory);
+
+		$body = 'original';
+		$out = new Rfc9421OutgoingSignedRequest($body, $signatoryManager, 'receiver.example.org', 'POST', 'https://receiver.example.org/ocm/shares');
+		$out->sign();
+
+		$req = $this->mockRequestFromOutgoing($out, 'POST', '/ocm/shares', 'receiver.example.org');
+		$this->expectException(IncomingRequestException::class);
+		new Rfc9421IncomingSignedRequest('tampered', $req);
+	}
+
+	public function testTamperedSignatureRejected(): void {
+		[$signatory, $jwk] = $this->ed25519Material('https://sender.example.org/ocm#ed25519');
+		$signatoryManager = $this->makeSignatoryManager($signatory);
+
+		$body = 'msg';
+		$out = new Rfc9421OutgoingSignedRequest($body, $signatoryManager, 'receiver.example.org', 'POST', 'https://receiver.example.org/ocm/shares');
+		$out->sign();
+
+		$headers = $out->getHeaders();
+		// Replace the inner base64 of the signature with a different valid base64.
+		$headers['Signature'] = preg_replace('/=:[^:]+:/', '=:' . base64_encode(random_bytes(64)) . ':', (string)$headers['Signature']);
+
+		$req = $this->mockRequest($headers, 'POST', '/ocm/shares', 'receiver.example.org');
+		$in = new Rfc9421IncomingSignedRequest($body, $req);
+		$in->setKey($jwk);
+
+		$this->expectException(InvalidSignatureException::class);
+		$in->verify();
+	}
+
+	public function testOutgoingUsesOcmLabel(): void {
+		[$signatory] = $this->ed25519Material('https://sender.example.org/ocm#ed25519');
+		$signatoryManager = $this->makeSignatoryManager($signatory);
+
+		$out = new Rfc9421OutgoingSignedRequest('msg', $signatoryManager, 'receiver.example.org', 'POST', 'https://receiver.example.org/ocm/shares');
+		$out->sign();
+
+		$headers = $out->getHeaders();
+		$this->assertStringStartsWith('ocm=(', (string)$headers['Signature-Input']);
+		$this->assertStringStartsWith('ocm=:', (string)$headers['Signature']);
+	}
+
+	public function testRequestWithoutOcmLabelRejected(): void {
+		[$signatory, $jwk] = $this->ed25519Material('https://sender.example.org/ocm#ed25519');
+		$signatoryManager = $this->makeSignatoryManager($signatory);
+
+		$out = new Rfc9421OutgoingSignedRequest('msg', $signatoryManager, 'receiver.example.org', 'POST', 'https://receiver.example.org/ocm/shares');
+		$out->sign();
+
+		// Rename the OCM label to something else; verifier MUST reject.
+		$headers = $out->getHeaders();
+		$headers['Signature-Input'] = preg_replace('/^ocm=/', 'sig1=', (string)$headers['Signature-Input']);
+		$headers['Signature'] = preg_replace('/^ocm=/', 'sig1=', (string)$headers['Signature']);
+
+		$req = $this->mockRequest($headers, 'POST', '/ocm/shares', 'receiver.example.org');
+		$this->expectException(SignatureNotFoundException::class);
+		new Rfc9421IncomingSignedRequest('msg', $req);
+	}
+
+	public function testDuplicateOcmLabelRejected(): void {
+		// RFC 8941 §4.2 last-wins on duplicate dictionary keys, but OCM
+		// mandates that duplicate `ocm` entries cause the request to be
+		// rejected outright. The model layer enforces that.
+		[$signatory, $jwk] = $this->ed25519Material('https://sender.example.org/ocm#ed25519');
+		$signatoryManager = $this->makeSignatoryManager($signatory);
+
+		$out = new Rfc9421OutgoingSignedRequest('msg', $signatoryManager, 'receiver.example.org', 'POST', 'https://receiver.example.org/ocm/shares');
+		$out->sign();
+
+		$headers = $out->getHeaders();
+		$headers['Signature-Input'] = (string)$headers['Signature-Input'] . ', ' . (string)$headers['Signature-Input'];
+		$headers['Signature'] = (string)$headers['Signature'] . ', ' . (string)$headers['Signature'];
+
+		$req = $this->mockRequest($headers, 'POST', '/ocm/shares', 'receiver.example.org');
+		$this->expectException(IncomingRequestException::class);
+		new Rfc9421IncomingSignedRequest('msg', $req);
+	}
+
+	public function testForeignSiblingLabelIgnored(): void {
+		[$signatory, $jwk] = $this->ed25519Material('https://sender.example.org/ocm#ed25519');
+		$signatoryManager = $this->makeSignatoryManager($signatory);
+
+		$out = new Rfc9421OutgoingSignedRequest('msg', $signatoryManager, 'receiver.example.org', 'POST', 'https://receiver.example.org/ocm/shares');
+		$out->sign();
+
+		// Splice in a sibling proxy_sig1 entry; the verifier must ignore it
+		// and still verify the ocm-labeled signature successfully.
+		$headers = $out->getHeaders();
+		$proxyParams = '("@method");created=1;keyid="proxy"';
+		$proxySig = base64_encode(random_bytes(64));
+		$headers['Signature-Input'] = (string)$headers['Signature-Input'] . ', proxy_sig1=' . $proxyParams;
+		$headers['Signature'] = (string)$headers['Signature'] . ', proxy_sig1=:' . $proxySig . ':';
+
+		$req = $this->mockRequest($headers, 'POST', '/ocm/shares', 'receiver.example.org');
+		$in = new Rfc9421IncomingSignedRequest('msg', $req);
+		$in->setKey($jwk);
+		$in->verify();
+		$this->addToAssertionCount(1);
+	}
+
+	public function testTooOldSignatureRejected(): void {
+		[$signatory] = $this->ed25519Material('https://sender.example.org/ocm#ed25519');
+		$signatoryManager = $this->makeSignatoryManager($signatory);
+
+		$body = 'msg';
+		$out = new Rfc9421OutgoingSignedRequest($body, $signatoryManager, 'receiver.example.org', 'POST', 'https://receiver.example.org/ocm/shares');
+		$out->sign();
+
+		// Backdate `created` in Signature-Input by 10 minutes.
+		$headers = $out->getHeaders();
+		$pastCreated = time() - 600;
+		$headers['Signature-Input'] = preg_replace('/created=\d+/', 'created=' . $pastCreated, (string)$headers['Signature-Input']);
+
+		$req = $this->mockRequest($headers, 'POST', '/ocm/shares', 'receiver.example.org');
+		$this->expectException(IncomingRequestException::class);
+		new Rfc9421IncomingSignedRequest($body, $req, ['ttl' => 300]);
+	}
+
+	public function testFutureCreatedRejected(): void {
+		[$signatory] = $this->ed25519Material('https://sender.example.org/ocm#ed25519');
+		$signatoryManager = $this->makeSignatoryManager($signatory);
+
+		$body = 'msg';
+		$out = new Rfc9421OutgoingSignedRequest($body, $signatoryManager, 'receiver.example.org', 'POST', 'https://receiver.example.org/ocm/shares');
+		$out->sign();
+
+		// Push `created` 10 minutes into the future, well past the
+		// 60-second skew tolerance.
+		$headers = $out->getHeaders();
+		$futureCreated = time() + 600;
+		$headers['Signature-Input'] = preg_replace('/created=\d+/', 'created=' . $futureCreated, (string)$headers['Signature-Input']);
+
+		$req = $this->mockRequest($headers, 'POST', '/ocm/shares', 'receiver.example.org');
+		$this->expectException(IncomingRequestException::class);
+		new Rfc9421IncomingSignedRequest($body, $req);
+	}
+
+	public function testMissingCreatedRejected(): void {
+		[$signatory] = $this->ed25519Material('https://sender.example.org/ocm#ed25519');
+		$signatoryManager = $this->makeSignatoryManager($signatory);
+
+		$body = 'msg';
+		$out = new Rfc9421OutgoingSignedRequest($body, $signatoryManager, 'receiver.example.org', 'POST', 'https://receiver.example.org/ocm/shares');
+		$out->sign();
+
+		// Strip the `;created=...` parameter so the signature loses its
+		// freshness anchor.
+		$headers = $out->getHeaders();
+		$headers['Signature-Input'] = preg_replace('/;created=\d+/', '', (string)$headers['Signature-Input']);
+
+		$req = $this->mockRequest($headers, 'POST', '/ocm/shares', 'receiver.example.org');
+		$this->expectException(IncomingRequestException::class);
+		new Rfc9421IncomingSignedRequest($body, $req);
+	}
+
+	public function testSignatureNotCoveringRequiredComponentsRejected(): void {
+		// A peer that signs only `@method` and `@target-uri`: the body and
+		// freshness window aren't bound. Even with a valid signature we
+		// must refuse it.
+		[$signatory] = $this->ed25519Material('https://sender.example.org/ocm#ed25519');
+		$signatoryManager = $this->makeSignatoryManagerWithComponents(
+			$signatory,
+			['@method', '@target-uri'],
+		);
+
+		$body = 'msg';
+		$out = new Rfc9421OutgoingSignedRequest($body, $signatoryManager, 'receiver.example.org', 'POST', 'https://receiver.example.org/ocm/shares');
+		$out->sign();
+		$req = $this->mockRequest($out->getHeaders(), 'POST', '/ocm/shares', 'receiver.example.org');
+
+		$this->expectException(IncomingRequestException::class);
+		new Rfc9421IncomingSignedRequest($body, $req);
+	}
+
+	private function makeSignatoryManagerWithComponents(Signatory $signatory, array $components): ISignatoryManager {
+		return new class($signatory, $components) implements ISignatoryManager {
+			public function __construct(
+				private Signatory $sig,
+				private array $components,
+			) {
+			}
+
+			public function getProviderId(): string {
+				return 'test';
+			}
+
+			public function getOptions(): array {
+				return [
+					'algorithm' => SignatureAlgorithm::RSA_SHA256,
+					'digestAlgorithm' => DigestAlgorithm::SHA256,
+					'rfc9421.coveredComponents' => $this->components,
+				];
+			}
+
+			public function getLocalSignatory(): Signatory {
+				return $this->sig;
+			}
+
+			public function getRemoteSignatory(string $remote): ?Signatory {
+				return null;
+			}
+		};
+	}
+
+	private function ed25519Material(string $kid): array {
+		$keypair = sodium_crypto_sign_keypair();
+		$publicKey = sodium_crypto_sign_publickey($keypair);
+		$secretKey = sodium_crypto_sign_secretkey($keypair);
+		$signatory = new Signatory(true);
+		$signatory->setKeyId($kid);
+		$signatory->setPublicKey($publicKey);
+		$signatory->setPrivateKey($secretKey);
+		$key = JWK::parseKey([
+			'kty' => 'OKP',
+			'crv' => 'Ed25519',
+			'kid' => $kid,
+			'alg' => 'EdDSA',
+			'x' => rtrim(strtr(base64_encode($publicKey), '+/', '-_'), '='),
+		], 'EdDSA');
+		return [$signatory, $key];
+	}
+
+	private function makeSignatoryManager(Signatory $signatory): ISignatoryManager {
+		return new class($signatory) implements ISignatoryManager {
+			public function __construct(
+				private Signatory $sig,
+			) {
+			}
+
+			public function getProviderId(): string {
+				return 'test';
+			}
+
+			public function getOptions(): array {
+				return [
+					'algorithm' => SignatureAlgorithm::RSA_SHA256,
+					'digestAlgorithm' => DigestAlgorithm::SHA256,
+				];
+			}
+
+			public function getLocalSignatory(): Signatory {
+				return $this->sig;
+			}
+
+			public function getRemoteSignatory(string $remote): ?Signatory {
+				return null;
+			}
+		};
+	}
+
+	private function mockRequestFromOutgoing(Rfc9421OutgoingSignedRequest $out, string $method, string $path, string $host): IRequest {
+		return $this->mockRequest($out->getHeaders(), $method, $path, $host);
+	}
+
+	private function mockRequest(array $headers, string $method, string $path, string $host): IRequest {
+		$lowered = [];
+		foreach ($headers as $name => $value) {
+			$lowered[strtolower($name)] = (string)$value;
+		}
+		$mock = $this->createMock(IRequest::class);
+		$mock->method('getHeader')->willReturnCallback(static fn (string $h) => $lowered[strtolower($h)] ?? '');
+		$mock->method('getMethod')->willReturn($method);
+		$mock->method('getRequestUri')->willReturn($path);
+		$mock->method('getServerProtocol')->willReturn('https');
+		$mock->method('getServerHost')->willReturn($host);
+		return $mock;
+	}
+}
diff --git a/tests/lib/Security/Signature/Rfc9421/AlgorithmTest.php b/tests/lib/Security/Signature/Rfc9421/AlgorithmTest.php
new file mode 100644
index 0000000000000..ba117ca99baf4
--- /dev/null
+++ b/tests/lib/Security/Signature/Rfc9421/AlgorithmTest.php
@@ -0,0 +1,197 @@
+<?php
+
+declare(strict_types=1);
+
+/**
+ * SPDX-FileCopyrightText: 2026 Nextcloud GmbH and Nextcloud contributors
+ * SPDX-License-Identifier: AGPL-3.0-or-later
+ */
+
+namespace Test\Security\Signature\Rfc9421;
+
+use Firebase\JWT\JWK;
+use Firebase\JWT\Key;
+use OC\Security\Signature\Rfc9421\Algorithm;
+use OCP\Security\Signature\Exceptions\SignatureException;
+use Test\TestCase;
+
+class AlgorithmTest extends TestCase {
+	public function testNormalizeNativeIsPassThrough(): void {
+		$this->assertSame('ed25519', Algorithm::normalize('ed25519'));
+		$this->assertSame('rsa-v1_5-sha256', Algorithm::normalize('rsa-v1_5-sha256'));
+		$this->assertSame('ecdsa-p256-sha256', Algorithm::normalize('ecdsa-p256-sha256'));
+	}
+
+	public function testNormalizeJoseAliases(): void {
+		$this->assertSame('ed25519', Algorithm::normalize('EdDSA'));
+		$this->assertSame('ecdsa-p256-sha256', Algorithm::normalize('ES256'));
+		$this->assertSame('ecdsa-p384-sha384', Algorithm::normalize('ES384'));
+		$this->assertSame('rsa-v1_5-sha256', Algorithm::normalize('RS256'));
+	}
+
+	public function testNormalizeRejectsUnknown(): void {
+		$this->expectException(SignatureException::class);
+		Algorithm::normalize('totally-not-real');
+	}
+
+	public function testNormalizeRejectsRsaPss(): void {
+		$this->expectException(SignatureException::class);
+		Algorithm::normalize('rsa-pss-sha512');
+	}
+
+	public function testNormalizeRejectsJosePsAlias(): void {
+		$this->expectException(SignatureException::class);
+		Algorithm::normalize('PS512');
+	}
+
+	public function testDeriveJoseAlgFromJwk(): void {
+		$this->assertSame('EdDSA', Algorithm::deriveJoseAlgFromJwk(['kty' => 'OKP', 'crv' => 'Ed25519']));
+		$this->assertSame('ES256', Algorithm::deriveJoseAlgFromJwk(['kty' => 'EC', 'crv' => 'P-256']));
+		$this->assertSame('ES384', Algorithm::deriveJoseAlgFromJwk(['kty' => 'EC', 'crv' => 'P-384']));
+		// RSA: hash function isn't determined by key shape.
+		$this->assertNull(Algorithm::deriveJoseAlgFromJwk(['kty' => 'RSA']));
+		$this->assertNull(Algorithm::deriveJoseAlgFromJwk([]));
+	}
+
+	public function testEd25519RoundTrip(): void {
+		[$priv, $key] = $this->ed25519KeyPair();
+		$base = 'arbitrary signature base';
+		$sig = Algorithm::sign($base, $priv, 'ed25519');
+		$this->assertSame(64, strlen($sig));
+		$this->assertTrue(Algorithm::verify($base, $sig, $key, 'ed25519'));
+		// JOSE alias accepted.
+		$this->assertTrue(Algorithm::verify($base, $sig, $key, 'EdDSA'));
+		// alg-omitted path resolves through Key alg.
+		$this->assertTrue(Algorithm::verify($base, $sig, $key, null));
+		// tamper detection
+		$this->assertFalse(Algorithm::verify($base . 'x', $sig, $key, 'ed25519'));
+	}
+
+	public function testRsaPkcs1RoundTrip(): void {
+		[$priv, $key] = $this->rsaKeyPair();
+		$sig = Algorithm::sign('payload', $priv, 'rsa-v1_5-sha256');
+		$this->assertSame(256, strlen($sig));
+		$this->assertTrue(Algorithm::verify('payload', $sig, $key, 'rsa-v1_5-sha256'));
+		$this->assertTrue(Algorithm::verify('payload', $sig, $key, 'RS256'));
+	}
+
+	public function testEcdsaP256RoundTrip(): void {
+		[$priv, $key] = $this->ecKeyPair('prime256v1', 'P-256', 'ES256');
+		$sig = Algorithm::sign('payload', $priv, 'ecdsa-p256-sha256');
+		$this->assertSame(64, strlen($sig));
+		$this->assertTrue(Algorithm::verify('payload', $sig, $key, 'ecdsa-p256-sha256'));
+		$this->assertTrue(Algorithm::verify('payload', $sig, $key, 'ES256'));
+	}
+
+	public function testEcdsaP384RoundTrip(): void {
+		[$priv, $key] = $this->ecKeyPair('secp384r1', 'P-384', 'ES384');
+		$sig = Algorithm::sign('payload', $priv, 'ecdsa-p384-sha384');
+		$this->assertSame(96, strlen($sig));
+		$this->assertTrue(Algorithm::verify('payload', $sig, $key, 'ecdsa-p384-sha384'));
+	}
+
+	public function testKeyTypeMismatchFailsClosed(): void {
+		[, $rsaKey] = $this->rsaKeyPair();
+		$this->expectException(SignatureException::class);
+		Algorithm::verify('payload', random_bytes(64), $rsaKey, 'ed25519');
+	}
+
+	public function testAlgHintConflictsWithJwkAlgRejected(): void {
+		// Ed25519 JWK, request claims ES256: RFC 9421 §3.2 step 6 disagreement.
+		[, $key] = $this->ed25519KeyPair();
+		$this->expectException(SignatureException::class);
+		Algorithm::verify('payload', random_bytes(64), $key, 'ES256');
+	}
+
+	public function testParseKeyRejectsContradictoryAlg(): void {
+		// kty=OKP/crv=Ed25519 with alg=ES256 is contradictory; firebase's
+		// parseKey rejects it before we ever build a Key.
+		$keypair = sodium_crypto_sign_keypair();
+		$this->expectException(\Throwable::class);
+		JWK::parseKey([
+			'kty' => 'OKP',
+			'crv' => 'Ed25519',
+			'kid' => 'k',
+			'alg' => 'ES256',
+			'x' => self::b64url(sodium_crypto_sign_publickey($keypair)),
+		], null);
+	}
+
+	public function testAlgHintAgreesViaJoseAlias(): void {
+		[$priv, $key] = $this->ed25519KeyPair();
+		$base = 'agreement check';
+		$sig = Algorithm::sign($base, $priv, 'ed25519');
+		$this->assertTrue(Algorithm::verify($base, $sig, $key, 'ed25519'));
+		$this->assertTrue(Algorithm::verify($base, $sig, $key, 'EdDSA'));
+	}
+
+	public function testEcdsaRawToDerProducesValidSignature(): void {
+		[$priv, $key] = $this->ecKeyPair('prime256v1', 'P-256', 'ES256');
+		$rawSig = Algorithm::sign('msg', $priv, 'ecdsa-p256-sha256');
+		$der = Algorithm::ecdsaRawToDer($rawSig, 32);
+		$this->assertNotNull($der);
+		$this->assertTrue(Algorithm::verify('msg', $rawSig, $key, 'ecdsa-p256-sha256'));
+	}
+
+	public function testEcdsaRawToDerWrongLength(): void {
+		$this->assertNull(Algorithm::ecdsaRawToDer('short', 32));
+	}
+
+	/**
+	 * @return array{0: string, 1: Key}
+	 */
+	private function ed25519KeyPair(): array {
+		$keypair = sodium_crypto_sign_keypair();
+		$publicKey = sodium_crypto_sign_publickey($keypair);
+		$secretKey = sodium_crypto_sign_secretkey($keypair);
+		$key = JWK::parseKey([
+			'kty' => 'OKP',
+			'crv' => 'Ed25519',
+			'kid' => 'k',
+			'alg' => 'EdDSA',
+			'x' => self::b64url($publicKey),
+		], 'EdDSA');
+		return [$secretKey, $key];
+	}
+
+	/**
+	 * @return array{0: string, 1: Key}
+	 */
+	private function rsaKeyPair(): array {
+		$pkey = openssl_pkey_new(['private_key_type' => OPENSSL_KEYTYPE_RSA, 'private_key_bits' => 2048]);
+		$priv = '';
+		openssl_pkey_export($pkey, $priv);
+		$details = openssl_pkey_get_details($pkey);
+		$key = JWK::parseKey([
+			'kty' => 'RSA',
+			'kid' => 'k',
+			'alg' => 'RS256',
+			'n' => self::b64url($details['rsa']['n']),
+			'e' => self::b64url($details['rsa']['e']),
+		], 'RS256');
+		return [$priv, $key];
+	}
+
+	/**
+	 * @return array{0: string, 1: Key}
+	 */
+	private function ecKeyPair(string $opensslCurve, string $jwkCurve, string $joseAlg): array {
+		$pkey = openssl_pkey_new(['private_key_type' => OPENSSL_KEYTYPE_EC, 'curve_name' => $opensslCurve]);
+		$priv = '';
+		openssl_pkey_export($pkey, $priv);
+		$details = openssl_pkey_get_details($pkey);
+		$key = JWK::parseKey([
+			'kty' => 'EC',
+			'crv' => $jwkCurve,
+			'kid' => 'k',
+			'alg' => $joseAlg,
+			'x' => self::b64url($details['ec']['x']),
+			'y' => self::b64url($details['ec']['y']),
+		], $joseAlg);
+		return [$priv, $key];
+	}
+
+	private static function b64url(string $bin): string {
+		return rtrim(strtr(base64_encode($bin), '+/', '-_'), '=');
+	}
+}
diff --git a/tests/lib/Security/Signature/Rfc9421/ContentDigestTest.php b/tests/lib/Security/Signature/Rfc9421/ContentDigestTest.php
new file mode 100644
index 0000000000000..4198acec5342b
--- /dev/null
+++ b/tests/lib/Security/Signature/Rfc9421/ContentDigestTest.php
@@ -0,0 +1,76 @@
+<?php
+
+declare(strict_types=1);
+
+/**
+ * SPDX-FileCopyrightText: 2026 Nextcloud GmbH and Nextcloud contributors
+ * SPDX-License-Identifier: AGPL-3.0-or-later
+ */
+
+namespace Test\Security\Signature\Rfc9421;
+
+use OC\Security\Signature\Rfc9421\ContentDigest;
+use Test\TestCase;
+
+class ContentDigestTest extends TestCase {
+	public function testComputeRoundTrip(): void {
+		$body = '{"hello":"world"}';
+		$header = ContentDigest::compute($body, ContentDigest::ALGO_SHA256);
+		$this->assertStringStartsWith('sha-256=:', $header);
+		$this->assertStringEndsWith(':', $header);
+		$this->assertTrue(ContentDigest::verify($header, $body));
+	}
+
+	public function testDifferentBodyFails(): void {
+		$header = ContentDigest::compute('hello', ContentDigest::ALGO_SHA256);
+		$this->assertFalse(ContentDigest::verify($header, 'goodbye'));
+	}
+
+	public function testSha512(): void {
+		$header = ContentDigest::compute('payload', ContentDigest::ALGO_SHA512);
+		$this->assertStringStartsWith('sha-512=:', $header);
+		$this->assertTrue(ContentDigest::verify($header, 'payload'));
+	}
+
+	public function testParseMultipleAlgorithmsAcceptsAnyMatch(): void {
+		$body = 'data';
+		$sha256 = ContentDigest::compute($body, ContentDigest::ALGO_SHA256);
+		$sha512 = ContentDigest::compute($body, ContentDigest::ALGO_SHA512);
+		$header = $sha256 . ', ' . $sha512;
+		$this->assertTrue(ContentDigest::verify($header, $body));
+	}
+
+	public function testFailsIfAnyRecognisedAlgorithmMismatches(): void {
+		// All recognised digests must agree. A correct sha-256 alongside a
+		// wrong sha-512 is treated as an attack on the weaker algorithm,
+		// not as a successful match on the stronger one.
+		$body = 'data';
+		$sha256 = ContentDigest::compute($body, ContentDigest::ALGO_SHA256);
+		$wrongSha512 = 'sha-512=:' . base64_encode(hash('sha512', 'tampered', true)) . ':';
+		$this->assertFalse(ContentDigest::verify($sha256 . ', ' . $wrongSha512, $body));
+		// And the inverse ordering.
+		$this->assertFalse(ContentDigest::verify($wrongSha512 . ', ' . $sha256, $body));
+	}
+
+	public function testUnknownAlgorithmIsIgnored(): void {
+		$body = 'data';
+		$sha256 = ContentDigest::compute($body, ContentDigest::ALGO_SHA256);
+		$header = 'md5=:abcd:, ' . $sha256;
+		$this->assertTrue(ContentDigest::verify($header, $body));
+	}
+
+	public function testEmptyHeaderFails(): void {
+		$this->assertFalse(ContentDigest::verify('', 'body'));
+	}
+
+	public function testGarbageHeaderFails(): void {
+		$this->assertFalse(ContentDigest::verify('not a digest', 'body'));
+	}
+
+	public function testParseExtractsRawBytes(): void {
+		$header = ContentDigest::compute('abc', ContentDigest::ALGO_SHA256);
+		$parsed = ContentDigest::parse($header);
+		$this->assertArrayHasKey('sha-256', $parsed);
+		$this->assertSame(hash('sha256', 'abc', true), $parsed['sha-256']);
+	}
+}
diff --git a/tests/lib/Security/Signature/Rfc9421/SignatureBaseTest.php b/tests/lib/Security/Signature/Rfc9421/SignatureBaseTest.php
new file mode 100644
index 0000000000000..d5aed5e9ab679
--- /dev/null
+++ b/tests/lib/Security/Signature/Rfc9421/SignatureBaseTest.php
@@ -0,0 +1,85 @@
+<?php
+
+declare(strict_types=1);
+
+/**
+ * SPDX-FileCopyrightText: 2026 Nextcloud GmbH and Nextcloud contributors
+ * SPDX-License-Identifier: AGPL-3.0-or-later
+ */
+
+namespace Test\Security\Signature\Rfc9421;
+
+use OC\Security\Signature\Rfc9421\SignatureBase;
+use OCP\Security\Signature\Exceptions\SignatureException;
+use Test\TestCase;
+
+class SignatureBaseTest extends TestCase {
+	public function testBuildBasicComponents(): void {
+		$base = SignatureBase::build(
+			method: 'POST',
+			uri: 'https://example.org/foo?bar=baz',
+			headers: [
+				'content-digest' => 'sha-256=:abcd:',
+				'date' => 'Mon, 04 May 2026 12:00:00 GMT',
+			],
+			components: ['@method', '@target-uri', 'content-digest', 'date'],
+			signatureParamsLine: '("@method" "@target-uri" "content-digest" "date");created=1;keyid="k"',
+		);
+
+		$expected = '"@method": POST' . "\n"
+			. '"@target-uri": https://example.org/foo?bar=baz' . "\n"
+			. '"content-digest": sha-256=:abcd:' . "\n"
+			. '"date": Mon, 04 May 2026 12:00:00 GMT' . "\n"
+			. '"@signature-params": ("@method" "@target-uri" "content-digest" "date");created=1;keyid="k"';
+		$this->assertSame($expected, $base);
+	}
+
+	public function testAuthorityStripsDefaultPort(): void {
+		$base = SignatureBase::build('GET', 'https://EXAMPLE.org:443/x', [], ['@authority'], '()');
+		$this->assertStringContainsString('"@authority": example.org' . "\n", $base);
+	}
+
+	public function testAuthorityKeepsCustomPort(): void {
+		$base = SignatureBase::build('GET', 'https://example.org:8443/x', [], ['@authority'], '()');
+		$this->assertStringContainsString('"@authority": example.org:8443' . "\n", $base);
+	}
+
+	public function testQueryComponent(): void {
+		$base = SignatureBase::build('GET', 'https://example.org/x?a=1', [], ['@query'], '()');
+		$this->assertStringContainsString('"@query": ?a=1' . "\n", $base);
+	}
+
+	public function testMissingFieldThrows(): void {
+		$this->expectException(SignatureException::class);
+		SignatureBase::build('GET', 'https://example.org/', [], ['x-missing'], '()');
+	}
+
+	public function testFieldValueIsTrimmed(): void {
+		$base = SignatureBase::build(
+			'GET',
+			'https://example.org/',
+			['date' => '  Mon, 04 May 2026 12:00:00 GMT  '],
+			['date'],
+			'()'
+		);
+		$this->assertStringContainsString('"date": Mon, 04 May 2026 12:00:00 GMT' . "\n", $base);
+	}
+
+	public function testSerializeSignatureParams(): void {
+		$line = SignatureBase::serializeSignatureParams(
+			['@method', '@target-uri'],
+			['created' => 100, 'keyid' => 'kid', 'expires' => 200],
+		);
+		$this->assertSame('("@method" "@target-uri");created=100;keyid="kid";expires=200', $line);
+	}
+
+	public function testSerializeBareItemEscapesQuotes(): void {
+		$this->assertSame('"\\"hi\\""', SignatureBase::serializeBareItem('"hi"'));
+		$this->assertSame('"\\\\"', SignatureBase::serializeBareItem('\\'));
+	}
+
+	public function testSerializeBareItemBoolean(): void {
+		$this->assertSame('?1', SignatureBase::serializeBareItem(true));
+		$this->assertSame('?0', SignatureBase::serializeBareItem(false));
+	}
+}
diff --git a/tests/lib/Security/Signature/SignatureManagerDispatchTest.php b/tests/lib/Security/Signature/SignatureManagerDispatchTest.php
new file mode 100644
index 0000000000000..ae5945fd9b5a9
--- /dev/null
+++ b/tests/lib/Security/Signature/SignatureManagerDispatchTest.php
@@ -0,0 +1,262 @@
+<?php
+
+declare(strict_types=1);
+
+/**
+ * SPDX-FileCopyrightText: 2026 Nextcloud GmbH and Nextcloud contributors
+ * SPDX-License-Identifier: AGPL-3.0-or-later
+ */
+
+namespace Test\Security\Signature;
+
+use Firebase\JWT\JWK;
+use Firebase\JWT\Key;
+use OC\Security\Signature\Db\SignatoryMapper;
+use OC\Security\Signature\Model\Rfc9421IncomingSignedRequest;
+use OC\Security\Signature\Model\Rfc9421OutgoingSignedRequest;
+use OC\Security\Signature\Rfc9421\IJwkResolvingSignatoryManager;
+use OC\Security\Signature\SignatureManager;
+use OCP\IAppConfig;
+use OCP\IRequest;
+use OCP\Security\Signature\Enum\DigestAlgorithm;
+use OCP\Security\Signature\Enum\SignatureAlgorithm;
+use OCP\Security\Signature\Exceptions\IncomingRequestException;
+use OCP\Security\Signature\ISignatoryManager;
+use OCP\Security\Signature\Model\Signatory;
+use PHPUnit\Framework\MockObject\MockObject;
+use Psr\Log\LoggerInterface;
+use Test\TestCase;
+
+class SignatureManagerDispatchTest extends TestCase {
+	private IRequest&MockObject $request;
+	private SignatoryMapper&MockObject $mapper;
+	private IAppConfig&MockObject $appConfig;
+	private LoggerInterface&MockObject $logger;
+	private SignatureManager $signatureManager;
+
+	#[\Override]
+	protected function setUp(): void {
+		parent::setUp();
+
+		$this->request = $this->createMock(IRequest::class);
+		$this->mapper = $this->createMock(SignatoryMapper::class);
+		$this->appConfig = $this->createMock(IAppConfig::class);
+		$this->logger = $this->createMock(LoggerInterface::class);
+
+		$this->signatureManager = new SignatureManager(
+			$this->request,
+			$this->mapper,
+			$this->appConfig,
+			$this->logger,
+		);
+	}
+
+	public function testOutgoingDispatchesToCavageByDefault(): void {
+		// Cavage signs with an RSA PEM, so we need a real RSA keypair here;
+		// the Ed25519 helper would produce libsodium bytes that openssl_sign
+		// can't consume.
+		$signatoryManager = $this->rsaSignatoryManager();
+
+		$signed = $this->signatureManager->getOutgoingSignedRequest(
+			$signatoryManager,
+			'{}',
+			'POST',
+			'https://receiver.example.org/ocm/shares',
+		);
+
+		$this->assertNotInstanceOf(Rfc9421OutgoingSignedRequest::class, $signed);
+	}
+
+	public function testOutgoingDispatchesToRfc9421WhenOptionSet(): void {
+		[$signatoryManager,] = $this->ed25519SignatoryManager(rfc9421Format: true);
+
+		$signed = $this->signatureManager->getOutgoingSignedRequest(
+			$signatoryManager,
+			'{}',
+			'POST',
+			'https://receiver.example.org/ocm/shares',
+		);
+
+		$this->assertInstanceOf(Rfc9421OutgoingSignedRequest::class, $signed);
+		$headers = $signed->getHeaders();
+		$this->assertArrayHasKey('Signature-Input', $headers);
+		$this->assertStringStartsWith('ocm=(', (string)$headers['Signature-Input']);
+	}
+
+	public function testInboundDispatchesToRfc9421WhenSignatureInputPresent(): void {
+		[$signatoryManager, $jwk, $secret] = $this->ed25519SignatoryManager(rfc9421Format: true);
+
+		// Build a real signed request and replay its headers as the inbound
+		// request to exercise the full inbound path including verification.
+		$body = '{"hello":"world"}';
+		$out = new Rfc9421OutgoingSignedRequest(
+			$body,
+			$signatoryManager,
+			'receiver.example.org',
+			'POST',
+			'https://receiver.example.org/ocm/shares',
+		);
+		$out->sign();
+		$headers = $out->getHeaders();
+
+		$this->primeRequest($headers, 'POST', '/ocm/shares', 'receiver.example.org');
+
+		$resolver = $this->makeKeyResolver($signatoryManager, $jwk, 'https://sender.example.org/ocm#ed25519');
+
+		$signed = $this->signatureManager->getIncomingSignedRequest($resolver, $body);
+		$this->assertInstanceOf(Rfc9421IncomingSignedRequest::class, $signed);
+	}
+
+	public function testInboundRejectsRfc9421WhenSignatoryManagerCannotResolve(): void {
+		[$signatoryManager,] = $this->ed25519SignatoryManager(rfc9421Format: true);
+
+		$body = '{"hello":"world"}';
+		$out = new Rfc9421OutgoingSignedRequest(
+			$body,
+			$signatoryManager,
+			'receiver.example.org',
+			'POST',
+			'https://receiver.example.org/ocm/shares',
+		);
+		$out->sign();
+		$this->primeRequest($out->getHeaders(), 'POST', '/ocm/shares', 'receiver.example.org');
+
+		// $signatoryManager does NOT implement IJwkResolvingSignatoryManager.
+		$this->expectException(IncomingRequestException::class);
+		$this->signatureManager->getIncomingSignedRequest($signatoryManager, $body);
+	}
+
+	private function rsaSignatoryManager(): ISignatoryManager {
+		$key = openssl_pkey_new(['private_key_type' => OPENSSL_KEYTYPE_RSA, 'private_key_bits' => 2048]);
+		$priv = '';
+		openssl_pkey_export($key, $priv);
+		$pub = openssl_pkey_get_details($key)['key'];
+
+		$signatory = new Signatory(true);
+		$signatory->setKeyId('https://sender.example.org/ocm#signature');
+		$signatory->setPublicKey($pub);
+		$signatory->setPrivateKey($priv);
+
+		return new class($signatory) implements ISignatoryManager {
+			public function __construct(
+				private Signatory $signatory,
+			) {
+			}
+
+			public function getProviderId(): string {
+				return 'test';
+			}
+
+			public function getOptions(): array {
+				return [
+					'algorithm' => SignatureAlgorithm::RSA_SHA256,
+					'digestAlgorithm' => DigestAlgorithm::SHA256,
+				];
+			}
+
+			public function getLocalSignatory(): Signatory {
+				return $this->signatory;
+			}
+
+			public function getRemoteSignatory(string $remote): ?Signatory {
+				return null;
+			}
+		};
+	}
+
+	/**
+	 * @return array{ISignatoryManager, Key, string} [manager, parsed verification key, raw secret key]
+	 */
+	private function ed25519SignatoryManager(bool $rfc9421Format): array {
+		$keypair = sodium_crypto_sign_keypair();
+		$publicKey = sodium_crypto_sign_publickey($keypair);
+		$secretKey = sodium_crypto_sign_secretkey($keypair);
+		$kid = 'https://sender.example.org/ocm#ed25519';
+
+		$signatory = new Signatory(true);
+		$signatory->setKeyId($kid);
+		$signatory->setPublicKey($publicKey);
+		$signatory->setPrivateKey($secretKey);
+
+		$key = JWK::parseKey([
+			'kty' => 'OKP',
+			'crv' => 'Ed25519',
+			'kid' => $kid,
+			'alg' => 'EdDSA',
+			'x' => rtrim(strtr(base64_encode($publicKey), '+/', '-_'), '='),
+		], 'EdDSA');
+
+		$manager = new class($signatory, $rfc9421Format) implements ISignatoryManager {
+			public function __construct(
+				private Signatory $signatory,
+				private bool $rfc9421,
+			) {
+			}
+
+			public function getProviderId(): string {
+				return 'test';
+			}
+
+			public function getOptions(): array {
+				return [
+					'algorithm' => SignatureAlgorithm::RSA_SHA256,
+					'digestAlgorithm' => DigestAlgorithm::SHA256,
+					'rfc9421.format' => $this->rfc9421,
+				];
+			}
+
+			public function getLocalSignatory(): Signatory {
+				return $this->signatory;
+			}
+
+			public function getRemoteSignatory(string $remote): ?Signatory {
+				return null;
+			}
+		};
+		return [$manager, $key, $secretKey];
+	}
+
+	private function makeKeyResolver(ISignatoryManager $delegate, Key $key, string $kid): IJwkResolvingSignatoryManager {
+		return new class($delegate, $key, $kid) implements IJwkResolvingSignatoryManager {
+			public function __construct(
+				private ISignatoryManager $delegate,
+				private Key $key,
+				private string $kid,
+			) {
+			}
+
+			public function getProviderId(): string {
+				return $this->delegate->getProviderId();
+			}
+
+			public function getOptions(): array {
+				return $this->delegate->getOptions();
+			}
+
+			public function getLocalSignatory(): Signatory {
+				return $this->delegate->getLocalSignatory();
+			}
+
+			public function getRemoteSignatory(string $remote): ?Signatory {
+				return $this->delegate->getRemoteSignatory($remote);
+			}
+
+			public function getRemoteKey(string $origin, string $keyId): ?Key {
+				return $keyId === $this->kid ? $this->key : null;
+			}
+		};
+	}
+
+	private function primeRequest(array $headers, string $method, string $path, string $host): void {
+		$lowered = [];
+		foreach ($headers as $name => $value) {
+			$lowered[strtolower($name)] = (string)$value;
+		}
+		$this->request->method('getHeader')
+			->willReturnCallback(static fn (string $name) => $lowered[strtolower($name)] ?? '');
+		$this->request->method('getMethod')->willReturn($method);
+		$this->request->method('getRequestUri')->willReturn($path);
+		$this->request->method('getServerProtocol')->willReturn('https');
+		$this->request->method('getServerHost')->willReturn($host);
+	}
+}