Merge bitcoin/bitcoin#28577: net: raise V1_PREFIX_LEN from 12 to 16

ba2e5bf net: raise V1_PREFIX_LEN from 12 to 16 (Pieter Wuille)

Pull request description:

  A "version" message in the V1 protocol starts with a fixed 16 bytes:
  * The 4-byte network magic
  * The 12-byte command string: "version" plus 5 0x00 bytes

  The current code detects incoming V1 connections by just looking at the first 12 bytes (matching an [earlier version](bitcoin/bips#1496) of BIP324), but 16 bytes is more precise. This isn't an observable difference right now, as a 12 byte prefix ought to be negligible already, but it may become observable with future extensions to the protocol, so make the code match the specification.

ACKs for top commit:
  achow101:
    ACK ba2e5bf
  theStack:
    re-ACK ba2e5bf
  mzumsande:
    Code review ACK ba2e5bf

Tree-SHA512: 64876b03613bd1c5dda82f4ca1b367014365f9ae4cfa30f45c5758a563c68cbea81a98d02ba616c264674c204517aac8b7de94da10f32e77b56267a43710c651

Author: achow101

src/net.cpp

@@ -1098,10 +1098,10 @@ void V2Transport::ProcessReceivedMaybeV1Bytes() noexcept
     AssertLockNotHeld(m_send_mutex);
     Assume(m_recv_state == RecvState::KEY_MAYBE_V1);
     // We still have to determine if this is a v1 or v2 connection. The bytes being received could
-    // be the beginning of either a v1 packet (network magic + "version\x00"), or of a v2 public
-    // key. BIP324 specifies that a mismatch with this 12-byte string should trigger sending of the
-    // key.
-    std::array<uint8_t, V1_PREFIX_LEN> v1_prefix = {0, 0, 0, 0, 'v', 'e', 'r', 's', 'i', 'o', 'n', 0};
+    // be the beginning of either a v1 packet (network magic + "version\x00\x00\x00\x00\x00"), or
+    // of a v2 public key. BIP324 specifies that a mismatch with this 16-byte string should trigger
+    // sending of the key.
+    std::array<uint8_t, V1_PREFIX_LEN> v1_prefix = {0, 0, 0, 0, 'v', 'e', 'r', 's', 'i', 'o', 'n', 0, 0, 0, 0, 0};
     std::copy(std::begin(Params().MessageStart()), std::end(Params().MessageStart()), v1_prefix.begin());
     Assume(m_recv_buffer.size() <= v1_prefix.size());
     if (!std::equal(m_recv_buffer.begin(), m_recv_buffer.end(), v1_prefix.begin())) {
@@ -1295,7 +1295,7 @@ size_t V2Transport::GetMaxBytesToProcess() noexcept
         // receive buffer.
         Assume(m_recv_buffer.size() <= V1_PREFIX_LEN);
         // As long as we're not sure if this is a v1 or v2 connection, don't receive more than what
-        // is strictly necessary to distinguish the two (12 bytes). If we permitted more than
+        // is strictly necessary to distinguish the two (16 bytes). If we permitted more than
         // the v1 header size (24 bytes), we may not be able to feed the already-received bytes
         // back into the m_v1_fallback V1 transport.
         return V1_PREFIX_LEN - m_recv_buffer.size();

src/net.h

@@ -473,7 +473,7 @@ class V2Transport final : public Transport
 
     /** The length of the V1 prefix to match bytes initially received by responders with to
      *  determine if their peer is speaking V1 or V2. */
-    static constexpr size_t V1_PREFIX_LEN = 12;
+    static constexpr size_t V1_PREFIX_LEN = 16;
 
     // The sender side and receiver side of V2Transport are state machines that are transitioned
     // through, based on what has been received. The receive state corresponds to the contents of,