fix(sdk-lib-mpc): throw in Dsg.endSession() when signature exists

Without the throw, endSession() silently reset dsgState to
Uninitialized after signing, allowing init() to succeed again
and enabling inadvertent session reuse on the same key share.

Ticket: WAL-383

Author: zahin-mohammad

modules/sdk-lib-mpc/src/tss/ecdsa-dkls/dsg.ts

@@ -164,7 +164,7 @@ export class Dsg {
    */
   endSession(): void {
     if (this._signature) {
-      new Error('Session already ended because combined signature was produced.');
+      throw new Error('Session already ended because combined signature was produced.');
     }
     if (this.dsgSession) {
       this.dsgSession.free();

modules/sdk-lib-mpc/test/unit/tss/ecdsa/dklsDsg.ts

@@ -217,6 +217,29 @@ describe('DKLS Dsg 2x3', function () {
     await party.init().should.be.rejectedWith(/Invalid messageHash length/);
   });
 
+  it('should throw in endSession() when signature has already been produced', async function () {
+    const vector = vectors[0];
+    const party1 = new DklsDsg.Dsg(
+      fs.readFileSync(shareFiles[vector.party1]),
+      vector.party1,
+      vector.derivationPath,
+      crypto.createHash('sha256').update(Buffer.from(vector.msgToSign, 'hex')).digest()
+    );
+    const party2 = new DklsDsg.Dsg(
+      fs.readFileSync(shareFiles[vector.party2]),
+      vector.party2,
+      vector.derivationPath,
+      crypto.createHash('sha256').update(Buffer.from(vector.msgToSign, 'hex')).digest()
+    );
+    const round4Messages = await executeTillRound(4, party1, party2);
+    party1.handleIncomingMessages({
+      p2pMessages: [],
+      broadcastMessages: round4Messages[1].broadcastMessages,
+    });
+    should.exist(party1.signature);
+    (() => party1.endSession()).should.throw('Session already ended because combined signature was produced.');
+  });
+
   it(`should fail when signing two different messages`, async function () {
     const party1 = new DklsDsg.Dsg(
       fs.readFileSync(`${__dirname}/fixtures/userShare`),