musig: test nonce_gen_counter with random counters #1947

The only output check of nonce_gen_counter uses nonrepeating_cnt = 0, so the following mutant replacing secp256k1_write_be64 with secp256k1_write_be32 (which could enable nonce reuse) survived because zero serializes the same either way.

diff --git a/src/modules/musig/session_impl.h b/src/modules/musig/session_impl.h
index d8a3cca..710c4c5 100644
--- a/src/modules/musig/session_impl.h
+++ b/src/modules/musig/session_impl.h
@@ -447,7 +447,7 @@ int secp256k1_musig_nonce_gen_counter(const secp256k1_context* ctx, secp256k1_mu
     memset(secnonce, 0, sizeof(*secnonce));
     ARG_CHECK(keypair != NULL);
 
-    secp256k1_write_be64(buf, nonrepeating_cnt);
+    secp256k1_write_be32(buf, nonrepeating_cnt);
     /* keypair_sec and keypair_pub do not fail if the arguments are not NULL */
     ret = secp256k1_keypair_sec(ctx, seckey, keypair);
     VERIFY_CHECK(ret);

This add a new test, which checks that for random 64-bit counters, nonce_gen_counter gives the same nonce as nonce_gen.

utACK b819a790f06122d5a53c0320e79c0dc486349fbd

<!-- ccr-overview-v2 -->

Copilot review overview

🟢 Approval recommended

The focused test correctly detects incorrect counter serialization with no unresolved issues.

Review effort: Balanced
Findings: None

<details> <summary><strong>What changed in this PR</strong></summary>

Adds regression coverage ensuring MuSig counter-based nonce generation uses the complete 64-bit counter representation.

Changes:

• Compares nonce_gen_counter against equivalent nonce_gen calls using random counters.
• Registers the new test.

</details>

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