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Reject weak entropy in nonce_gen when seckey isn't provided #156

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Reject weak entropy in nonce_gen when seckey isn't provided #156

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10 changes: 7 additions & 3 deletions src/modules/musig/session_impl.h
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Original file line number Diff line number Diff line change
Expand Up @@ -283,10 +283,14 @@ int secp256k1_musig_nonce_gen(const secp256k1_context* ctx, secp256k1_musig_secn
ARG_CHECK(session_id32 != NULL);
ARG_CHECK(secp256k1_ecmult_gen_context_is_built(&ctx->ecmult_gen_ctx));
if (seckey == NULL) {
/* Check in constant time that the session_id is not 0 as a
* defense-in-depth measure that may protect against a faulty RNG. */
/* Check in constant time that if seckey isn't provided, then the
* middle 16 bytes of session_id are not all 0.
*
* This is a defense-in-depth measure that may protect against a
* faulty RNG, as well invalid use of a 64-bit counter (in any
* endian-ness) instead of a secure RNG. */
unsigned char acc = 0;
for (i = 0; i < 32; i++) {
for (i = 8; i < 24; i++) {
acc |= session_id32[i];
}
ret &= !!acc;
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13 changes: 13 additions & 0 deletions src/modules/musig/tests_impl.h
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Expand Up @@ -126,6 +126,7 @@ void musig_api_tests(secp256k1_scratch_space *scratch) {
unsigned char max64[64];
unsigned char zeros68[68] = { 0 };
unsigned char session_id[2][32];
unsigned char invalid_session_id[32];
secp256k1_musig_secnonce secnonce[2];
secp256k1_musig_secnonce secnonce_tmp;
secp256k1_musig_secnonce invalid_secnonce;
Expand Down Expand Up @@ -293,6 +294,18 @@ void musig_api_tests(secp256k1_scratch_space *scratch) {
CHECK(secp256k1_musig_nonce_gen(sign, &secnonce[0], &pubnonce[0], zeros68, NULL, msg, &keyagg_cache, max64) == 0);
CHECK(ecount == 5);
CHECK(memcmp_and_randomize(secnonce[0].data, zeros68, sizeof(secnonce[0].data)) == 0);
/* no seckey and session_id looks like a counter (little-endian) */
memset(invalid_session_id, 0x55, 8);
memset(invalid_session_id + 8, 0, 24);
CHECK(secp256k1_musig_nonce_gen(sign, &secnonce[0], &pubnonce[0], invalid_session_id, NULL, msg, &keyagg_cache, max64) == 0);
CHECK(ecount == 5);
CHECK(memcmp_and_randomize(secnonce[0].data, zeros68, sizeof(secnonce[0].data)) == 0);
/* no seckey and session_id looks like a counter (big-endian) */
memset(invalid_session_id, 0, 24);
memset(invalid_session_id + 24, 0x55, 8);
CHECK(secp256k1_musig_nonce_gen(sign, &secnonce[0], &pubnonce[0], invalid_session_id, NULL, msg, &keyagg_cache, max64) == 0);
CHECK(ecount == 5);
CHECK(memcmp_and_randomize(secnonce[0].data, zeros68, sizeof(secnonce[0].data)) == 0);
/* session_id 0 is fine when a seckey is provided */
CHECK(secp256k1_musig_nonce_gen(sign, &secnonce[0], &pubnonce[0], zeros68, sk[0], msg, &keyagg_cache, max64) == 1);
CHECK(secp256k1_musig_nonce_gen(sign, &secnonce[0], &pubnonce[0], session_id[0], NULL, msg, &keyagg_cache, max64) == 1);
Expand Down