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fix: add missing domain separation to Bulletproof Fiat-Shamir challenges #143

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fix: add missing domain separation to Bulletproof Fiat-Shamir challenges #143

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efc2610
fix: add missing domain tag to Bulletproof x challenge
tesseract-ripple Sep 17, 2026
5ff7f24
fix: domain-separate the two IPA Fiat-Shamir challenges
tesseract-ripple Sep 17, 2026
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16 changes: 12 additions & 4 deletions src/bulletproof_aggregated.c
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Original file line number Diff line number Diff line change
Expand Up @@ -683,7 +683,7 @@ int secp256k1_bulletproof_ipa_compress_step(const secp256k1_context *ctx,
* ux is the fixed IPA binding scalar.
*
* It MUST be derived exactly once from:
* ux = H(commit_inp || <a,b>)
* ux = H(domain || commit_inp || <a,b>)
*
* and reused consistently throughout the IPA:
* - L/R cross-term construction
Expand Down Expand Up @@ -712,6 +712,8 @@ int derive_ipa_binding_challenge(const secp256k1_context *ctx,
/* 2. Hash */
if (EVP_DigestInit_ex(mdctx, EVP_sha256(), NULL) != 1)
goto cleanup;
if (EVP_DigestUpdate(mdctx, "MPT_BULLETPROOF_IPA", 19) != 1)
goto cleanup;
if (EVP_DigestUpdate(mdctx, hash_input, 2 * kMPT_SCALAR_SIZE) != 1)
goto cleanup;
if (EVP_DigestFinal_ex(mdctx, hash_output, NULL) != 1)
Expand All @@ -732,7 +734,7 @@ int derive_ipa_binding_challenge(const secp256k1_context *ctx,
}

/**
* Derive u = H(last_challenge || L || R) reduced to a valid scalar.
* Derive u = H(domain || last_challenge || L || R) reduced to a valid scalar.
* IMPORTANT: use the SAME exact logic in verifier.
*/

Expand Down Expand Up @@ -764,6 +766,8 @@ int derive_ipa_round_challenge(

if (EVP_DigestInit_ex(mdctx, EVP_sha256(), NULL) != 1)
goto cleanup;
if (EVP_DigestUpdate(mdctx, "MPT_BULLETPROOF_IPA", 19) != 1)
goto cleanup;
if (EVP_DigestUpdate(mdctx, last_challenge, kMPT_SCALAR_SIZE) != 1)
goto cleanup;
if (EVP_DigestUpdate(mdctx, L_ser, kMPT_PUBKEY_SIZE) != 1)
Expand Down Expand Up @@ -1785,7 +1789,7 @@ int secp256k1_bulletproof_prove_agg(
}

/* ---- 9. Challenge x ---- */
/* x = H(A || S || y || z || T1 || T2 || context_id) */
/* x = H(domain || A || S || y || z || T1 || T2 || context_id) */
{
unsigned char A_ser[kMPT_PUBKEY_SIZE], S_ser[kMPT_PUBKEY_SIZE],
T1_ser[kMPT_PUBKEY_SIZE], T2_ser[kMPT_PUBKEY_SIZE];
Expand Down Expand Up @@ -1819,6 +1823,8 @@ int secp256k1_bulletproof_prove_agg(

if (EVP_DigestInit_ex(mdctx, EVP_sha256(), NULL) != 1)
goto fs_x_cleanup;
if (EVP_DigestUpdate(mdctx, "MPT_BULLETPROOF_RANGE", 21) != 1)
goto fs_x_cleanup;
if (EVP_DigestUpdate(mdctx, A_ser, kMPT_PUBKEY_SIZE) != 1)
goto fs_x_cleanup;
if (EVP_DigestUpdate(mdctx, S_ser, kMPT_PUBKEY_SIZE) != 1)
Expand Down Expand Up @@ -2471,10 +2477,12 @@ int secp256k1_bulletproof_verify_agg(
scalar_vector_powers(ctx, (unsigned char (*)[kMPT_SCALAR_SIZE])y_inv_powers,
y_inv, n);

/* ---------------- x = H(A || S || y || z || T1 || T2 || context)
/* ---------------- x = H(domain || A || S || y || z || T1 || T2 || context)
* ---------------- */
if (EVP_DigestInit_ex(mdctx, EVP_sha256(), NULL) != 1)
goto fs_fail;
if (EVP_DigestUpdate(mdctx, "MPT_BULLETPROOF_RANGE", 21) != 1)
goto fs_fail;
if (EVP_DigestUpdate(mdctx, A_ser, kMPT_PUBKEY_SIZE) != 1)
goto fs_fail;
if (EVP_DigestUpdate(mdctx, S_ser, kMPT_PUBKEY_SIZE) != 1)
Expand Down
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