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Make signing follow BIP340 exactly w.r.t. aux randomness

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libsecp256k1's secp256k1_schnorrsig_sign only follows BIP340 exactly
if an aux_rand32 argument is passed. When no randomness is used
(as is the case in the current codebase here), there is no impact
on security between not providing aux_rand32 at all, or providing
an empty one. Yet, for repeatability/testability it is simpler
to always use an all-zero one.
This commit is contained in:
Pieter Wuille 2021-10-28 13:46:52 -04:00
parent c9dd5c8d6e
commit 2478c6730a
4 changed files with 8 additions and 7 deletions
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4
src/key.cpp
View file

@ -275,7 +275,7 @@ bool CKey::SignCompact(const uint256 &hash, std::vector<unsigned char>& vchSig)
return true;
}
bool CKey::SignSchnorr(const uint256& hash, Span<unsigned char> sig, const uint256* merkle_root, const uint256* aux) const
bool CKey::SignSchnorr(const uint256& hash, Span<unsigned char> sig, const uint256* merkle_root, const uint256& aux) const
{
assert(sig.size() == 64);
secp256k1_keypair keypair;
@ -288,7 +288,7 @@ bool CKey::SignSchnorr(const uint256& hash, Span<unsigned char> sig, const uint2
uint256 tweak = XOnlyPubKey(pubkey_bytes).ComputeTapTweakHash(merkle_root->IsNull() ? nullptr : merkle_root);
if (!secp256k1_keypair_xonly_tweak_add(GetVerifyContext(), &keypair, tweak.data())) return false;
}
bool ret = secp256k1_schnorrsig_sign(secp256k1_context_sign, sig.data(), hash.data(), &keypair, aux ? (unsigned char*)aux->data() : nullptr);
bool ret = secp256k1_schnorrsig_sign(secp256k1_context_sign, sig.data(), hash.data(), &keypair, (unsigned char*)aux.data());
if (ret) {
// Additional verification step to prevent using a potentially corrupted signature
secp256k1_xonly_pubkey pubkey_verify;

4
src/key.h
View file

@ -130,7 +130,7 @@ public:
/**
* Create a BIP-340 Schnorr signature, for the xonly-pubkey corresponding to *this,
* optionally tweaked by *merkle_root. Additional nonce entropy can be provided through
* optionally tweaked by *merkle_root. Additional nonce entropy is provided through
* aux.
*
* merkle_root is used to optionally perform tweaking of the private key, as specified
@ -143,7 +143,7 @@ public:
* (this is used for key path spending, with specific
* Merkle root of the script tree).
*/
bool SignSchnorr(const uint256& hash, Span<unsigned char> sig, const uint256* merkle_root = nullptr, const uint256* aux = nullptr) const;
bool SignSchnorr(const uint256& hash, Span<unsigned char> sig, const uint256* merkle_root, const uint256& aux) const;
//! Derive BIP32 child key.
bool Derive(CKey& keyChild, ChainCode &ccChild, unsigned int nChild, const ChainCode& cc) const;

3
src/script/sign.cpp
View file

@ -81,7 +81,8 @@ bool MutableTransactionSignatureCreator::CreateSchnorrSig(const SigningProvider&
uint256 hash;
if (!SignatureHashSchnorr(hash, execdata, *txTo, nIn, nHashType, sigversion, *m_txdata, MissingDataBehavior::FAIL)) return false;
sig.resize(64);
if (!key.SignSchnorr(hash, sig, merkle_root, nullptr)) return false;
// Use uint256{} as aux_rnd for now.
if (!key.SignSchnorr(hash, sig, merkle_root, {})) return false;
if (nHashType) sig.push_back(nHashType);
return true;
}

4
src/test/key_tests.cpp
View file

@ -321,7 +321,7 @@ BOOST_AUTO_TEST_CASE(bip340_test_vectors)
key.Set(sec.begin(), sec.end(), true);
XOnlyPubKey pubkey(key.GetPubKey());
BOOST_CHECK(std::equal(pubkey.begin(), pubkey.end(), pub.begin(), pub.end()));
bool ok = key.SignSchnorr(msg256, sig64, nullptr, &aux256);
bool ok = key.SignSchnorr(msg256, sig64, nullptr, aux256);
BOOST_CHECK(ok);
BOOST_CHECK(std::vector<unsigned char>(sig64, sig64 + 64) == sig);
// Verify those signatures for good measure.
@ -337,7 +337,7 @@ BOOST_AUTO_TEST_CASE(bip340_test_vectors)
BOOST_CHECK(tweaked);
XOnlyPubKey tweaked_key = tweaked->first;
aux256 = InsecureRand256();
bool ok = key.SignSchnorr(msg256, sig64, &merkle_root, &aux256);
bool ok = key.SignSchnorr(msg256, sig64, &merkle_root, aux256);
BOOST_CHECK(ok);
BOOST_CHECK(tweaked_key.VerifySchnorr(msg256, sig64));
}