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Reject Ed25519 signatures with S >= L (GH #1352) #1355

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Merged
noloader merged 1 commit into from
Oct 2, 2026
Merged

Reject Ed25519 signatures with S >= L (GH #1352) #1355

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26 changes: 25 additions & 1 deletion xed25519.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -65,6 +65,25 @@ bool HasSmallOrder(const byte y[32])
return (bool)((k >> 8) & 1);
}

// Order of the Ed25519 base point, little-endian
const byte order[32] = {
0xed, 0xd3, 0xf5, 0x5c, 0x1a, 0x63, 0x12, 0x58, 0xd6, 0x9c, 0xf7, 0xa2, 0xde, 0xf9, 0xde, 0x14,
0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x10
};

// The signature scalar S must be less than the order, GH #1352
bool IsCanonicalScalar(const byte s[32])
{
for (size_t i = 32; i-- > 0; ) {
if (s[i] < order[i])
return true;
if (s[i] > order[i])
return false;
}

return false;
}

ANONYMOUS_NAMESPACE_END

NAMESPACE_BEGIN(CryptoPP)
Expand Down Expand Up @@ -884,7 +903,9 @@ bool ed25519Verifier::VerifyAndRestart(PK_MessageAccumulator &messageAccumulator
{
ed25519_MessageAccumulator& accum = static_cast<ed25519_MessageAccumulator&>(messageAccumulator);
const ed25519PublicKey& pk = dynamic_cast<const ed25519PublicKey&>(GetPublicKey());
int ret = Donna::ed25519_sign_open(accum.data(), accum.size(), pk.GetPublicKeyBytePtr(), accum.signature());
int ret = -1;
if (IsCanonicalScalar(accum.signature() + 32))
ret = Donna::ed25519_sign_open(accum.data(), accum.size(), pk.GetPublicKeyBytePtr(), accum.signature());
accum.Restart();

return ret == 0;
Expand All @@ -895,6 +916,9 @@ bool ed25519Verifier::VerifyStream(std::istream& stream, const byte *signature,
CRYPTOPP_ASSERT(signatureLen == SIGNATURE_LENGTH);
CRYPTOPP_UNUSED(signatureLen);

if (IsCanonicalScalar(signature + 32) == false)
return false;

const ed25519PublicKey& pk = static_cast<const ed25519PublicKey&>(GetPublicKey());
int ret = Donna::ed25519_sign_open(stream, pk.GetPublicKeyBytePtr(), signature);

Expand Down
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