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Add an optimisation to succeed fast for scalars whose 4th MSB is unset.

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This is only done during signature verification.
This commit is contained in:
Isis Lovecruft 2019-10-11 21:33:55 +00:00
parent de3eb13dcf
commit deca36d074
No known key found for this signature in database
GPG key ID: AB41313533E8E812
1 changed files with 12 additions and 0 deletions
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12
src/signature.rs
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@ -90,6 +90,18 @@ fn check_scalar(bytes: [u8; 32]) -> Result<Scalar, SignatureError> {
#[cfg(not(feature = "legacy_compatibility"))]
#[inline(always)]
fn check_scalar(bytes: [u8; 32]) -> Result<Scalar, SignatureError> {
// Since this is only used in signature deserialisation (i.e. upon
// verification), we can do a "succeed fast" trick by checking that the most
// significant 4 bits are unset. If they are unset, we can succeed fast
// because we are guaranteed that the scalar is fully reduced. However, if
// the 4th most significant bit is set, we must do the full reduction check,
// as the order of the basepoint is roughly a 2^(252.5) bit number.
//
// This succeed-fast trick should succeed for roughly half of all scalars.
if bytes[31] & 240 == 0 {
return Ok(Scalar::from_bits(bytes))
}
match Scalar::from_canonical_bytes(bytes) {
None => return Err(SignatureError(InternalError::ScalarFormatError)),
Some(x) => return Ok(x),