fn check_scalar()

in src/signature.rs [85:102]

• 17 lines of code
• 4 McCabe index (conditional complexity)

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(InternalError::ScalarFormatError.into()),
        Some(x) => return Ok(x),
    };
}