verify.rs

/// Name:  verify
///
/// Description: Compare two arrays for equality in constant time.
///
/// Arguments:   const [u8] a: first byte array
///  const [u8] b: second byte array
///  size_t len:   length of the byte arrays
///
/// Returns 0 if the byte arrays are equal, 1 otherwise
pub(crate) fn verify(a: &[u8], b: &[u8], len: usize) -> u8 {
    let mut r = 0u64;
    for i in 0..len {
        r |= (a[i] ^ b[i]) as u64;
    }
    r = r.wrapping_neg() >> 63;
    r as u8
}

/// Name:  cmov
///
/// Description: Copy len bytes from x to r if b is 1;
///  don't modify x if b is 0. Requires b to be in {0,1};
///  assumes two's complement representation of negative integers.
///  Runs in constant time.
///
/// Arguments:   [u8] r:   output byte array
///  const [u8] x: input byte array
///  size_t len:   Amount of bytes to be copied
///  [u8]  b:  Condition bit; has to be in {0,1}
pub(crate) fn cmov(r: &mut [u8], x: &[u8], len: usize, mut b: u8) {
    b = b.wrapping_neg();
    for i in 0..len {
        r[i] ^= b & (x[i] ^ r[i]);
    }
}