private static int eq(int a, int b) {
    int r = ~(a ^ b) & 0xff;
    r &= r << 4;
    r &= r << 2;
    r &= r << 1;
    return (r >> 7) & 1;
  }
Compares two byte values in constant time.

Please note that this doesn't reuse {@link Curve25519#eq} method since the below inputs are
byte values.
java_src/src/main/java/com/google/crypto/tink/internal/Ed25519.java