private static boolean isNonZeroVarTime(long[] in) {
    long[] inCopy = new long[in.length + 1];
    System.arraycopy(in, 0, inCopy, 0, in.length);
    Field25519.reduceCoefficients(inCopy);
    byte[] bytes = Field25519.contract(inCopy);
    for (byte b : bytes) {
      if (b != 0) {
        return true;
      }
    }
    return false;
  }
Returns true if {@code in} is nonzero.

Note that execution time might depend on the input {@code in}.
java_src/src/main/java/com/google/crypto/tink/internal/Ed25519.java