private static long load32(byte[] in, int idx) {
    return ((in[idx] & 0xff)
            | ((in[idx + 1] & 0xff) << 8)
            | ((in[idx + 2] & 0xff) << 16)
            | ((in[idx + 3] & 0xff) << 24))
        & 0xffffffffL;
  }
Poly1305 one-time MAC based on RFC 7539.

<p>This is not an implementation of the MAC interface on purpose and it is not equivalent to
HMAC.

<p>The implementation is based on poly1305 implementation by Andrew Moon
(https://github.com/floodyberry/poly1305-donna) and released as public domain.
java_src/src/main/java/com/google/crypto/tink/aead/internal/Poly1305.java