private static long load26(byte[] in, int idx, int shift) {
    return (load32(in, idx) >> shift) & 0x3ffffff;
  }
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