private byte[] getMacKey(final byte[] nonce) throws GeneralSecurityException {
    ByteBuffer firstBlock = macKeyChaCha20.chacha20Block(nonce, 0 /* counter */);
    byte[] result = new byte[MAC_KEY_SIZE_IN_BYTES];
    firstBlock.get(result);
    return result;
  }
The MAC key is the first 32 bytes of the first key stream block
java_src/src/main/java/com/google/crypto/tink/aead/internal/InsecureNonceChaCha20Poly1305Base.java