private synchronized byte[] computeNonceAndIncrementSequenceNumber()
      throws GeneralSecurityException {
    byte[] nonce = computeNonce();
    incrementSequenceNumber();
    return nonce;
  }
Returns the next nonce to use for seal/open. Also, increments the sequence number.
java_src/src/main/java/com/google/crypto/tink/hybrid/internal/HpkeContext.java