@GuardedBy("this")
  private byte[] computeNonce() throws GeneralSecurityException {
    return Bytes.xor(
        baseNonce,
        BigIntegerEncoding.toBigEndianBytesOfFixedLength(sequenceNumber, aead.getNonceLength()));
  }
ComputeNonce() from https://www.rfc-editor.org/rfc/rfc9180.html#section-5.2-11.
java_src/src/main/java/com/google/crypto/tink/hybrid/internal/HpkeContext.java