@Override
  public byte[] engineSeal(byte[] plaintext, byte[] aad) {
    checkIsSender();
    Preconditions.checkNotNull(plaintext, "null plaintext");
    return NativeCrypto.EVP_HPKE_CTX_seal(ctx, plaintext, aad);
  }
Implementation of {@link HpkeSpi}.  Should not be used directly, but rather by one
of the subclasses of {@link HpkeContext}.
common/src/main/java/org/conscrypt/HpkeImpl.java