public byte[] seal(byte[] plaintext, byte[] associatedData) throws GeneralSecurityException {
    byte[] nonce = computeNonceAndIncrementSequenceNumber();
    return aead.seal(key, nonce, plaintext, associatedData);
  }
Performs AEAD encryption of {@code plaintext} with {@code associatedData} according to
ContextS.Seal() defined in https://www.rfc-editor.org/rfc/rfc9180.html#section-5.2-8.

@return ciphertext
java_src/src/main/java/com/google/crypto/tink/hybrid/internal/HpkeContext.java