public byte[] seal(byte[] plaintext, byte[] aad) {
        return spi.engineSeal(plaintext, aad);
    }
Seals a message, using the internal key schedule maintained by this HpkeContextSender.

@param plaintext the plaintext
@param aad optional associated data, may be null or empty
@return the ciphertext
@throws NullPointerException if the plaintext is null
@throws IllegalStateException if this HpkeContextSender has not been initialised
common/src/main/java/org/conscrypt/HpkeContextSender.java