public byte[] open(byte[] ciphertext, byte[] aad) throws GeneralSecurityException {
        return spi.engineOpen(ciphertext, aad);
    }
Opens a message, using the internal key schedule maintained by this HpkeContextRecipient.

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