encrypt
abstract fun encryptAES256by16(plaintext: ByteArray, secret: ByteArray, initial: ByteArray): ByteArray
AES256 encryption if you KNOW the message length is a multiple of 16. This is a lower level API that offers greater size efficiency if you know what you are doing. For variable sized messages, use encryptAES256.
Return
ciphertext or an empty array. The ciphertext length will be the same as the plaintext.