Hey! Lots of people end up building things like this. You have a problem though. Your program produces linear ciphertexts. The key "a" and the message "a" will

Hey! Lots of people end up building things like this. You have a problem though. Your program produces linear ciphertexts. The key "a" and the message "a" will always create the ciphertext "136". Everyone writing code should be using what's called an AEAD. Libraries like nacl will provide this for you in most languages. AEADs abstract away all the usage of primitives and implement encryption and decryption safely (just remember to always pass in a unique nonce. In my opinion this should be abstracted out of the AEAD interface)