SHA3_Sym_Enc(key,message)
{
base_key=SHA3(key); //not really needed, but in case we will ever allow the key to be "anything", we'll standardize the initial value like this

keystream="";
int counter=0;

for(read_message=0;read_message<message_length;read_message+=base_key.length()) //work in blocks of SHA3 outputs
    {
    hash_block=SHA3(base_key+string(counter)); //H(base_key+#) -> block#
    keystream+=hash_block; //add blocks into one big pseudorandom output
    counter++;
    }

keystream=keystream.Left(message.length()); //our keystream works in blocks, and might output bigger than the message, so trim any excess

return message^keystream; //XOR the keystream with the message, output
}

void main()
{
scanf(key); //input user key

nonce=generate_random_bytes(sizeof(SHA3)); //generate a nonce, SHA3-length should be enough

message_key=SHA3(key+nonce); //generate a random nonce, then build the actual key that is going to encrypt the message via concat and hash

ciphertext=SHA3_Sym_Enc(plaintext,message_key); //output the encrypted message via the function described earlier

final_message=ciphertext+SHA3(ciphertext+message_key)+nonce; //append a MAC and the nonce to the message, note that we are not doing HMAC because SHA3 is supposedly immune to length extension attacks
}