In Java using commented code:

Create a cryptosystem that encrypts and decrypts a file for a given key that the user give. This key will only use 2 characters instead of the user creating their own binary string.

These are the paramaters:

Keys are 16-bit values.

Messages are strings with an even number of characters. If the input string is odd, add a blank at the end of the string.

The encryption of a message M of length n (in bytes) is given by:

E_k (M) = M (K || K || ),

where the key K is repeated n/2 times.

The decryption algorithm for a ciphertext C is the same as the encryption algorithm:

D_k (M) = M (K || K || ).