Please assist with this program using Python only. Should be pretty simple, also please explain what you did.

# Shift cipher # Shift a single plaintext byte by k (mod 256) def shift(b, k): return (b + k) % 256 # Shift a whole byte string by k, returning an array of numbers def shiftstring(plaintext, k): return [ shift(p, k) for p in plaintext ] # Shift a whole byte string by k, returning a byte string def shiftcipher (plaintext, k): return bytes( shiftstring(plaintext, k) ) ### Write the three functions to implement the linear cipher below