#!/usr/bin/python3 plaintext = [b"This may seem relatively simple, but it's the same basic " b"principles as used in s-boxes, a technique used in many modern " b"cryptographic algoritms. Of course, instead of letter substitution, " b"you're doing byte substitution.", b"The next two puzzles are a bit different; Frequency counts (of characters) " b"will just reveal random noise. " b"Oh, by the way, the key this time is: 'the s is for sucks'."] key = b"thequickbrownfxjmpdvlazygs" def encode(text): ukey = key.upper() lkey = key.lower() assert len(set(key)) == 26, 'invalid key' assert key.isalpha(), 'non alpha character in key' out = bytearray() for t in text: if t in lkey: out.append(lkey[t - ord('a')]) elif t in ukey: out.append(ukey[t - ord('A')]) else: out.append(t) return bytes(out) def decode(text): ukey = key.upper() lkey = key.lower() assert len(set(key)) == 26, 'invalid key' assert key.isalpha(), 'non alpha character in key' out = bytearray() for t in text: if t in lkey: out.append(ord('a') + lkey.index(bytes([t]))) elif t in ukey: out.append(ord('A') + ukey.index(bytes([t]))) else: out.append(t) return bytes(out) c = encode(plaintext[0]) print('
Alice
', str(c, 'utf-8')) assert decode(c) == plaintext[0] c = encode(plaintext[1]) print('
Bob
', str(c, 'utf-8'), '
') assert decode(c) == plaintext[1]