import string import itertools def chunker(seq, size): it = iter(seq) while True: chunk = tuple(itertools.islice(it, size)) if not chunk: return yield chunk def prepare_input(dirty): """ Prepare the plaintext by uppcasing it and seperating repeated letters with X's """ dirty = ''.join([c.upper() for c in dirty if c in string.ascii_letters]) clean = "" if len(dirty) < 2: return dirty for i in range(len(dirty)-1): clean += dirty[i] if dirty[i] == dirty[i+1]: clean += 'X' clean += dirty[-1] if len(clean) & 1: clean += 'X' return clean def generate_table(key): # I and J are used interchangably to allow # us to use a 5x5 table (25 letters) alphabet = "ABCDEFGHIKLMNOPQRSTUVWXYZ" # we're using a list instead of a '2d' array because it makes the math # for setting up the table and doing the actual encoding/decoding simpler table = [] # copy key chars into the table if they are in `alphabet` ignoring duplicates for char in key.upper(): if char not in table and char in alphabet: table.append(char) # fill the rest of the table in with the remaining alphabet chars for char in alphabet: if char not in table: table.append(char) return table def encode(plaintext, key): table = generate_table(key) plaintext = prepare_input(plaintext) ciphertext = "" # https://en.wikipedia.org/wiki/Playfair_cipher#Description for char1, char2 in chunker(plaintext, 2): row1, col1 = divmod(table.index(char1), 5) row2, col2 = divmod(table.index(char2), 5) if row1 == row2: ciphertext += table[row1*5+(col1+1)%5] ciphertext += table[row2*5+(col2+1)%5] elif col1 == col2: ciphertext += table[((row1+1)%5)*5+col1] ciphertext += table[((row2+1)%5)*5+col2] else: # rectangle ciphertext += table[row1*5+col2] ciphertext += table[row2*5+col1] return ciphertext def decode(ciphertext, key): table = generate_table(key) plaintext = "" # https://en.wikipedia.org/wiki/Playfair_cipher#Description for char1, char2 in chunk(ciphertext, 2): row1, col1 = divmod(table.index(char1), 5) row2, col2 = divmod(table.index(char2), 5) if row1 == row2: plaintext += table[row1*5+(col1-1)%5] plaintext += table[row2*5+(col2-1)%5] elif col1 == col2: plaintext += table[((row1-1)%5)*5+col1] plaintext += table[((row2-1)%5)*5+col2] else: # rectangle plaintext += table[row1*5+col2] plaintext += table[row2*5+col1] return plaintext