porta cipher in python

porta.py

'''
implements porta cipher
Author: James Lyons 
Created: 2012-06-27
'''
from .base import Cipher

####################################################################################
class Porta(Cipher):
    def __init__(self,key='CODE'):
        self.key = [k.upper() for k in key]
        
    def encipher(self,string):
        string = self.remove_punctuation(string)
        ret = ''
        for (i,c) in enumerate(string):
            i = i%len(self.key)
            if   self.key[i] in 'AB': ret += 'NOPQRSTUVWXYZABCDEFGHIJKLM'[self.a2i(c)]
            elif self.key[i] in 'CD': ret += 'ZNOPQRSTUVWXYBCDEFGHIJKLMA'[self.a2i(c)]
            elif self.key[i] in 'EF': ret += 'YZNOPQRSTUVWXCDEFGHIJKLMAB'[self.a2i(c)]
            elif self.key[i] in 'GH': ret += 'XYZNOPQRSTUVWDEFGHIJKLMABC'[self.a2i(c)]
            elif self.key[i] in 'IJ': ret += 'WXYZNOPQRSTUVEFGHIJKLMABCD'[self.a2i(c)]
            elif self.key[i] in 'KL': ret += 'VWXYZNOPQRSTUFGHIJKLMABCDE'[self.a2i(c)]
            elif self.key[i] in 'MN': ret += 'UVWXYZNOPQRSTGHIJKLMABCDEF'[self.a2i(c)]
            elif self.key[i] in 'OP': ret += 'TUVWXYZNOPQRSHIJKLMABCDEFG'[self.a2i(c)]
            elif self.key[i] in 'QR': ret += 'STUVWXYZNOPQRIJKLMABCDEFGH'[self.a2i(c)]
            elif self.key[i] in 'ST': ret += 'RSTUVWXYZNOPQJKLMABCDEFGHI'[self.a2i(c)]
            elif self.key[i] in 'UV': ret += 'QRSTUVWXYZNOPKLMABCDEFGHIJ'[self.a2i(c)]
            elif self.key[i] in 'WX': ret += 'PQRSTUVWXYZNOLMABCDEFGHIJK'[self.a2i(c)]
            elif self.key[i] in 'YZ': ret += 'OPQRSTUVWXYZNMABCDEFGHIJKL'[self.a2i(c)]
        return ret    

    def decipher(self,string):
        return self.encipher(string)

if __name__ == '__main__': 
    print 'use "import pycipher" to access functions'