Letter Stats Decipher

from operator import itemgetter
import urllib

def getStats(string):
    letters = [chr(i) for i in xrange(0,256)]
    output = {}

    string = string.lower()
    string = string.replace(" ", "").replace(".", "").replace(",","")
    string = string.replace("\n","").replace("\r","")

    for letter in letters:
        count = string.count(letter)
        if count > 0 and ord(letter) >= ord('a') and ord(letter) <= ord('z'):
            output[letter] = float(count)/len(string) * 100
    return output

def decipher(statsStr, cipherStr):
    statsStr = statsStr.lower()
    cipherStr = cipherStr.lower()

    stats = sorted(getStats(statsStr).items(), key=itemgetter(1))
    stats = convertList(stats)
    cipher = sorted(getStats(cipherStr).items(), key=itemgetter(1))
    cipher = convertList(cipher)

    cipherList = list(cipherStr)
    outputList = list(cipherStr)

    for i in xrange(0, len(stats)):
        outputList = listReplace(outputList, cipherList, cipher[i], stats[i])
    return "".join(outputList)

def convertList(list):
    return  [item[0] for item in list]

def listReplace(output, input, replaced, replacing):
    for i in xrange(0, len(input)):
        if input[i] == replaced:
            output[i] = replacing
    return output

def main():
    string = urllib.urlopen("http://home.agh.edu.pl/~kanczula/kryptografia/lab/lab02/text.txt").read()
    cipher = urllib.urlopen("http://home.agh.edu.pl/~kanczula/kryptografia/lab/lab02/szyfrogram.txt").read()
    print decipher(string, cipher)

if __name__ == "__main__":
    main()