Question

I’m stuck with a decryption problem I’m having. I have a really basic cipher that is only the alphabet and is offset by 1 letter, like this:

A    B
B    C
C    D
D    E
to z  to A

the right column is the letters I’m given, and I need to turn them to the letters on the left.

I’m reading this from a file, and saving each column into a list like this

#!/usr/bin/python

key = "key.txt"
encrypted = "encrypted.txt"
decrypted = "decrypted.txt"

encryptedList = []
decryptedList = []

with open(key, "r") as file:
    for line in file:
        currentLine = line.split()

        currentDecrypted = currentLine[0]
        currentEncrypted = currentLine[1]

        decryptedList.append(currentEncrypted)
        encryptedList.append(currentDecrypted)

file.close()

counter = 0
with open(encrypted, "r") as file:
for line in file:
    currentLine = line
    for letter in currentLine:
        currentLetter = letter
        for item in encryptedList:
            if(item == currentLetter):
            ####here's where the problem starts####
            ####I've tried just printing counter, and I get mostly go    
                printencryptedList[counter-1]
                counter = 0
                break
            counter += 1

what I’m trying to decrypt is a text file that looks like this

FMMP
NZ OBNF JT KSMBB
KPIO TVDLT BQQ
GWWWWWWBMT PG DJJJJJH

I get the correct count numbers for FMMP, (5, 12, 12, 15) with counter -1, but then i get 39, 25,40 and so on. Any help would be greatly appreciated, and let me know if you need more info.

I also welcome ideas on better/easier ways to do this, but I would also like a solution similar to this, so I can figure out what’s going on here. Thanks

Alright, thanks for all the answers and info. I’m posting what I finally did and it works. I’m sure it’s not as pythonic as it should be, but I implemented a few things that people mentioned. Thanks for the info.

import string

key = "key.txt"
encrypted = ""encrypted.txt"
decrypted = "decrypted.txt


encryptedString = ""
decryptedString = ""

keyDict = {}

with open(key, "r") as file:
    for line in file:
    currentLine = line.split()

    currentDecrypted = currentLine[0]
    currentEncrypted = currentLine[1]

    keyDict[currentDecrypted] = currentEncrypted


with open(encrypted, "r") as file:
    for line in file:
    currentLine = line
        for letter in currentLine:
        currentLetter = letter
        encryptedString += letter
            for key in keyDict:
            if(keyDict[key] == letter):
                decryptedString += key
                break
            elif(letter == " "):
                decryptedString += " "
                break
            elif(letter == "\n"):
                decryptedString += "\n"
                break




with open(decrypted, "a") as file:
    file.write(decryptedString)

Answer 1

You definitely should use dictionaries. Together with Pythons map(), your task is really simple:

import string

secret = """FMMP
NZ OBNF JT KPTIVB
KPIO TVDLT BTT
GVOEBNFOUBMT PG DPNQVUJOH"""

characters = list(string.uppercase)
cipher = dict(zip(characters[1:]+[characters[0]], characters))

decrypted = "".join(map(lambda x: cipher.get(x,x), secret))

print decrypted

I won’t post the result of the decryption here as I don’t have a “Parental Advisory”-sticker atm. 😉 Just try it!

Of course there are helper functions in the string-module, but OP wants to learn python, not implement a “bullet-proof” cryptographic system.