Very Simple Caesar Cipher (Python)

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During a recent CTF I needed to decrypt a page encrypted with a Caesar Cipher. I didn’t know the what the ROT was and I was struggling to find an easy yet useful one around so I wrote this one.

Feel free to borrow it, steal it, claim it as your own or do whatever with it.

norsec0de

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#!/usr/bin/python
###############################
#                             #
#  Very Simple Caesar Cipher  #
#        by norsec0de         #
#                             #
#          caesar.py          #
#                             #
###############################
import sys
# Encrypt Plain Text
def caesar(plainText, key=0):
    cipherText = ""
    for c in plainText:
        if c.isalpha():
            if c.isupper():
                caps = True
            else:
                caps = False
            alphabet = ord(c.lower()) + key
            if alphabet > ord('z'):
                alphabet -= 26
            letter = chr(alphabet)
            if caps is True:
                letter = letter.upper()
            cipherText += letter
        else:
            cipherText += c
    return cipherText
# Brute force all 26 possibilities.
def guess_caesar(plainText):
    print 'Showing all combinations for text ' + plainText
    key = 1
    for c in range(0,26):
        cipherText = caesar(plainText, key)
        print 'Key ' + str(key) + ' => ' + cipherText
        key += 1
# Main Program
# Usage
if len(sys.argv) == 1:
    print '\nCaesar Cypher Tool'
    print '\n Usage: ' + sys.argv[0] + ' (options) <text> <rot key>'
    print '\t encrypt         => \"Text to encrypt\" <key>'
    print '\t decrypt         => -d \"Text to decrypt\" <key>'
    print '\t decrypt file    => -f <filename> <key>'
    print '\t brute force     => -b \"Cyphertext\" \n'
# Brute force
elif sys.argv[1] == '-b':
    plainText = sys.argv[2]
    cipherText = guess_caesar(plainText)
    print cipherText
# Decode a string
elif sys.argv[1] == '-d':
    plainText = sys.argv[2]
    key = int(sys.argv[3])
    cipherText = caesar(plainText, key)
    print cipherText
# Decode a File
elif sys.argv[1] == '-f':
    infile = sys.argv[2]
    key = int(sys.argv[3])
    with open(infile, "r") as f:
        while True:
            line = f.readline()
            if not line:
                break
            print caesar(line, key)
# Encode a String
else:
    plainText = str(sys.argv[1])
    key = int(sys.argv[2])
    cipherText = caesar(plainText, key)
    print '\nEncrypting \"' + plainText + '\" with ROT' + str(key) + " => " + c
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