#Caesar cipher

import random

#the plaintext string is assumed to consist
#of lower-case letters
#with no spaces or other punctuation

#The key k is an integer--we only use the value of k
#mod 26.

def caesar_encrypt(plaintext,k):
    ciphertext = ''
    for ch in plaintext:
        ciphertext += shiftletter(ch,k)
    return ciphertext

#The actual computation of adding k mod 26 to a lower-case
#letter value is done here. Note we use functions chr and ord
#to convert between characters and their ASCII encodings, and
#use ord('a') as an offset so that 'a' corresponds to 0, 'b' to 1,
#etc.

def shiftletter(letter,k):
    return chr(ord('a')+(ord(letter)-ord('a')+k)%26)

def caesar_decrypt(ciphertext,k):
    return caesar_encrypt(ciphertext,-k)

#Return a list of all decryptions of the given ciphertext under
#every possible key.  Since there are only 26 of them, visual inspection
#reveals the correct plaintext.

def all_decryptions(ciphertext):
    dec_list=[]
    for k in range(26):
        dec_list.append(caesar_encrypt(ciphertext,k))
    return dec_list

#Run this function to see a demonstration
def caesar_demo():
    plaintext=raw_input('Enter plaintext string\n')
    #convert to lower case and strip non-letters
    plaintext=plaintext.lower()
    newplaintext=''
    for ch in plaintext:
        if ord('a')<=ord(ch)<=ord('z'):  #yes, this is legal Python!
            newplaintext+= ch
    print 'Modified plaintext:'
    print newplaintext
    print
    print 'Encrypt under random key'
    key=random.randint(0,25)
    ciphertext = caesar_encrypt(newplaintext,key)
    print
    print ciphertext
    print
    print 'Decrypt under every possible key'
    print
    decs=all_decryptions(ciphertext)
    for text in decs:
        print text