Miscellany

import sys
import string

d={}
for i in range(26):
    d[chr(i+97)]=i

e={}
for i in range(26):
    e[i]=chr(i+97)

plain_text=raw_input("enter plain text")
key=input("enter key")

cipher_text=[None]*len(plain_text)
l=list(plain_text)
cipher_list=[None]*len(plain_text)
for i in range(len(plain_text)):
    cipher_list[i]=e[((d[l[i]]+key)%26)]

print string.join(cipher_list, '')