implementation of vigenere cipher in python

vigenere cipher

letters = ['a','b','c','d','e','f','g','h','i','j','k','l','m','n','o','p','q','r','s','t','u','v','w','x','y','z',' ','.',',','?','1','2','3','4','5','6','7','8','9','0']
cipher = []
pos_key = []
pos_text = []
pos_cipher = 0
key = []
plain = []
letters_len = 40
import os
def find_pos(value):
        temp = []
        i=0
        while i<len(value):
                j=0
                while j<letters_len:
                        if value[i] == letters[j]:
                                temp.append(j)
                        j = j+1
                i= i+1
        return temp
        
def chek_end(m,n,size):
        if m==size and n<size:
                m=0
                return m
        if m==size and n==size:
                return m,n
        return m

def vigenere_encrypt(n):
       
        
        K = raw_input('Enter The Key ::::')
        P= raw_input('Enter The String To encrypt or decrypt ::::')
        key = map(lambda k:k.lower(),K)
        plain = map(lambda p:p.lower(),P)
        pos_key = find_pos(key)
        pos_text = find_pos(plain)                                      
        
        if len(pos_key) > len(pos_text):
                range_list = pos_key
        else :
                range_list = pos_text
        i=0
        j=0
        loop_pass = 0
        
        while i<len(range_list) and j<len(range_list) and loop_pass<len(pos_text):      
                try:
                        pos_cipher = n*pos_key[i] + pos_text[j]
                        if pos_cipher<letters_len:
                                cipher.append(letters[pos_cipher])
                        else:
                                pos_cipher = pos_cipher-letters_len
                                cipher.append(letters[pos_cipher])
                        i = i+1
                        j = j+1
                        
                        if i==len(pos_key) and j<len(pos_text):
                                i=0

                        elif j==len(pos_text) and i<len(pos_key):
                                j=0
                        loop_pass = loop_pass + 1
                except:
                        print("Oops Something went wrong")
        
        result = "" .join(cipher)
        print result
       
if __name__=='__main__':
        n = input (' 1 for encrypt \n-1 for decrypt\n::::')
        vigenere_encrypt(n)
        end=input ('press <enter>')
        pass