Day 19: Vigenère cipher

The Vigenère cipher is a method of encrypting alphabetic text by using a series of interwoven Caesar ciphers, based on the letters of a keyword. It employs a form of polyalphabetic substitution.

import re
import string

alphabet = "abcdefghijklmnopqrstuvwxyz"
def encrypt(p, k):
    c = ""
    kpos = []
    for x in k:
        kpos.append(alphabet.find(x))
    i = 0
    for x in p:
      if i == len(kpos):
          i = 0
      pos = alphabet.find(x) + kpos[i] 
      print(pos)
      if pos > 25:
          pos = pos-26               
      c += alphabet[pos].capitalize()
      i +=1
    return c

def decrypt(c, k):
    p = ""
    kpos = []
    for x in k:
        kpos.append(alphabet.find(x))
    i = 0
    for x in c:
      if i == len(kpos):
          i = 0
      pos = alphabet.find(x.lower()) - kpos[i]
      if pos < 0:
          pos = pos + 26
      p += alphabet[pos].lower()
      i +=1
    return p
try:
    print("The message must contain only letters and the key should be a single word with no numbers. \n"
          "Press 1 to Enrypt a message \npress 2 to Decrypt a message")
    choose = input("Choice: ")
    if choose == '1':
       p = input("enter the plain text: ")
       p = p.replace(" ", "")
       if p.isalpha():
           k = input("Enter the key: ")
           k = k.strip()
           if k.isalpha():
              # print(k)
               c = encrypt(p, k)
               print("The cipher text is: ", c)

           else:
               print(k)
               print("Enter valid key.")
       else:
           print("only letters are allowed")

    elif choose == '2':
        c = input("enter the cipher text: ")
        c = c.replace(" ", "")
        if c.isalpha():
            k = input("Enter the key: ")
            if not k.isalpha():
                print("Enter valid key.")
            else:
                p = decrypt(c, k)
                print("The plain text is: ", p)
        else:
            print("only letters are allowed")

    else:
        print("Please enter a selection")
except Exception as e:
    print(e)
    exit("Enter a valid text")