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# 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: