vigenere.py

import cs50
import sys

if len(sys.argv) == 1 or len(sys.argv) > 2:
    print("Usage: python vigenere.py <key>")
    exit(1)

key = sys.argv[1]
for i in range(len(key)):
    if key[i].isalpha() is not True:
        print("Keyword must only contain letters A-Z or a-z")
        exit(2)

print("plaintext: ", end="")
plaintext = cs50.get_string()
plaintext_size = len(plaintext)
key_size = len(key)

ciphertext = []
j = 0
for i in range(plaintext_size):
    if j >= key_size:
        j = 0
    if plaintext[i].isalpha():
        k = ord(key[j].upper()) % 65

        c = (ord(plaintext[i]) + k)
        if c >= 0 or c <= 25:
            c %= 26
        ciphertext.append(chr(c))
        j += 1

    else:
        c = ord(plaintext[i])
        ciphertext.append(chr(c))

print("ciphertext: ", end="")
print("".join(ciphertext))
exit(0)