polyalphabetic_cipher

def encrypt(text, key):

    #Encrypts the plaintext using the polyalphabetic cipher.

    encrypted_text = []
    key = key.upper()
    key_length = len(key)
    key_index = 0

    for char in text:
        if char.isalpha():
            shift = ord(key[key_index]) - ord('A')
            if char.isupper():
                new_char = chr((ord(char) - ord('A') + shift) % 26 + ord('A'))
            else:
                new_char = chr((ord(char) - ord('a') + shift) % 26 + ord('a'))
            encrypted_text.append(new_char)
            key_index = (key_index + 1) % key_length
        else:
            encrypted_text.append(char)

    return ''.join(encrypted_text)


def decrypt(text, key):
    #Decrypts the ciphertext using the polyalphabetic cipher

    decrypted_text = []
    key = key.upper()
    key_length = len(key)
    key_index = 0

    for char in text:
        if char.isalpha():
            shift = ord(key[key_index]) - ord('A')
            if char.isupper():
                new_char = chr((ord(char) - ord('A') - shift) % 26 + ord('A'))
            else:
                new_char = chr((ord(char) - ord('a') - shift) % 26 + ord('a'))
            decrypted_text.append(new_char)
            key_index = (key_index + 1) % key_length
        else:
            decrypted_text.append(char)

    return ''.join(decrypted_text)


plaintext = "HELLO, WORLD!"
key = "KEY"

ciphertext = encrypt(plaintext, key)
print("Encrypted Text:", ciphertext)

decrypted_text = decrypt(ciphertext, key)
print("Decrypted Text:", decrypted_text)