running_key_cipher.py

"""
https://en.wikipedia.org/wiki/Running_key_cipher
"""


defrunning_key_encrypt(key: str, plaintext: str) ->str:
"""
 Encrypts the plaintext using the Running Key Cipher.

 :param key: The running key (long piece of text).
 :param plaintext: The plaintext to be encrypted.
 :return: The ciphertext.
 """
plaintext=plaintext.replace(" ", "").upper()
key=key.replace(" ", "").upper()
key_length=len(key)
ciphertext= []
ord_a=ord("A")

fori, charinenumerate(plaintext):
p=ord(char) -ord_a
k=ord(key[i%key_length]) -ord_a
c= (p+k) %26
ciphertext.append(chr(c+ord_a))

return"".join(ciphertext)


defrunning_key_decrypt(key: str, ciphertext: str) ->str:
"""
 Decrypts the ciphertext using the Running Key Cipher.

 :param key: The running key (long piece of text).
 :param ciphertext: The ciphertext to be decrypted.
 :return: The plaintext.
 """
ciphertext=ciphertext.replace(" ", "").upper()
key=key.replace(" ", "").upper()
key_length=len(key)
plaintext= []
ord_a=ord("A")

fori, charinenumerate(ciphertext):
c=ord(char) -ord_a
k=ord(key[i%key_length]) -ord_a
p= (c-k) %26
plaintext.append(chr(p+ord_a))

return"".join(plaintext)


deftest_running_key_encrypt() ->None:
"""
 >>> key = "How does the duck know that? said Victor"
 >>> ciphertext = running_key_encrypt(key, "DEFEND THIS")
 >>> running_key_decrypt(key, ciphertext) == "DEFENDTHIS"
 True
 """


if__name__=="__main__":
importdoctest

doctest.testmod()
test_running_key_encrypt()

plaintext=input("Enter the plaintext: ").upper()
print(f"\n{plaintext=}")

key="How does the duck know that? said Victor"
encrypted_text=running_key_encrypt(key, plaintext)
print(f"{encrypted_text=}")

decrypted_text=running_key_decrypt(key, encrypted_text)
print(f"{decrypted_text=}")