Krypto_Grundlagen/chapter_one/Multiplikative_Chiffre.py

from utils import AlphabetUtils as au

ALLOWED_KEYS = [1, 3, 5, 7, 9, 11, 15, 17, 19, 21, 23, 25]


def encrypt_text(cleartext: str, key: int):
	"""
	Encrypts the given text with the given key
	:param cleartext: The text to encrypt
	:param key: The key to use. Has to be between 1 and 25 and a coprime to 26.
	:return: The encrypted text
	"""
	# Key has to be coprime to 26
	if key not in ALLOWED_KEYS:
		raise AttributeError

	resulting = ''

	for char in cleartext:
		index = au.get_index_of_letter(char)
		new_index = (index * key) % 26
		resulting += au.get_letter_at_index(new_index)

	return resulting


def decrypt_text(ciphertext: str, key: int):
	"""
	Decrypts the given ciphertext with the given key
	:param ciphertext: The text to decrypt
	:param key: The key to use. Has to be between 1 and 25 and a coprime to 26.
	:return: The decrypted text
	"""
	# Key has to be coprime to 26
	if key not in ALLOWED_KEYS:
		raise AttributeError

	decrypt_key = _get_key_reverse(key)

	resulting = ''

	for char in ciphertext:
		index = au.get_index_of_letter(char)
		new_index = (index * decrypt_key) % 26
		resulting += au.get_letter_at_index(new_index)

	return resulting


def _get_key_reverse(key: int):
	"""
	Calculates the decryption key (inverse of the original key) based on the following formula:
	s * s' mod 26 = 1

	:param key: The key to reverse
	:return: The decryption key
	"""
	for possibility in ALLOWED_KEYS:
		if (key * possibility) % 26 == 1:
			return possibility


if __name__ == '__main__':
	print(encrypt_text('BonkRocks', 11))
	print(decrypt_text('lyngfywgq', 11))