Krypto_Grundlagen/chapter_one/Tausch-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, key1: int, key2: int):
	"""
	Encrypts the given text with the given keys
	:param cleartext: The text to encrypt
	:param key1: The first key. Has to be between 1 and 25 and a coprime to 26
	:param key2: The second key. Has to be between 0 and 26
	:return: The encrypted text
	"""
	# key1 has to be coprime to 26
	if key1 not in ALLOWED_KEYS:
		raise AttributeError

	# key2 has to be between 0 and 25
	if key2 < 0 or key2 > 25:
		raise AttributeError

	resulting = ''

	for char in cleartext:
		index = au.get_index_of_letter(char)
		newIndex = (index * key1 + key2) % 26
		resulting += au.get_letter_at_index(newIndex)

	return resulting


def decrypt_text(ciphertext: str, key1: int, key2: int):
	"""
	Decrypts the given ciphertext with the given keys
	:param ciphertext: The text to decrypt
	:param key1: The first key. Has to be between 1 and 25 and a coprime to 26
	:param key2: The second key. Has to be between 0 and 26
	:return: The decrypted text
	"""
	# key1 has to be coprime to 26
	if key1 not in ALLOWED_KEYS:
		raise AttributeError

	# key2 has to be between 0 and 25
	if key2 < 0 or key2 > 25:
		raise AttributeError

	decrypt_key1 = _get_key_reverse(key1)

	resulting = ''

	for char in ciphertext:
		index = au.get_index_of_letter(char)
		newIndex = ((index - key2) * decrypt_key1) % 26
		resulting += au.get_letter_at_index(newIndex)

	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, 7))
	print(decrypt_text('sfunmfdnx', 11, 7))