From 7f9a4f7adcc0aa18077d2ab43443411d87dc6d23 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Patrick=20M=C3=BCller?= <patrick@mueller-patrick.tech>
Date: Fri, 15 Oct 2021 16:40:23 +0200
Subject: [PATCH] #5: Implementing Tausch-Chiffre

---
 chapter_one/Tausch-Chiffre.py | 75 +++++++++++++++++++++++++++++++++++
 1 file changed, 75 insertions(+)
 create mode 100644 chapter_one/Tausch-Chiffre.py

diff --git a/chapter_one/Tausch-Chiffre.py b/chapter_one/Tausch-Chiffre.py
new file mode 100644
index 0000000..761d30c
--- /dev/null
+++ b/chapter_one/Tausch-Chiffre.py
@@ -0,0 +1,75 @@
+from utils import AlphabetUtils as au
+
+ALLOWED_KEYS = [1, 3, 5, 7, 9, 11, 15, 17, 19, 21, 23, 25]
+
+
+def encrypt_text(key1: int = 11, key2: int = 7, cleartext: str = 'BonkRocks'):
+	"""
+	Encrypts the given text with the given keys
+	: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
+	:param cleartext: The text to encrypt
+	: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(key1: int = 11, key2: int = 7, ciphertext: str = 'sfunmfdnx'):
+	"""
+	Decrypts the given ciphertext with the given keys
+	: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
+	:param ciphertext: The text to decrypt
+	: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())
+	print(decrypt_text())