From 588507bcf5fa4cd7a3baf4be10713833d5a9eca8 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Patrick=20M=C3=BCller?= <patrick@mueller-patrick.tech>
Date: Fri, 15 Oct 2021 16:10:44 +0200
Subject: [PATCH] #4: Implementing Multiplikative Chiffre

---
 chapter_one/Multiplikative_Chiffre.py | 65 +++++++++++++++++++++++++++
 chapter_one/__init__.py               |  0
 utils/AlphabetUtils.py                |  7 +--
 3 files changed, 66 insertions(+), 6 deletions(-)
 create mode 100644 chapter_one/Multiplikative_Chiffre.py
 create mode 100644 chapter_one/__init__.py

diff --git a/chapter_one/Multiplikative_Chiffre.py b/chapter_one/Multiplikative_Chiffre.py
new file mode 100644
index 0000000..5157422
--- /dev/null
+++ b/chapter_one/Multiplikative_Chiffre.py
@@ -0,0 +1,65 @@
+from utils import AlphabetUtils as au
+
+ALLOWED_KEYS = [1, 3, 5, 7, 9, 11, 15, 17, 19, 21, 23, 25]
+
+
+def encrypt_text(key: int = 11, cleartext: str = 'BonkRocks'):
+	"""
+	Encrypts the given text with the given key
+	:param key: The key to use. Has to be between 1 and 25 and a coprime to 26.
+	:param cleartext: The text to encrypt
+	: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)
+		newIndex = (index * key) % 26
+		resulting += au.get_letter_at_index(newIndex)
+
+	return resulting
+
+
+def decrypt_text(key: int = 11, ciphertext: str = 'lyngfywgq'):
+	"""
+	Decrypts the given ciphertext with the given key
+	:param key: The key to use. Has to be between 1 and 25 and a coprime to 26.
+	:param ciphertext: The text to decrypt
+	: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)
+		newIndex = (index * decrypt_key) % 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())
diff --git a/chapter_one/__init__.py b/chapter_one/__init__.py
new file mode 100644
index 0000000..e69de29
diff --git a/utils/AlphabetUtils.py b/utils/AlphabetUtils.py
index bc2d099..045556a 100644
--- a/utils/AlphabetUtils.py
+++ b/utils/AlphabetUtils.py
@@ -8,14 +8,9 @@ def get_letter_at_index(idx: int, capital: bool = False):
 
 	return LETTERS[idx] if capital else LETTERS[idx].lower()
 
+
 def get_index_of_letter(letter: str):
 	if letter.upper() not in LETTERS:
 		raise AttributeError
 
 	return LETTERS.index(letter.upper())
-
-
-if __name__ == '__main__':
-	print(get_letter_at_index(25, True))
-	print(get_index_of_letter('ä'))
-