affine cipher

practice/affine-cipher/affine_cipher.py

@@ -1,6 +1,60 @@
-def encode(plain_text, a, b):
-    pass
+"""
+Approach:
+- The provided code implements a simple affine cipher, which is a type of substitution cipher.
+- The `encode` function encodes plain text using the formula E(x) = (ax + b) mod m, where 'a' and 'm' are coprime, and 'b' is any integer.
+- The `decode` function decodes the ciphered text using the formula D(x) = a^(-1)(x - b) mod m.
+- The `is_coprime` function checks whether two numbers are coprime.
+- The implementation ensures that 'a' and the length of the alphabet are coprime to ensure that every letter is mapped to a unique value.
+"""
+
+import math
+import string
 
+def is_coprime(x, y):
+    return math.gcd(x, y) == 1
+
+def encode(plain_text, a, b):
+    alphabet = list(string.ascii_lowercase)
+    numbers = list(string.digits)
+    if not is_coprime(a, len(alphabet)):
+        raise ValueError("a and m must be coprime.")
+    indexes = []
+    for letter in plain_text.lower():
+        if letter in alphabet:
+            indexes.append((a * alphabet.index(letter) + b) % len(alphabet))
+        elif letter in numbers:
+            indexes.append(int(letter) + 1000)
+    out = ""
+    for place, index in enumerate(indexes):
+        if place % 5 == 0 and place > 1:
+            out += " "
+            if index < len(alphabet):
+                out += alphabet[index]
+            else:
+                out += str(index - 1000)
+        else:
+            if index < len(alphabet):
+                out += alphabet[index]
+            else:
+                out += str(index - 1000)
+    return out
 
 def decode(ciphered_text, a, b):
-    pass
+    alphabet = list(string.ascii_lowercase)
+    numbers = list(string.digits)
+    if not is_coprime(a, len(alphabet)):
+        raise ValueError("a and m must be coprime.")
+    indexes = []
+    for letter in ciphered_text:
+        if letter in numbers:
+            indexes.append(int(letter) + 1000)
+        elif letter in alphabet:
+            indexes.append(
+                pow(a, -1, len(alphabet)) * (alphabet.index(letter) - b) % len(alphabet)
+            )
+    return "".join(
+        [
+            str(letter - 1000) if letter > len(alphabet) else alphabet[letter]
+            for letter in indexes
+        ]
+    )