Simple Cipher2026-04-27

My Solution: Exercism Simple Cipher solution

Instructions

Build a simple substitution cipher using a lowercase key.

• If no key is provided, generate one randomly.
• To encode, shift each plaintext letter forward by the matching key letter.
• To decode, shift each ciphertext letter backward by the matching key letter.
• The key repeats when the text is longer than the key.

Solution

import random
import string

class Cipher:
    def __init__(self, key=None):
        if key:
            self.key = key
        else:
            self.key = ''.join(random.choices(string.ascii_lowercase, k=100))
    
    def encode(self, text):
        res = ''
        for i, ch in enumerate(text):
            shift = ord(self.key[i % len(self.key)]) - ord('a')

            res += chr((ord(ch) - ord('a') + shift) % 26 + ord('a'))

        return res

    def decode(self, text):
        res = ''
        for i, ch in enumerate(text):
            shift = ord(self.key[i % len(self.key)]) - ord('a')

            res += chr((ord(ch) - ord('a') - shift) % 26 + ord('a'))
            
        return res

Syntax Notes

• ord(char) gives the Unicode number for a character.
• chr(number) converts a number back to a character.
• ord('a') is used as a zero point so letters map cleanly to 0..25.
• shift = ord(key_char) - ord('a') turns each key letter into a shift amount.
• % 26 wraps around the alphabet (for example, after z comes a).
• enumerate(text) gives both index i and character ch.
• i % len(self.key) cycles through the key when text is longer than the key.
• random.choices(string.ascii_lowercase, k=100) picks 100 random lowercase letters.
• ''.join(...) combines those letters into one key string.