peer-graded-vignere-cipher.md

Honors Peer Graded Assignment: Pseudo-Code Vignere Cipher

Which programming language, if any, are you familiar with?

I am familiar with Java and Python.

Write your answer for the pseudo-code for the Vigenere Cipher encryption.

We take the plaintext as p and ciphertext as c which we produce from plaintext during encryption and we produce the plaintext back from the ciphertext using the decryption steps. We encrypt the plaintext p of arbitrary length using key stream generated from key k of arbitrary length.

such that c_i = (p_i + k_i) mod 26 where k_i = key_{i mod m} where m is the length of the key. The pseudo-code is represented as:

m <-- length of key 
for index, character in plaintext:
    ciphertext[index] <-- (character + key[index % m]) % 26

return ciphertext

This encryption algorithm can be represented in Python as

def encrypt(plaintext: str, key: str) -> str:
    ciphertext = ''
    m = len(key)
    for i, letter in enumerate(plaintext.lower()):
        ciphertext += chr((char_2_num(letter) + char_2_num(key[i % m])) % 26 + ord('a'))
    return ciphertext.upper()

def char_2_num(character: str) -> int:
    return ord(character.lower()) - ord('a')

where char_2_num is a utility function. Similarly we decrypt the ciphertext using the key by the following pseudo-code:

m = length of key
for index, character in ciphertext:
    plaintext[i] <-- (character - key[index % m]) % 26

return plaintext

The algorithm in Python is represented as:

def decrypt(ciphertext: str, key: str) -> str:
    plaintext = ''
    m = len(key)
    for index, letter in enumerate(ciphertext.lower()):
        plaintext += chr(
            (char_2_num(letter) - char_2_num(key[index % m])) % 26 + ord('a'))
    return plaintext.lower()