caesars cipher encryption (snippet)

Python example 'caesars cipher encryption'

Functions in program:

• def caesars_cipher_encoding(s, k, lowercase = True, uppercase = False):

Modules used in program:

• import string, sys

caesars cipher encryption

Python beginners example: caesars cipher encryption

#!/usr/bin/python
# -*- coding: utf-8 -*-

import string, sys

# Caesar's cipher is type of shift cipher, 
# an encryption technique
# First used by roman rular Julius Caesar
# To see how it works refer:
# web-page/article: https://en.wikipedia.org/wiki/Caesar_cipher OR 
#                   https://www.khanacademy.org/computing/computer-science/cryptography/crypt/v/caesar-cipher


def caesars_cipher_encoding(s, k, lowercase = True, uppercase = False):

    # To encrypt plain text in either 
    # uppercase letters or lowercase letters
    if lowercase:
        alphas = list(string.lowercase)
    elif uppercase:
        alphas = list(string.uppercase)

    encrypted = ""
    # List characters in orignal string
    char_s = list(s)

    # Shifting of each single 
    # character in orignal string
    # using given key
    for char in char_s:

        # avoid encryption of spaces
        if char == " ":
            encrypted += " "

        else:  
            # n_index or new index is
            # the index of encrypted character
            n_index = k + alphas.index(char)

            # if encrypted character is greater than
            # length of english alphabets
            if n_index > 25:

                # Read explaination on line 59
                while not n_index <= 25:
                    n_index = n_index - 26
                encrypted += alphas[n_index]

            # simple shift
            else:
                encrypted += alphas[n_index]

    return encrypted

# Explaination of key bigger than 26:
# say we have key 54, 
# we add index of char(say 'a') to key, i.e. key = 54 + 1 = 55
# so after first iteration over array of length(26), 
# i.e. 55(total iterations to perform) - 26(iterations completed) = 29(iterations left)
# but, 29 is still a big index i.e. 29 > length of array(26)
# so we continue to subtract more 26(iterations), 
# i.e. 29(iterations to perform) - (26 iterations done) = 3(remaining)
# so 3 is smaller than length of array and can be used as key, 
# i.e. 3 < 26(length of array)
# thus, we reached at index of 3 after 54 iterations over an array of length 26
# bit complicated but read it twice, you'll master it!

# Test, Playing UI
i = 0
while True:
    if raw_input("[{}] Exit(press e), To continue(press c): ".format(i)).lower() == "c":
        # Number of times 
        i += 1

        # Input for String and key for char shift
        S, K = raw_input("\nString: "), int(raw_input("Key: "))

        # Results
        print("\nOrignal string: " + S + " ,  Key: " + str(K))
        print("Encrypted text: " +  caesars_cipher_encoding(S, K, True, False) + "\n")

    else:
        sys.exit()

Useful links

• Learn Python: https://pythonbasics.org
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