Python: Cryptography Caesar shift encryption (shift cipher)

I have always had a keen interest in cryptography and rather than give a brief history of cryptography I will recommend reading Simon Singh’s The code book or for a modern and hands on approach Applied Cryptography by Bruce Schneier (Who also made a brilliant book on security, more of descriptive approach but very interesting Secrets and Lies: Digital Security in a Networked World).

This post aims to detail the creation (in python) of one of the simplest forms of encryption; the simple Caesar shift (or shift cipher). The Caesar shift takes the normal alphabet and maps it to a an identical alphabet with a rotation. The cipher will be written in such a way that it can be easily expanded on to create more complex encryption schemes with little modification.

The above image shows a diagrammatic representation of a Caesar shift of 3 (alphabet transposed onto a rotation of itself with a displacement of 3). Below shows the entire alphabet in plaintext and in ciphertext, followed by a simple sentence in plaintext and in cipher text.

Plaintext:	abcdefghijklmnopqrstuvwxyz
Ciphertext:	defghijklmnopqrstuvwxyzabc

Plaintext:	the cat sat on the mat
Ciphertext:	wkh fdw vdw rq wkh pdw

Below is the code to convert plaintext into ciphertext along with an example of the usage:

def caesar(plaintext,shift):

	alphabet=["a","b","c","d","e","f","g","h","i","j","k","l",
	"m","n","o","p","q","r","s","t","u","v","w","x","y","z"]

	#Create our substitution dictionary
	dic={}
	for i in range(0,len(alphabet)):
		dic[alphabet[i]]=alphabet[(i+shift)%len(alphabet)]

	#Convert each letter of plaintext to the corrsponding
	#encrypted letter in our dictionary creating the cryptext
	ciphertext=""
	for l in plaintext.lower():
		if l in dic:
			l=dic[l]
		ciphertext+=l

	return ciphertext

#Example useage
plaintext="the cat sat on the mat"
print "Plaintext:", plaintext
print "Cipertext:",caesar(plaintext,3)
#This will result in:
#Plaintext: the cat sat on the mat
#Cipertext: wkh fdw vdw rq wkh pdw

Explanation
Here we have written a function that is given the plaintext and an arbitary rotation (including negative) and returns a ciphertext. The function creates a dictionary, mapping each letter to another letter that is ‘shift’ letters away from it in the alphabet (the modulus (%) is used so to wrap the alphabet back to the start if it falls off the end). It then converts each letter of the plain text into the corresponding encrypted letter using the dictionary.

Future
In the next post I will discuss methods to improve the Caesar shift and how to turn it into a full substitution cipher (where the alphabet is shuffled rather than rotated). The Caesar shift is very susceptible to brute forcing and frequency analysis which I shall explain and create a program to defeat these encryptions in a future post.