Lab 6: The Caesar Cipher

The Caesar (or shift) cipher with integer key n encodes a string of letters by doing the following to each letter:

  1. Translate the letter to an integer using the 26 character alphabet

    A = 00, B = 01, …, Z = 25

  2. Add n

  3. Reduce the result modulo 26 (take the remainder after dividing by 26)

  4. Translate the result back to a letter

The effect is to shift each letter forward in the alphabet by n spaces, wrapping around at the end of the alphabet. For example, to shift the letter T by n=12, we would do the following:

  1. T -> 19

  2. 19 -> 19+12 = 31

  3. 31 % 26 = 5

  4. 5 -> F

Task 1

Write a function shift_encrypt_letter(n,letter) that takes an integer n and a capital letter (or space character) letter and encrypts the letter using the Caesar cipher with key n. If letter is a space character, your function should return it unchanged.

>>> shift_encrypt_letter(3, `C')
'F'
>>> shift_encrypt_letter(3, 'A')
'D'
>>> shift_encrypt_letter(3, 'T')
'W'
>>> shift_encrypt_letter(9, 'T')
'C'
>>> shift_encrypt_letter(5, ' ')
' '

Task 2

Write a function shift_encrypt(n,s) that takes an integer n and a string of capital letters (or space characters) s and encrypts the string using the Caesar cipher with key n. Use list comprehension, then string join:

L = [shift_encrypt_letter(n,let) for let in s]
''.join(L)

>>> shift_encrypt(3, 'CAT')
'FDW'
>>> shift_encrypt(-23, 'CAT')
'FDW'
>>> shift_encrypt(7, 'CAT IN THE HAT')
'FDW LQ WKH KDW'

Task 3

Write a function shift_decrypt(n,s) that takes an integer n and a string of capital letters s and decrypts the string. Can you think of a way to do this by just calling shift_encrypt with a different n?

>>> shift_decrypt(3, 'FDW')
'CAT'

Task 4

Decrypt the coded message KYV TRBV ZJ R CZV by running your shift_decrypt function for every n in 1,2,3,...,25 and reading off the only one that makes sense.