As usual, create a directory to hold today's files. All programs that you write today should be stored in this directory.
$ cd ~/cs120/labs $ mkdir lab22 $ cd lab22
Cryptography is the science of hiding secrets. A typical cryptographic protocol takes some plaintext message P and some key k, and produces a ciphertext C that is unreadable by any entity that does not know the key k
We observed a very simplistic encryption scheme in the last class: Caesar cipher. This encryption scheme has been in use since Roman times, and is very easily breakable. As such, we likely want a stronger encryption scheme to make our messages even more secure.
The key in caesar is \(3\). To encrypt, you chose the letter \(3\) spaces further in the alphabet of the entered character. In Vigenere, the key is a word. For each character in the plaintext, you use the corresponding letter in the key to determine how much to shift that character by.
Consider the plaintext "hello", with a key of "hi". This means to encrypt the "h" in hello, you use the letter "h" as the shift (Which is a shift of \(7\), since h is the 7th character of the alphabet). Then, for the letter "e" in hello, you would use the letter "i" as the shift (which is a shift of \(8\)). But, now we have run out of letters in the key. In this case, you wrap back around to the beginning of the key string, using "h" as the key for the first "l" in hello.
In a file called vigenere.py, write a function
called encode_vigenere(plaintext, key). This function
takes two strings as a parameter, and returns a string which is the
encryption of plaintext using key.
Make sure you test your program well. How many test cases do you need? Include your test cases in your program file.
| Function Parameters | Expected Output |
|---|---|
| "hello", "hi" | omstv |
| "goodbye", "hello" | nszopfi |
The ord function gives you the ASCII
representation of a character. If you subtract
the ord('a') from this, you will get the
position in alphabet a character is.
The actual encryption of this will be very similar to the caesar cipher, with a change to the shift that occurs.
The chr function takes an integer, and returns
to you the character that represents the ASCII value.
% operator. You will likely want to mod by
the length of your key.
The vigenere cipher is possibly the strongest classical cipher you
will encounter. Which means that writing a decryption function is
of the utmost importance. Write a function
called decode_vigenere(ciphertext, key), which
decrypts the ciphertext with the associated key.
Another common technique for encryption is known as the Substitution Cipher. Instead of defining a linear equation between the plaintext and ciphertext, you can define a map that translates characters using a specified order. Crypograms use this technique, for example.
The key in the substitution cipher is a string of characters in the range [a, z]. For example, the string "abcdefghijklmnopqrstuvwxyz" is one possible key in the substitution cipher. It maps the letter 'a' to the letter 'a', the letter 'b' to 'b', and so on. A more interesting example would be "zyxwvutsrqponmlkjihgfedcba", which maps 'a' to 'z', 'b' to 'y', and so on. For example, "hello world" would become "svool dliow" under this key. Notice that characters that are not alphabetic are not encoded, and their normal value is included.
In a file called substitution.py, define a function
called encode_substitution(plaintext, key). This
function takes two
parameters: the plaintext as a string of lowercase characters, and
a key as a string of lowercase characters. Your function will
return a string of lowercase characters, which is the plaintext
with each character substituted with its corresponding letter.
| Function Parameters | Expected Output |
|---|---|
| "hello", "zyxwvutsrqponmlkjihgfedcba" | svool |
| "goodbye", "acegikmoqsuwybdfhjlnprtvxz" | mddgcxi |
Given a character, you can compute its position in the
alphabet by subtracting the ord('a') from the
ordinal value of that character.
Write a function decode_substitution(ciphertext,
key), which returns the decryption of the specified
ciphertext with the associated key.