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.
Details
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.
Sample Test Cases
| Function Parameters | Expected Output |
|---|---|
| "hello", "hi" | omstv |
| "goodbye", "hello" | nszopfi |
Hint
-
The actual encryption of this will be very similar to the caesar cipher, with a change to the shift that occurs.
-
In order to account for the wrapping around of the key, use
the
%operator. You will likely want to mod by the length of your key.
Challenge
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.
Details
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.
Sample Test Cases
| Function Parameters | Expected Output |
|---|---|
| "hello", "zyxwvutsrqponmlkjihgfedcba" | svool |
| "goodbye", "acegikmoqsuwybdfhjlnprtvxz" | mddgcxi |
Hint
-
Given a character, you can compute its position in the alphabet by using the index method on
string.ascii_lowercase. This returns an integer, the position the character exists in the string. You will use this as your index into the key string.
Challenge
Write a function decode_substitution(ciphertext,
key), which returns the decryption of the specified
ciphertext with the associated key.
Submission
Please show your source code and run your programs for the instructor or lab assistant. Only a programs that have perfect style and flawless functionality will be accepted as complete.