Lab 3: Cryptography Due: 8:00:00AM on 2022-10-07



For this assignment, we’re going to be experimenting with basic cryptography. Cryptography is a field at the intersection of math and computer science that studies ways of securely communicating information. Throughout history, it has played an important role in a variety of cultures in a variety of contexts. We’ll experiment with some basic encryption techniques, but there is a wealth of information on modern encryption out there (Wikipedia is a good place to start if you’re interested). In cryptography, the two main functions are encryption, where we take a plaintext message and encrypt it into the cyphertext, and decryption, where we take the encrypted cyphertext and extract the original plaintext message. In this assignment, we’ll experiment with a few different ways of doing both of these operations.

An important skill for any programmer is the ability to utilize and extend previously written code. For this assignment, I’m going to give you some code to get you started. This program has some variables and functions written for you already and you will be filling in some of the details.

Download the starter code

Right-click on the above link, select “Save as…”, and save the file for this assignment. Then open the saved file with Thonny. This file does not have any syntax errors, however, until you fill in some of the details not all of the functions will run (i.e. the keygen function will throw an error).


Warming up

To get you warmed up we’re going to try a very simple method for encryption. You have a friend who says they have a great idea for encrypting a message. To encrypt it, you repeat each letter 3 times. To decrypt it, you reverse the process. For example: “secret message” would become “ssseeecccrrreeettt mmmeeessssssaaagggeee”

Write a function called fools_encrypt that takes a message as a parameter and returns the message encrypted using this scheme.

If you want a challenge (not required), try to write a function named fools_decrypt that decrypts this. For example, you could execute:

>>> fools_decrypt("ssseeecccrrreeettt   mmmeeessssssaaagggeee")
'secret message'
>>> secret = fools_encrypt("secret message")
>>> secret
'ssseeecccrrreeettt   mmmeeessssssaaagggeee'
>>> fools_decrypt(secret)
'secret message'

If you’re not up for the challenge but want to test decrypting the results of your encryption function, you can copy my sample solution decryption function into your file: (although really give it a try before downloading the sample code).

Caesar’s method

Now, let’s try to implement Caesar’s encryption scheme. If you need a review of how it works, take a look at the prelab again. In the starter, I’ve given you a function called shift_letter. Play with this function and try some different letters (lowercase letters only) and different number offsets to make sure you understand how it works. Notice that you can also pass it negative numbers to shift the other direction.


Once you’re comfortable with how shift_letter works, write a function called caesar_encrypt that takes two parameters: the message to be encrypted and the size of the shift for the encryption. You will very likely want to utilize the shift_letter function.


One of the nice things about this encryption scheme is that we can use the same function for encrypting and decrypting a message. Before reading on, think about how we could do this…

To decrypt a message, we can use our caesar_encrypt function, but now, give it a negative number. Encrypt a message or two and make sure that you can decrypt them.

Decrypt the following message with number offset 5:


It’s a quote from a famous computer scientist (Dijsktra). You don’t have to write it down, but it’s a good check to make sure you’ve got things working.

Warning: our encryption schemes will only work for lowercase letters (specifically the letters a-z and space), so make sure that your message doesn’t have any other characters otherwise, it won’t work correctly.

Substitution ciphers

Caesar’s method is just an example of a more general type of encryption scheme called substitution ciphers. For a general substitution cipher, a key is used. The key has all of the same letters as the alphabet, but the order has been shuffled.

When encrypting, you encrypt one letter at a time. For each letter in the message you find that letter in the alphabet and then replace it with the corresponding letter in the key. For example, say you have following alphabet and key (I’ve put the indices in to make it easier to understand):

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  ' '
          0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26
key:      h v i e k s y r b d a  j  q  w  n  c  x  m  g  u  f  l  t  p ' ' o  z

and you see the character ‘k’, you look that up in the alphabet and notice that it is at index 10. You then use this index in the key to get ‘a’ as the letter. This process is repeated for each letter in the message until you have encrypted the entire message. Make sure you understand this process before moving on to the coding.

Key generation

Before we can actually encrypt anything, we first need to generate a key. We could do this by hand, but it’s difficult to do. In the starter code, I’ve included a helper function named keygen for generating the kind of keys needed by your substitution encryption and decryption functions. This function takes any string as a parameter and generates a random key based on that input string (think of it like a password). I’ve given you all the code for this function except it relies on the splice function, which needs to be implemented. Fill in the details of the splice function.

When you’re done, you should be able to generate random keys using the keygen function. Try out a few examples and make sure that it is a permutation of all of the letters in the alphabet and that you get the same thing back if you give it the same password. For example:

>>> keygen("doglover")
'deyvhaflcirmtjsgoxbpzukwnq '
>>> keygen("doglover")
'deyvhaflcirmtjsgoxbpzukwnq '

Note: due to various computer-specific factors you may not get the exact same keys as I have included here. If, on your computer, you call keygen twice with the same string, however, you should get the same string back.


Now that you can generate random keys, you’re ready to encrypt something. Write a function called subst_encrypt that takes two parameters: the message to be encrypted and the key (not the password). The function should return the message encrypted using the key. Notice that there is a constant called ALPHABET which has all of the letters and space in order. You’ll likely need to use this.


For a general substitution cipher we also need to write a decryption method. Write a function called subst_decrypt that takes two parameters: the encrypted message and the key (not the password). The function should return the message decrypted using the key (i.e. the original message before encryption). This function should be very similar to the encryption function with just one or two minor changes (it’s the same basic idea, but in the other direction). When you’re done, you can generate a new key then encrypt and decrypt messages. For example:

>>> key = keygen("mypassword")
>>> key
'jf rdesquztkwhoipcbamlvxgny'
>>> encrypted = subst_encrypt("this is a secret message", key)
>>> encrypted
'aqubyubyjybd cdaywdbbjsd'
>>> subst_decrypt(encrypted, key)
'this is a secret message'

As before, keygen might work differently on your machine, so your encrypted text might look different.

Decrypt the following message using the password “i like cs”

ao us jauaoiwpuxsrtauxusrrf xwuiju g wuiduqrtuxh uyhrwpuqrtuxh urwfqurddusquxusia

Note: If it doesn’t work, check to see if you random key is 'xscl dpoimkfzwrevhjatgynqbu'. If it’s not, it’s most likely just a difference in computers. Try your decrypt message function with this key then to make sure it works.

Again, you don’t have to write it down, but it’s a good check to make sure you’ve got things working.


At a minimum your submission should have:

  1. A function named fools_encrypt that takes a message as a parameter and returns the message encrypted using the scheme described in the Guide section.
  2. A function namecaesar_encrypt that takes two parameters, the message to be encrypted and the size of the shift for the encryption, and implements Caesar’s encryption method.
  3. A fully implemented splice function.
  4. A function named subst_encrypt that takes two parameters, the message to be encrypted and the key, and performs substitution encryption.
  5. A function named subst_decrypt that takes two parameters, the encrypted message and the key, and performs substitution decryption.

Creativity Suggestions

When you’re done

When you’re done you should have three different encryption/decryption methods and a method for generating random keys. Make sure that your program is properly commented:

In addition, make sure that you’ve used good coding style (including meaningful variable names, constants where relevant, vertical white space, removing “dead code” that doesn’t do anything, removing testing code, etc.).

Submit your program via Gradescope. Your program program file must be named You can submit multiple times, with only the most recent submission (before the due date) graded. Note that the tests performed by Gradescope are limited. Passing all of the visible tests does not guarantee that your submission correctly satisfies all of the requirements of the assignment.


Features Points
fools_encrypt 4
caesar_encrypt 4
splice 4
subst_encrypt 4
subst_decrypt 4
Code design and style 5
Creativity points 2
Total 27

FAQ Excerpts Click entry title for more information

How do I use keygen?

I have received several questions in office hours about how keygen is used. keygen is a helper function for generating valid keys for your subst_encrypt and subst_decrypt functions. You should not use keygen within the subst_encrypt and subst_decrypt functions. Instead, as shown in the lab examples, both of those functions take the key, generated previously by keygen, as a parameter. For example: