CS 150 - Assignment 3 - Encryption
Due: Wednesday 3/8 at the beginning of class
For this assignment, we're going to be playing 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 play 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
In cryptography, the two key functions are encryption, where we take a
message and encrypt it, and decryption, where we take an encrypted
message and recover the original message. In this assignment, we'll
play with a few different ways of doing this.
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 code 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 in a new folder for this assignment. Then open Spyder and
open the saved file. This file does not have any syntax errors,
however, until you fill in some of the details not all of the
functions will run.
2 Warming up
To get you warmed up we're going to try a very simple method for
encryption. You have a friend who says she has 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
Write a function called fools_encrypt that takes the message
as a parameter and returns the message encrypted using this scheme.
(Hint: remember the '*' operation for strings.)
If you want a challenge (not required), try to write a function
called fools_decrypt that decrypts this. For example, you
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:
(really, though, give it a try before you click on this!).
>>> fools_decrypt("ssseeecccrrreeettt mmmeeessssssaaagggeee")
>>> secret = fools_encrypt("secret message")
3 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
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 number offset for the encryption. You
will very likely want to utilize the shift_letter
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... I really mean it,
think about it...
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
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
4 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 a 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):
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.
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
Before we can actually encrypt anything, we first need to generate a
key. We could do this by hand, but it's a pain. In the starter code,
I've included a function called keygen. 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
(Note: due to various computer specific factors you may not get the
exact same keys as I have included here, however, if you call
keygen twice with the same string, you should get the same
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
For a general substitution cipher we also need to write a decryption
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:
(As before, note that keygen might work differently on your machine, so
your encrypted text might look different.)
>>> key = keygen("mypassword")
>>> encrypted = subst_encrypt("this is a secret message", key)
>>> subst_decrypt(encrypted, key)
'this is a secret message'
Decrypt the following message using the password "i like cs"
(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 with this key then to make sure it works).
'ao us jauaoiwpuxsrtauxusrrf xwuiju g wuiduqrtuxh uyhrwpuqrtuxh urwfqurddusquxusia'
Again, you don't have to write it down, but it's a good check to make
sure you've got things working.
5 Extra points
If you do extra points, please include a comment at the top stating
what you did.
- [1 point] Caesar's method can actually be written using our
code for the general substitution cypher. Write a function called
shift that takes as input a number. The function should
return a key that is the inorder alphabet shifted up by the number
input. For example:
If you then use this key with your subst_encrypt function you
should get the same answer as if you'd used your
caesar_encrypt method with the number offset
- [1 point] Write a pair of functions my_encrypt
and my_decrypt that implement your own encryption algorithm.
You may rely on whatever input you would like, but make it clear in
the documentation how they should be run. Points will be awarded
based on difficulty and creativity.
6 When you're done
When you're all 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 style.
- You should have comments at the very beginning of the file
stating your name, course (including section number), assignment
number and the date.
- Each function should have an appropriate docstring
- Other miscellaneous comments to make things clear
What to hand in:
You should have implemented:
Submit your .py file online using the digital submission link on the
course web page. You must have submitted it online before the
beginning of class on Wednesday.
| || points |
| fools_encrypt || 3|
|caesar_encrypt || 3|
|splice || 3|
|subst_encrypt || 3|
|subst_decrypt || 3|
|Prelab || 3|
|Comments, style || 5|
|Extra points || 2|
|Total || 23 + 2|