Encryption #

In this lab you will continue to practice functions and are introduced to encryption.

Syllabus Topics [SL] #

B2.1.3 Describe how programs use common exception handling techniques.
B2.5.1 Construct code to perform file-processing operations.
A2.4.4 Describe the process of encryption and digital certificates.

Key Vocabulary #

Word	Definition
Encryption	Converting plain text into a secure format, cipher text, that cannot be easily understood by unauthorized people.
Encryption Key	A string of characters or numbers used by an encryption algorithm to encode or decode data.
Symmetric Cryptography	The same key is used to encrypt and decrypt data
Asymmetric Cryptography	The public key is used to encrypt and the private key is used to decrypt data
Exception	An event that interrupts the execution of a program

[0] Set up #

💻 Go to your dpcs/unit01_cryptography folder.

cd ~/desktop/dpcs/unit01_cryptography/

💻 Clone your repo and go into the directory. Be sure to replace yourGithubUsername with your actual username.

git clone https://github.com/isf-dp-cs/lab_encryption_yourGithubUsername

cd lab_encryption_yourGithubUsername

💻 Enter the Poetry Shell to start the lab. As a reminder, we will run this command at the start of each lab, but only when we are inside a lab folder.

poetry shell

👾 💬 Exiting the poetry shell
When you want to exit the shell, you can type exit or ^D

[1] Symmetric Cryptography #

Symmetric cryptography is when the same key is used to encrypt and decrypt the data.

Atbash Cipher #

The atbash cipher is a simple cipher where each letter in the alphabet is reversed. A = Z, B = Y, and so forth.

💻 In atbash_cipher.py, construct the function atbash() to encrypt or decrypt a message that has been encrypted by the atbash cipher.

Caesar Cipher #

The caesar cipher is a type of symmetric cryptography used by ancient Romans. It takes a message, the plain text and transforms it by shifting each letter by a set value, the encryption key.

The plain text "beg" with the encryption key 3, becomes "ehj".

b shifts by 3, becoming e
e shifts by 3, becoming h
g shifts by 3, becoming j

How to use modulo ↕

The modulo operator (%). This operator takes two values, divides them, and returns the remainder of the division.

For example:
5/2 has a remainder of 1
5%2 = 1

Here are some more modulo examples:

print(5%2)
>> 1
print(3%3)
>> 0
print(6%2)
>> 0
print(9%2)
>> 1
print(3%4)
>> 3

💻 In caesar_cipher.py, construct the function decrypt_caesar_cipher() to decrypt a message that has been encrypted by a caesar cipher.

💻 Test your decryption function decrypt_caesar_cipher() on words and short phrases.

💻 Use your decrypt_caesar_cipher() function and file handling methods to decrypt the message in caesar_encrypted_text.txt. You should then create a new file with the decrypted text.

✅ Check your work by opening the created file and ensuring it makes sense as English text. Do you recogonize the text?

👾 💬
Incorporate exception handling into your program by ensuring it doesn’t crash if the file does not exist.
Be sure to include try, except, and finally.

[2] Deliverables #

⚡✨ Once you complete the lab, be sure to complete these two steps:
✏️ Go to your Syllabus Content Checklist in your Google Drive and update it accordingly.
💻 Push your work to Github
git status
git add -A
git status
git commit -m "describe your code here"
git push
remote

[3] Extension: Vigenere Cipher #

The Vigenere cipher is another substitution cipher. It takes a message, the plain text and transforms it by shifting each letter by a set value according to a repeating encryption key. Unlike the caesar cipher, the encryption key is a string.

For example, imagine that encryption key is 'be'.

b	e
1	5

'b' is number 1 in the alphabet and 'e' is number 5.

Therefore, we will shift our letters by 1 and 5, in an alternating pattern. For example the plain text "apple" with the encryption key "be", becomes "buqpf".

a->b	shift by 1
p->u	shift by 5
p->q	shift by 1
l->q	shift by 5
e->f	shift by 1

💻 In vigenere_cipher.py, construct the function decrypt_vigenere_cipher() to decrypt a message that has been encrypted by a vigenere cipher.

💻 Test your decryption function decrypt_vigenere_cipher() on words and short phrases

💻 Use your decrypt_vigenere_cipher() function and file handling methods to decrypt the message in "vigenere_encrypted_text.txt". The encryption key is the encryption key from the caesar_cipher problem written in English (e.g. 1 is one). You should then create a new file with the decrypted text.

✅ Check your work by opening the created file and ensuring it makes sense as English text. Do you recogonize the text?