Encryption 🔐

Alice and Bob want to share messages privately but Eve the evesdropper wants to read them

TLDR; (too long didn’t read)

Encryption = lock & unlock with a key! 🔐

  • Encryption can be reversed - you can decrypt messages back to the original
  • You need a key to encrypt AND decrypt
  • Without the key, encrypted messages look like gibberish
  • This protects messages while they’re being sent (like emails or texts)
  • Different from hashing: encryption is two-way, hashing is one-way only

Background Information

Remember Alice and Bob from the book cipher? They want to send secret messages to each other, but Eve (the eavesdropper) is trying to read them!

Encryption is like putting your message in a locked box. Only someone with the right key can unlock it and read the message.

How does encryption work?

  1. Encrypt: Take your original message (called “plaintext”) and scramble it using a key
  2. Send: Send the scrambled message (called “ciphertext”) - even if Eve intercepts it, she can’t read it!
  3. Decrypt: The receiver uses the same key to unscramble the message back to plaintext

Why is this important?

Every day, encryption protects your data: - 🔒 HTTPS websites encrypt your browsing so hackers can’t see what you’re doing - 💬 Messaging apps encrypt your messages so only you and the recipient can read them - 💳 Online shopping encrypts your payment details so they can’t be stolen - 📧 Email can be encrypted to keep your private conversations safe

Encryption vs Hashing

Feature Encryption 🔐 Hashing 🔒
Can it be reversed? ✅ Yes (with the key) ❌ No (one-way only)
What’s it used for? Protecting data in transit Protecting stored passwords
Do you need a key? ✅ Yes (to encrypt AND decrypt) ❌ No (just hash it)
Example Encrypted email message Hashed password in database

Both are important for security, but they solve different problems!

Practical Activity: Caesar Cipher Encryption 🔐

Objective

Learn how encryption works by creating your own Caesar cipher in Python. Discover how keys work and why encryption can be reversed (unlike hashing).

You will need

  • A computer with Python installed
  • The Python code below
  • Paper and pen to record your findings

What is a Caesar Cipher?

The Caesar cipher is one of the oldest encryption methods! It was used by Julius Caesar over 2000 years ago. It works by “shifting” each letter in the alphabet by a certain number of positions.

Example with shift of 3: - A → D (A is the 1st letter, shift 3 → D is the 4th letter) - B → E - C → F - … - X → A (wraps around to the beginning) - Y → B - Z → C

The shift number is the key!

Step 1: Set up your encryption tool

Run this Python code to create your Caesar cipher encryption and decryption tool:

Code 
def caesar_encrypt(message, shift):
    """Encrypt a message using Caesar cipher with a given shift (key)"""
    encrypted = ""
    
    for char in message:
        if char.isalpha():  # Only shift letters, not spaces or punctuation
            # Determine if it's uppercase or lowercase
            if char.isupper():
                # Shift uppercase letters
                shifted = chr((ord(char) - ord('A') + shift) % 26 + ord('A'))
            else:
                # Shift lowercase letters
                shifted = chr((ord(char) - ord('a') + shift) % 26 + ord('a'))
            encrypted += shifted
        else:
            # Keep spaces, punctuation, and numbers as they are
            encrypted += char
    
    return encrypted


def caesar_decrypt(encrypted_message, shift):
    """Decrypt a message using Caesar cipher with a given shift (key)"""
    # Decrypting is just encrypting with the opposite shift!
    return caesar_encrypt(encrypted_message, -shift)


# Test it out!
original_message = "Hello Alice"
key = 3  # The shift/key

encrypted = caesar_encrypt(original_message, key)
decrypted = caesar_decrypt(encrypted, key)

print(f"Original message: {original_message}")
print(f"Key (shift):      {key}")
print(f"Encrypted:        {encrypted}")
print(f"Decrypted:        {decrypted}")
print(f"\n✅ Success! Encryption and decryption work!")
Original message: Hello Alice
Key (shift):      3
Encrypted:        Khoor Dolfh
Decrypted:        Hello Alice

✅ Success! Encryption and decryption work!

Step 2: Send secret messages!

Now you can encrypt messages and send them to a partner. Try encrypting different messages:

Code 
# Try encrypting these messages
messages = [
    "Meet me at the park",
    "The password is secret",
    "Happy birthday Alice"
]

key = 5  # You and your partner must agree on the same key!

print("ENCRYPTING MESSAGES")
print("=" * 80)

for message in messages:
    encrypted = caesar_encrypt(message, key)
    print(f"Original:  {message}")
    print(f"Encrypted: {encrypted}")
    print()
ENCRYPTING MESSAGES
================================================================================
Original:  Meet me at the park
Encrypted: Rjjy rj fy ymj ufwp

Original:  The password is secret
Encrypted: Ymj ufxxbtwi nx xjhwjy

Original:  Happy birthday Alice
Encrypted: Mfuud gnwymifd Fqnhj

Step 3: Decrypt messages from your partner

If your partner sends you an encrypted message, you can decrypt it using the same key:

Code 
# Your partner sends you this encrypted message
encrypted_from_partner = "Mjqqt rj fy ymj ufwp"  # What does this say?
shared_key = 5  # You both agreed on this key

decrypted = caesar_decrypt(encrypted_from_partner, shared_key)

print("DECRYPTING A MESSAGE")
print("=" * 80)
print(f"Encrypted message: {encrypted_from_partner}")
print(f"Key:                {shared_key}")
print(f"Decrypted message: {decrypted}")
print("\n✅ You can read the secret message!")
DECRYPTING A MESSAGE
================================================================================
Encrypted message: Mjqqt rj fy ymj ufwp
Key:                5
Decrypted message: Hello me at the park

✅ You can read the secret message!

Step 4: Investigate encryption properties

Now try to answer these investigation questions by experimenting:

Code 
# Investigation 1: What happens with different keys?
print("INVESTIGATION 1: Different keys = different encryption")
print("=" * 80)

message = "Secret message"
keys = [1, 3, 5, 13, 26]

for key in keys:
    encrypted = caesar_encrypt(message, key)
    print(f"Key {key:2}: {message} → {encrypted}")

print("\nNotice: Different keys produce completely different encrypted messages!")
INVESTIGATION 1: Different keys = different encryption
================================================================================
Key  1: Secret message → Tfdsfu nfttbhf
Key  3: Secret message → Vhfuhw phvvdjh
Key  5: Secret message → Xjhwjy rjxxflj
Key 13: Secret message → Frperg zrffntr
Key 26: Secret message → Secret message

Notice: Different keys produce completely different encrypted messages!
Code 
# Investigation 2: Can we reverse it?
print("\nINVESTIGATION 2: Encryption is reversible!")
print("=" * 80)

original = "Meet at three o clock"
key = 7

encrypted = caesar_encrypt(original, key)
decrypted = caesar_decrypt(encrypted, key)

print(f"Original:  {original}")
print(f"Encrypted: {encrypted}")
print(f"Decrypted: {decrypted}")
print(f"\n✅ Encryption CAN be reversed! This is different from hashing.")

INVESTIGATION 2: Encryption is reversible!
================================================================================
Original:  Meet at three o clock
Encrypted: Tlla ha aoyll v jsvjr
Decrypted: Meet at three o clock

✅ Encryption CAN be reversed! This is different from hashing.
Code 
# Investigation 3: What happens without the key?
print("\nINVESTIGATION 3: Can Eve read the message without the key?")
print("=" * 80)

original = "The meeting is cancelled"
correct_key = 4
wrong_key = 10  # Eve guesses the wrong key

encrypted = caesar_encrypt(original, correct_key)
wrong_decrypt = caesar_decrypt(encrypted, wrong_key)
correct_decrypt = caesar_decrypt(encrypted, correct_key)

print(f"Original message:     {original}")
print(f"Encrypted:            {encrypted}")
print(f"\nEve's wrong key ({wrong_key}): {wrong_decrypt} ❌ Gibberish!")
print(f"Bob's correct key ({correct_key}): {correct_decrypt} ✅ Correct!")
print("\nNotice: Without the correct key, the message is unreadable!")

INVESTIGATION 3: Can Eve read the message without the key?
================================================================================
Original message:     The meeting is cancelled
Encrypted:            Xli qiixmrk mw gergippih

Eve's wrong key (10): Nby gyyncha cm wuhwyffyx ❌ Gibberish!
Bob's correct key (4): The meeting is cancelled ✅ Correct!

Notice: Without the correct key, the message is unreadable!
Code 
# Investigation 4: Compare to hashing
print("\nINVESTIGATION 4: Encryption vs Hashing")
print("=" * 80)

import hashlib

message = "secret"
encryption_key = 5

# Encrypt it
encrypted = caesar_encrypt(message, encryption_key)
decrypted = caesar_decrypt(encrypted, encryption_key)

# Hash it
hashed = hashlib.sha256(message.encode()).hexdigest()

print(f"Original message: {message}")
print(f"\nENCRYPTION (reversible):")
print(f"  Encrypted: {encrypted}")
print(f"  Decrypted: {decrypted} ✅ Can get original back!")
print(f"\nHASHING (one-way only):")
print(f"  Hashed:    {hashed}")
print(f"  Reversed:  ❌ IMPOSSIBLE - can't get original back!")
print(f"\nKey difference: Encryption = two-way, Hashing = one-way")

INVESTIGATION 4: Encryption vs Hashing
================================================================================
Original message: secret

ENCRYPTION (reversible):
  Encrypted: xjhwjy
  Decrypted: secret ✅ Can get original back!

HASHING (one-way only):
  Hashed:    2bb80d537b1da3e38bd30361aa855686bde0eacd7162fef6a25fe97bf527a25b
  Reversed:  ❌ IMPOSSIBLE - can't get original back!

Key difference: Encryption = two-way, Hashing = one-way

Step 5: Create your own secret messages!

Now it’s your turn! Create encrypted messages and swap them with a partner.

Code 
# Create your own encrypted messages
print("CREATE YOUR OWN SECRET MESSAGES")
print("=" * 80)

# Choose a key (share it with your partner secretly!)
my_key = 8  # Change this to your chosen key

# Write your messages here
my_messages = [
    "Your message here",
    "Another secret message",
    "One more message"
]

print(f"Using key: {my_key}\n")
print("ENCRYPTED MESSAGES (send these to your partner):")
print("-" * 80)

for msg in my_messages:
    encrypted = caesar_encrypt(msg, my_key)
    print(f"{encrypted}")

print("\n" + "-" * 80)
print("Share the key and encrypted messages with your partner!")
print("They should be able to decrypt them using the same key.")
CREATE YOUR OWN SECRET MESSAGES
================================================================================
Using key: 8

ENCRYPTED MESSAGES (send these to your partner):
--------------------------------------------------------------------------------
Gwcz umaaiom pmzm
Ivwbpmz amkzmb umaaiom
Wvm uwzm umaaiom

--------------------------------------------------------------------------------
Share the key and encrypted messages with your partner!
They should be able to decrypt them using the same key.

Discussion Questions

After completing the investigations, discuss these questions:

  1. Two-way process: Why can encryption be reversed, but hashing cannot?

  2. The key: Why is the key so important? What happens if someone intercepts your encrypted message but doesn’t have the key?

  3. Key security: If Eve steals your encrypted message, can she figure out the key by trying different numbers? (This is called a “brute force attack”)

  4. Real world: When would you use encryption instead of hashing? When would you use hashing instead of encryption?

  5. Caesar cipher weakness: The Caesar cipher is easy to break. Can you think of why? How do modern encryption methods protect against this?

  6. Alice and Bob: How does encryption help Alice and Bob communicate privately, even if Eve is listening?

Key Takeaways

✅ Encryption is two-way - You can encrypt AND decrypt messages

✅ Keys are essential - You need the same key to encrypt and decrypt

✅ Protects data in transit - Encryption keeps messages safe while being sent

✅ Different from hashing - Encryption is reversible, hashing is one-way only

✅ Both are important - Encryption and hashing solve different security problems

✅ Without the key = gibberish - Encrypted messages are useless without the correct key

Optional Stretch Activities 🌟

Challenge 1: Brute force attack

Try to break an encrypted message by testing all possible keys (0-25). How long does it take? Why is this a weakness of the Caesar cipher?

Challenge 2: Frequency analysis

Research “frequency analysis” - a method used to break Caesar ciphers. The letter ‘E’ is the most common in English. Can you use this to crack a message?

Challenge 3: Create a better cipher

Design your own encryption method. Can you make it harder to break than the Caesar cipher?

Challenge 4: Encryption vs Hashing comparison

Create a table comparing encryption and hashing. Include: - When each is used - Whether they can be reversed - Whether they need a key - Real-world examples of each

Challenge 5: Research modern encryption

Look up “AES encryption” or “RSA encryption”. How are these different from the Caesar cipher? Why are they more secure?

Challenge 6: Key exchange problem

Alice and Bob need to share a key to encrypt messages. But how do they share the key securely if Eve is listening? Research “Diffie-Hellman key exchange” to find out!