7. Radio shift cipher

7.1. Caesar cipher

The Caesar cipher, also known as Caesar’s cipher, the shift cipher, Caesar’s code, or Caesar shift, is one of the simplest and most widely known encryption techniques.
It is a type of substitution cipher in which each letter in the plaintext is replaced by a letter some fixed number of positions down the alphabet.
For example, with a right shift of 3, A would be replaced by D, B would become E, and so on.
Deciphering is done in reverse, with a left shift of 3.
../_images/shift3.png
Here’s an example of how it works:
Plaintext: THE QUICK BROWN FOX JUMPS OVER THE LAZY DOG
Ciphertext: WKH TXLFN EURZQ IRA MXPSV RYHU WKH ODCB GRJ

7.2. Break the code

The gif below has a caesar cipher shift of 1 place. Can you decode it?
../_images/caesar_shift_time_is_key.gif
This script uses radio communication for sending and receiving encrypted messages.
The script sets up a radio group, defines a list of secret phrases, and implements a Caesar shift cipher function for encryption.
When the A-button is pressed, it selects a random secret message, applies the Caesar cipher with a small random shift, sends the encrypted message via radio, and starts a timer.
If button B is pressed, it stops the timer and displays the elapsed time.
For the receiver, the script also checks for incoming radio messages and displays them on the microbit’s LED display.
from microbit import *
import radio
import random

# Turn on the radio
radio.on()
# Choose own group in pairs 0-255
radio.config(group=8)

# Caesar cipher letters
ALPHABET = "ABCDEFGHIJKLMNOPQRSTUVWXYZ"
# secrets to be chosen from
SECRETS = [
    "MEET AT DAWN",
    "THE BIRD HAS FLOWN",
    "TRUST NO ONE",
    "SEEK THE TRUTH",
    "SECURE THE ASSET",
    "AVOID DETECTION",
    "KEEP MOVING",
    "STAY HIDDEN",
    "WATCH YOUR BACK",
    "TIME IS KEY",
]
# Caesar cipher shift to be chosen from
SHIFTS = [-1, 1]


def caesar_cipher(message, shift):
    """
    Apply a Caesar cipher to a message.
    """
    cipher_text = ""
    for char in message:
        if char in ALPHABET:
            # Shift character
            index = (ALPHABET.index(char) + shift) % len(ALPHABET)
            cipher_text += ALPHABET[index]
        else:
            cipher_text += char
    return cipher_text


# Initialize timer
timer = 0

while True:
    # Check button presses to send a secret message
    if button_a.was_pressed():
        # Select a random shift
        shift = random.choice(SHIFTS)
        # Select a random secret message
        secret = random.choice(SECRETS)
        cipher_text = caesar_cipher(secret, shift)
        radio.send(cipher_text)
        # Display the secret message on the sender's microbit
        display.scroll(secret, delay=100)
        # Start the timer
        timer = running_time()
    elif button_b.was_pressed() and timer:
        # time must be not 0; so A-button must be pressed first
        # Stop the timer and display the elapsed time in seconds
        elapsed_time = int((running_time() - timer) / 1000)
        display.scroll(str(elapsed_time))
        timer = 0
    # Check for incoming messages
    incoming = radio.receive()
    if incoming:
        # Display the shifted secret message
        display.scroll(incoming, delay=100)

Exercises

  1. Challenge your partner to crack the code. Press A, read the message sent, and then press B when your partner has worked out the message correctly. Write down the time and reverse rolls to see who has the best time.

7.3. Code breaker: brute force

The term “brute force” in the context of decryption refers to the method of trying all possible keys until the correct one is found.
In the case of a Caesar cipher, which is used in the code below, the key is the shift value used to encrypt the original message.
Since there are only 25 possible shifts (for the 26 letters of the alphabet), it’s feasible to try all of them to decrypt the message.
This is exactly what the code does when it receives an encrypted message: it applies each possible shift and displays the result, effectively “brute forcing” the decryption.
Sender:
If button A is pressed, a random secret message is selected, encrypted using the Caesar cipher with a random shift, and then sent via the radio.
The original message is also scrolled on the microbit’s LED display.
If button B is pressed, the elapsed time since button A was pressed is calculated and displayed.
Receiver:
The program continuously checks for incoming radio messages.
If a message is received, it attempts to decrypt it by applying all possible shifts in the Caesar cipher and scrolling each attempt on the display.
If button B is pressed during this process, the decryption attempts stop and the current decrypted message is displayed.
from microbit import *
import radio
import random

# Turn on the radio
radio.on()
# Choose own group in pairs 0-255
radio.config(group=8)

# Caesar cipher letters
ALPHABET = "ABCDEFGHIJKLMNOPQRSTUVWXYZ"
# secrets to be chosen from
SECRETS = [
    "MEET AT DAWN",
    "THE BIRD HAS FLOWN",
    "TRUST NO ONE",
    "SEEK THE TRUTH",
    "SECURE THE ASSET",
    "AVOID DETECTION",
    "KEEP MOVING",
    "STAY HIDDEN",
    "WATCH YOUR BACK",
    "TIME IS KEY",
]
# Caesar cipher shift to be chosen from
SHIFTS = [-1, 1, -2, 2, -3, 3, -4, 4, -5, 5, -6, 6, -7, 7, -8, 8, -9, 9, -10, 10, -11, 11, -12, 12, -13, 13]


def caesar_cipher(message, shift):
    """
    Apply a Caesar cipher to a message.
    """
    cipher_text = ""
    for char in message:
        if char in ALPHABET:
            # Shift character
            index = (ALPHABET.index(char) + shift) % len(ALPHABET)
            cipher_text += ALPHABET[index]
        else:
            cipher_text += char
    return cipher_text


# Initialize timer
timer = 0

while True:
    # Check button presses to send a secret message
    if button_a.was_pressed():
        # Select a random shift
        shift = random.choice(SHIFTS)
        # Select a random secret message
        secret = random.choice(SECRETS)
        cipher_text = caesar_cipher(secret, shift)
        radio.send(cipher_text)
        # Display the secret message on the sender's microbit
        display.scroll(secret, delay=100)
        # Start the timer
        timer = running_time()
    elif button_b.was_pressed() and timer:
        # time must be not 0; so A-button must be pressed first
        # Stop the timer and display the elapsed time in seconds
        elapsed_time = int((running_time() - timer) / 1000)
        display.scroll(str(elapsed_time))
        timer = 0
    # Check for incoming messages
    # Brute force decode and display the message
    incoming = radio.receive()
    if incoming:
        message = incoming
        display.scroll(message, delay=100)
        # Brute force decode and display the message; stop loop with B-button
        for shift_i in SHIFTS:
            message = caesar_cipher(incoming, shift_i)
            display.scroll(message, delay=100)
            if button_b.was_pressed():
                break
            sleep(100)
        display.scroll(message, delay=100)

Exercises

  1. Challenge your partner to crack the code using the brute force decrypting. Press A, read the message sent, and then press B when your partner has worked out the message correctly. Write down the time and reverse rolls to see who has the best time.