How Using the Random Key Works

How It Works: radio_send_images_caesar_key_unknown

This section only explains modifications that make it use a random key and send strings at a slower rate.

Using set key to (pick random (1 to 25)) sets the key equal to a randomly generated integer in the 1 through 25 range. This statement was placed inside the on start block so that it gets executed before the main loop starts sending repeated encrypted messages. That way, the project sets the key value once, and then uses it with every repetition of the forever loop. 

Each time you restart the micro:bit, there is a 1 in 25 chance you will get the same key. Conversely, there is a 24 out of 25 chance that the key will be different.

In the main loop, set packet to caesar(key, packet) makes it so that the main loop repeatedly uses the one key value that was randomly determined before the main loop started repeating.

Pause (6000) is set to 6 seconds to intentionally slow down the broadcast rate and gives the online editor enough time to take a 0.2 s break between each line printed. This will reduce the WebUSB tendency to make printing errors.

How It Works: radio_receive_images_caesar_brute_force

The brute force receiver uses a loop that decrypts and prints the decrypted packet’s result to the terminal with each of the 25 possible keys. The for key in (range start(-1) end (-26) step (-1)) loop repeats the indented-below statements with key set to -1, then -2, -3, and so on up through -25. Each time through the loop, set result to (call caesar (key, packet)) decrypts the packet for that repetition’s key value. Then, serial write line (join (“key: “)(key)(“, result: “) (result)) displays the key and decrypted string for each iteration of the loop. 

Lastly, pause (200) delays before repeating the loop to give the online editor’s WebUSB connection extra time to transfer information to the terminal (since it seems to need it at the time of this writing). After all 25 loop repetitions, serial new line displays an empty line before the forever loop repeats and another 25 keys and decrypted strings are displayed.