Radio is a shared space, so how do you keep a message private, know it was not changed, and know who sent it? Build ciphers, a checksum and a challenge–response lock with micro:bits, then defend a radio door lock in a capture-the-flag finale.
★ Challenge completeRadio reaches everyone in the group, so a plain message is a public one.
A radio group is a channel, not a lock. Every micro:bit in the group hears every message, so anything sent in plain text is public to the group.
A scrambling method works only while nobody knows the method. A real code has a public method and a secret key.
Build a Caesar cipher, see why it is easy to break, then lock messages with a shared key.
Shift every letter along the alphabet by a secret number. The method is public; the number is the key.
Start lessonA Caesar cipher has only 25 useful keys, so trying them all breaks it in moments. Seeing why is how you learn what a strong key needs.
XOR mixes a number with a key. Doing it again with the same key brings the number back, so one shared key works in both directions.
Share a key without sending it, spot a changed message with a checksum, and prove who you are with a challenge that changes every time.
Both ends need the same key, but anything sent by radio is public. Each end mixes a private number with public numbers, swaps the mixes, and both arrive at the same key without it ever being sent.
A checksum is a small number worked out from a message and sent with it. If the number the receiver works out is different, the message was damaged or changed on the way.
A password sent again and again is the same every time. A challenge that changes each time makes each answer good for one use only.
Solve capture-the-flag puzzles, then find the weaknesses in a radio door lock and fix each one.
Solve three puzzles that use the ciphers and checks you have built. Each answer is a flag you type in to prove you solved it.
A radio door lock has three weaknesses. Find each one, name the tool from this course that fixes it, and build the fixes in milestones.
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