K-5 ideas

Talk with students about junk mail vs real mail (e.g., bills to pay). What is their or their parents' strategy to separate junk from real mail? Write down good ideas on the board.

Ask for a volunteer student to act as "server". They will do some simple math.

Privately choose 2-3 students in the class to act as "attackers", and choose other 5 students to act as "legitimate clients". Others in the class can see who's sending messages but not if they are an attacker or a legitimate client.

Instruct each legitimate client to send folded notes that contain two-number addition problems, with 1-digit numbers, roughly one per minute

Instruct each attacker to send folded notes that contain two-number addition problems with 9-digit numbers as fast as they can

Server should calculate each result (without a calculator) as accurately as possible and send it back.

The rest of the class is engaged in passing notes between clients and the server without opening them

After a few minutes the notes should start piling up at the server. Ask them if they can come up with a strategy to weed out bad notes but keep answering good ones. Ask the class if they can tell who attackers are. Discuss possible strategies. Assume note handlers cannot open notes.

Reveal who the attackers were and privately select new attackers and legitimate clients. Ask the server to implement one defense strategy they came up with. Then run the scenario again. This time attackers are aware of the defense strategy and should try to circumvent it. Did they succeed? Discuss with the class what they did and how well it worked.