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  • Beginner
  • English
  • About 75 min
  • 10 steps · 2 quizzes

Encryption Explained — From Secret Codes to the Lock in Your Browser

How do you keep a secret when your message passes through strangers' computers? Students turn a cipher wheel, break a code…

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10 steps · 2 quizzes · about 75 minutes · open any step

  1. Your message takes a long tripStart hereOn its way to a website, your message passes through computers you don't control — the café Wi-Fi, your internet provider, the exchanges between networks. Without encryption it travels like a postcard. Try it: send the postcard.InteractiveHands-on · 6 min
  2. The Caesar cipher: shift every letterAbout 2,000 years ago, Julius Caesar hid his letters by shifting each letter a few places along the alphabet. The number of places is the key. Try it: type a message, turn the wheel, then press Decrypt.InteractiveHands-on · 5 min
  3. Breaking itEve intercepted a Caesar message. She has two easy attacks. Attack 1 — try every key. There are only 25.InteractiveHands-on · 11 min
  4. Bigger keysModern ciphers such as AES mix the bits so thoroughly that no pattern leaks — the only attack left is guessing the key. A key is a row of bits, and each extra bit doubles the number of possible keys.InteractiveHands-on · 6 min
  5. Check: ciphers and keysQuiz · 4 questions
  6. The key problemWith one key to lock and unlock, Alice and Bob first have to share that key. But every path between them is public — Eve sees everything. Try it: try each idea for getting the key to Bob.InteractiveHands-on · 5 min
  7. Agreeing on a secret in publicTheir trick: two people can build the same secret while everything they send is public. Paint shows the idea: mixing colours is easy, un-mixing them is practically impossible. Try it: press Next step six times. Then pick other secret colours and run it again.InteractiveHands-on · 6 min
  8. Public and private keysA related idea gives everyone two keys. A public key is like an open padlock handed out to anyone: they can snap it shut on a box. Only the matching private key opens it. Try it: step through Alice sending Bob a message.InteractiveHands-on · 6 min
  9. The lock in your browserPut it together and you get HTTPS — the lock beside a web address. In a split second your browser checks the site's signed certificate, agrees on a shared key (the paint trick), then encrypts everything.InteractiveHands-on · 7 min
  10. Passwords: store a fingerprint, not the passwordA good website never keeps your password. It stores a hash: a fingerprint made by a one-way function. Same input, same hash; a tiny change, a totally different hash; and no way back. Try it: change one letter.InteractiveHands-on · 6 min

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