Crypto Lab

An interactive course in twelve lessons

How twenty digits switch your lights on

You pay with mobile money, twenty digits come back by SMS, and you type them into the keypad by your door. The meter on the pole outside decides they are genuine — with no internet connection, no SIM card and no call to the utility.

Crypto Lab explains how that works, from the ground up: ciphers, keys, hashes and signatures first, then the Yaka meter itself. Every lesson is a 3D scene you can turn over and a set of controls you can change.

Start with lesson 1 Go straight to the meter

Free, and it runs in your browser. Nothing to install, no account.

Four things to try on the keypad

  1. Load credit with the token on the receipt. How a token is built
  2. Type the same token again. Why the meter refuses it
  3. Change one digit before you press ↵. Why one digit ruins it
  4. Try your neighbour’s token on your meter. One key per meter

Your house

Lights off — no credit

CIU
NO CREDITRELAY OPEN ENTER TOKEN 0.0 kWh

Meter 0412 2334 455

The keypad by your door only forwards digits. The meter on the pole does the checking. You can also type on your keyboard.

Token receipt

Meter — yours

—

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Inside the meter on the pole

  1. Twenty digits?
  2. Decrypt with this meter’s key
  3. Checksum matches?
  4. Newer than the last token?
  5. Add credit, close the relay

Waiting for a token.

The course

Twelve lessons in three acts

Each lesson builds on the one before, but every one opens on its own — start at the beginning, or jump to whatever you are curious about.

Act I

Foundations

What a key does, how a block cipher scrambles everything at once, and how a hash fingerprints data.

  1. 01What encryption actually isReadable text goes in, unreadable text comes out — and a secret decides how.
  2. 02One shared keySymmetric encryption: both sides hold the same secret — its strength and its problem.
  3. 03Inside a block cipherRounds of substitution and diffusion, until one changed bit changes half the output.
  4. 04Hashing: the one-way doorAny amount of data in, a fixed fingerprint out — and no way back.

Act II

Two keys

Public keys and signatures — and the trick the meter actually relies on: deriving a key instead of sending it.

  1. 05Two keys instead of onePublish one half of the pair, and the distribution problem disappears.
  2. 06Digital signaturesSign the hash with the private key — anyone can prove who sent it.
  3. 07Deriving a key instead of sending oneTwo machines end up with the same secret without it crossing a wire.

Act III

The Yaka meter

The meter on the pole and the keypad by the door: how a token is built, checked and refused, and the 2024 deadline every meter in the world had to meet.

  1. 08Inside a Yaka installationWhy the meter is split in two, and why the half that matters is out of reach.
  2. 09Building a tokenFrom shillings to twenty digits, one field at a time.
  3. 10The meter decidesFive checks, no network — and the one that stops you reusing yesterday’s token.
  4. 11Re-keying, rollover and tamperChanging a key through the keypad, and the day the TID counter ran out.
  5. 12The whole thing, runningMobile money to light bulb, with every step visible.

How it works

Every lesson is something to do, not just something to read

1Look
A 3D scene of the idea — a cipher engine, a token, the meter on its pole. Drag to turn it, scroll to zoom.
2Read
The Learn tab explains what you are looking at in plain language. No maths background needed.
3Change
The Controls tab lets you type a message, flip a bit or enter a token, and watch the scene respond.
4Inspect
The Data tab shows the raw numbers underneath: keys, blocks, digests, a token’s fields.

Nothing is faked. Every cipher, hash, signature and token on this site — including the meter above — is computed live by code running in your browser. It is an educational model: the meter follows the structure of the STS standard (IEC 62055-41), but the licensed STS encryption algorithm is replaced by a teaching cipher.

Start with lesson 1