About 90 minutes
Take the crash course
Ten short lessons for the big picture, with check questions.
Start lesson 1
The game · early access
Learn PKI by playing
Keystone is a pixel-art town you walk through with Ken and Rosa. Every building is one lesson, and in every level you do the real thing: encrypt, sign, verify, build a chain, renew in time.
Plays in your browser, on desktop and phone. No sign-up.
Early access
Available now
- Level 0The Royal ExamRoyal Exam HallPlay now
- Level 1The secret keyHarbour Post OfficePlay now
- Level 2The key-sharing problemCourier StablesPlay now
In developmentThe other 25 levels are in development and will be released soon.
Where it starts
The Royal Exam
The first thing you play. In the Royal Exam Hall on the town's plaza, Rosa hosts and Ken answers ten short questions drawn from every part of PKI. After every answer you see the right one and one plain sentence of why, so you leave knowing ten true things.
- 10 questionsThree options each, plus a practice round that is not scored.
- About 7 to 10 minutesNo timer, and nothing to sign up for.
- A rankApprentice, Squire, Knight or Hero, on a card you can share without spoilers.
- A map of what to learnEach question you missed lights up the building in town that teaches it.
Prefer reading? The crash course covers the same ground in ten short lessons, with check questions.
How a level works
Explain, show, then you do it
Every level teaches one idea in the same rhythm, with every line advanced by you and nothing on a timer while you learn.
The idea, in real words
A short story moment, then the real names of the parts: secret key, public key, certificate.
Watch it done
Ken or Rosa does the operation step by step, with every step labelled.
Do it yourself
The tempting wrong move is playable too, and you see its real consequence.
Prove it, then play
A short untimed quiz, then an optional arcade challenge with relaxed, normal and no-timer modes.
The town of Keystone
27 levels in eight districts
The districts ring the Royal Plaza in the order you learn. Rosa will be crowned in thirty days, and the kingdom's trust must run on maths, not wax seals. Levels 1 to 26 each match a handbook chapter you can read today.
Royal Plaza: where it starts
The Royal Exam Hall with its gold dome, in the middle of town.
- 0The Royal ExamRoyal Exam Hall: ten questions that show what you already knowPlay now
IHarbour Quarter: Secrets
Docks, the post office, the stables and the lighthouse.
- 1The secret keyHarbour Post OfficePlay now
- 2The key-sharing problemCourier StablesPlay now
- 3Two keys: the open padlockThe LocksmithIn development
- 4Agreeing a secret in publicThe LighthouseIn development
IIMarket Square: Proof
The scribes, the signet workshop and the town gate.
- 5FingerprintsScribes' HallIn development
- 6The signatureSignet WorkshopIn development
- 7The impostorThe Town GateIn development
IIIChancery Row: Certificates
The notary, the registry, the petition office and the surveyors.
- 8The certificateRoyal NotaryIn development
- 9Names and jobsName RegistryIn development
- 10Asking for a certificatePetition OfficeIn development
- 11Proving you own the nameSurveyors' OfficeIn development
IVCastle Approach: Trust and HTTPS
The registry of roots, the chain bridge, the crossroads inn and the barbican.
- 12Who do we trust?Registry of RootsIn development
- 13The chainChain BridgeIn development
- 14Two roads to the same rootCrossroads InnIn development
- 15The handshakeThe BarbicanIn development
VWatchtower Hill: When trust breaks
The watchtower, the public ledger hall and the old archive.
- 16Taking it backWatchtowerIn development
- 17Everyone can seePublic Ledger HallIn development
- 18Famous failuresOld ArchiveIn development
VIClockwork Works: Running PKI
The clocktower, the robot workshop, the royal vault and the chancery.
- 19Expiry dayClocktowerIn development
- 20Robots that renewRobot WorkshopIn development
- 21Guarding the crown jewelsRoyal VaultIn development
- 22A CA of your ownThe ChanceryIn development
VIIGuild Quarter: Beyond the browser
The guild hall, the printing press and the clockmaker's factory.
- 23Both sides show IDGuild HallIn development
- 24Signed and sealedPrinting PressIn development
- 25Machines with passportsClockmaker's FactoryIn development
VIIICastle Heights: The future
The observatory and the Throne Hall.
- 26The quantum stormObservatoryIn development
- 27The coronationThrone Hall: make your own key pair and request a real certificateIn development
Before you play
Keyboard, touch or gamepad
The game runs in your web browser, on a desktop or laptop and on a phone or tablet. There is nothing to install.
- KeyboardThe arrow keys or WASD walk, Space or Enter chooses, and Esc opens the menu.
- TouchHold an on-screen arrow to walk and tap the buttons. Phones work upright and sideways.
- GamepadA standard gamepad works too: the left stick walks and the buttons choose.
- Your progressSaved in this browser. Settings has larger text, up to 200%.
Watch the teaser
Learn the real thing, in the same order
Everything the game teaches, including the levels still in development, is already here in plain words.
One chapter per level
Read the handbook
From a single shared secret to post-quantum cryptography, in 26 chapters.
Start reading
In your browser
Try the tools
Decode a certificate, check a chain, match a hostname or paste your error.
Open the tools
Get notified about new levels
The kingdom is still being built. Send us an email and we will let you know when new levels are released.