June 24, 2024

Mental Poker Part 8: Rock-Paper-Scissors

For an overview on Mental Poker, see Mental Poker Part 0: An Overview. Other articles in this series: https://vladris.com/writings/index.html#mental-poker. In the previous post in the series we looked at some low-level building blocks. It this post, we’ll finally see how to implement a game end-to-end using the toolkit. We’ll start with a simple game: rock-paper-scissors.


We’ll build this game as a React app, using the toolkit. We’ll be using Redux for state management - Redux provides a good way of binding game state to the UI, which works well with our toolkit.

The full code for this is in the demos/rock-paper-scissors app.


Since we got a lot of the primitives out of the way in the previous post (Fluid connection, getting a SignedTransport etc.), in this post we can focus on the higher level semantics of modeling the game.

We’ll play a round of rock-paper-scissors as follows:

This 2-step protects against cheating: before the game proceeds, both players need to make a selection. But the other player doesn’t know what the selection is until the decryption key is provided. Note for this particular game, turn order doesn’t matter.

We’ll start with a few type definitions:

type PlaySelection = "Rock" | "Paper" | "Scissors";

type EncryptedSelection = string;

PlaySelection represents the possible plays, EncryptedSelection is the string representation of an encrypted PlaySelection.

Our game model will have 2 actions:

type PlayAction = {
    clientId: ClientId;
    type: "PlayAction";
    encryptedSelection: EncryptedSelection;

type RevealAction = {
    clientId: ClientId;
    type: "RevealAction";
    key: SerializedSRAKeyPair;

type Action = PlayAction | RevealAction;

PlayAction is the first step, when players post their encrypted choice. RevealAction is the second step, revealing the encryption key. We’ll use the SRA algorithm for encryption since we have it in our toolkit, but for this game any encryption algorithm would work.

We’ll also need a couple more type definitions for the game state:

type GameStatus = "Waiting" | "Ready" | "Win" | "Loss" | "Draw";

type PlayValue =
    | { type: "Selection"; value: PlaySelection }
    | { type: "Encrypted"; value: EncryptedSelection }
    | { type: "None"; value: undefined };

The GameStatus represents the different states the client can be in:

The PlayValue represents the current state of a player’s pick. It can be either an encrypted selection, a revealed selection, or nothing (at the start of the game).

Before implementing the game state machine, let’s look at the Redux store.


I won’t go into the details of Redux in this post - please refer to the Redux documentation for that. We’ll be using the Redux Toolkit to streamline setting up our store.

We will maintain 6 pieces of state:

We’ll use the Redux Toolkit createAction helper to define the update functions for these:

const updateId = createAction<string>("id/update");
const updateOtherPlayer = createAction<string>("otherPlayer/update");
const updateQueue = createAction<IQueue<Action>>("queue/update");
const updateGameStatus = createAction<GameStatus>("gameStatus/update");
const updateMyPlay = createAction<PlayValue>("myPlay/update");
const updateTheirPlay = createAction<PlayValue>("theirPlay/update");

We’ll also need reducers (another Redux concept) for updating the values. We can implement a helper function to create these:

function makeUpdateReducer<T>(
    initialValue: T,
    updateAction: ReturnType<typeof createAction>
) {
    return createReducer({ value: initialValue }, (builder) => {
        builder.addCase(updateAction, (state, action) => {
            state.value = action.payload;

Finally, we set up our Redux store as:

const store = configureStore({
    reducer: {
        id: makeUpdateReducer("", updateId),
        otherPlayer: makeUpdateReducer("Not joined", updateOtherPlayer),
        queue: makeUpdateReducer<IQueue<Action> | undefined>(
        myPlay: makeUpdateReducer<PlayValue>(
            { type: "None", value: undefined },
        theirPlay: makeUpdateReducer<PlayValue>(
            { type: "None", value: undefined },
        gameStatus: makeUpdateReducer("Waiting", updateGameStatus),
    middleware: (getDefaultMiddleware) =>
            serializableCheck: false,

We initialize the store with the default values:

That’s about it for Redux setup - again, I won’t cover what reducers are, how Redux manages state changes etc.

Playing a round

We’ll implement playing a round of rock-paper-scissors in the function async function playRound(selection: PlaySelection). We invoke this with our selection (rock, paper, or scissors).

First, we need to get a few references:

const context = store;

await context.dispatch(updateGameStatus("Waiting"));

const queue = context.getState().queue.value!;

const kp = SRA.genereateKeyPair(BigIntUtils.randPrime());

First, we get a reference to the Redux store. Then we update the game status to Waiting. We get a reference to the async queue from the Redux store and, finally, we generate an SRA key pair. The generateKeyPair() and randPrime() functions we discussed all the way in part 1, when we covered cryptography. The dispatch() and getState() are standard Redux calls.

Now let’s look at the state machine modeling a round. It consists of the following sequence:

  1. Post our encrypted selection.
  2. Expect to receive 2 encrypted selections (ours and the opponent’s).
  3. Post our encryption key to reveal our selection.
  4. Expect to receive 2 encryption keys (ours and the opponent’s).

We can run this state machine with the Redux store as context:

await sm.run(sm.sequence([
        sm.local(async (queue) => {
            const playAction = {
                clientId: context.getState().id.value,
                type: "PlayAction",
                encryptedSelection: SRA.encryptString(selection, kp),

            await queue.enqueue(playAction);
        sm.repeat(sm.transition(async (play: PlayAction, context: RootStore) => {
            const action =
            play.clientId === context.getState().id.value
                ? updateMyPlay
                : updateTheirPlay;

            await context.dispatch(
                action({ type: "Encrypted", value: play.encryptedSelection })
        }), 2),
        sm.local(async (queue) => {
            const revealAction = {
                clientId: context.getState().id.value,
                type: "RevealAction",
                key: SRAKeySerializationHelper.serializeSRAKeyPair(kp),
            await queue.enqueue(revealAction);
        sm.repeat(sm.transition(async (reveal: RevealAction, context: RootStore) => {
            const action =
                reveal.clientId === context.getState().id.value
                    ? updateMyPlay
                    : updateTheirPlay;
            const originalValue =
                reveal.clientId === context.getState().id.value
                    ? context.getState().myPlay.value
                    : context.getState().theirPlay.value;

            await context.dispatch(
                    type: "Selection",
                    value: SRA.decryptString(
                        originalValue.value as EncryptedSelection,
                    ) as PlaySelection,
        }), 2)
    ]), queue, context);

We first define a local transition - we enqueue our PlayAction.

We then repeat 2 times a transition. We update the Redux store accordingly: if the received client ID is ours, we call updateMyPlay(), otherwise we call updateTheirPlay() with the encrypted value.

Next, we enqueue our RevealAction.

We then again repeat 2 times a transition. If the incoming client ID is ours, we call updateMyPlay() and decrypt the originalValue (myPlay.value) with the received key, otherwise we call updateTheirPlay() and decrypt the originalValue (theirPlay.value) with the received key.

Note how this code updates the Redux store directly, by using it as the context for the state machine.

Once the state machine finishes, we should have both our play and the opponent’s play, so we can determine the winner and update the game state accordingly:

const myPlay = context.getState().myPlay.value;
const theirPlay = context.getState().theirPlay.value;

if (myPlay.value === theirPlay.value) {
    await context.dispatch(updateGameStatus("Draw"));
} else if (
    (myPlay.value === "Rock" && theirPlay.value === "Scissors") ||
    (myPlay.value === "Paper" && theirPlay.value === "Rock") ||
    (myPlay.value === "Scissors" && theirPlay.value === "Paper")
) {
    await context.dispatch(updateGameStatus("Win"));
} else {
    await context.dispatch(updateGameStatus("Loss"));

And that’s it in terms of game mechanics. Finally, let’s look at a simple UI for the game.


We’ll build the UI using React. First, let’s create a component that provides the rock-paper-scissors options as 3 buttons:

type ButtonsViewProps = {
    disabled: boolean;
    onPlay: (play: PlaySelection) => void;

const ButtonsView = ({ disabled, onPlay }: ButtonsViewProps) => {
    return <div>
        <button disabled={disabled} onClick={() => onPlay("Rock")} style={{ width: 200}}>🪨</button>
        <button disabled={disabled} onClick={() => onPlay("Paper")} style={{ width: 200 }}>📄</button>
        <button disabled={disabled} onClick={() => onPlay("Scissors")} style={{ width: 200 }}>✂️</button>

Our properties are a boolean that determines whether buttons should be enabled or disabled and an onPlay() callback.

Our view is also very simple:

const useSelector: TypedUseSelectorHook<RootState> = useReduxSelector;

const MainView = () => {
    const idSelector = useSelector((state) => state.id);
    const otherPlayer = useSelector((state) => state.otherPlayer);
    const gameStateSelector = useSelector((state) => state.gameStatus);

    return <div>
        <p>Id: {idSelector.value}</p>
        <p>Other player: {otherPlayer.value}</p>
        <p>Status: {gameStateSelector.value}</p>
        <ButtonsView disabled={gameStateSelector.value === "Waiting"} onPlay={playRound}></ButtonsView>

The first line is some React-Redux plumbing (via the react-redux package), which allows us to grab data from the Redux store and put it in the UI.

We’ll be showing our ID, the other player’s ID, the game status, and the 3 buttons. The buttons are enabled as long as the game state is no Waiting. Once the user clicks a button, we simply call the playRound() function we looked at in the previous section.

Rendering all of this on the page:

const root = ReactDOM.createRoot(document.getElementById("root")!);
    <Provider store={store}>
        <MainView />

Here, Provider comes from the react-redux package and makes the Redux store available to the React components.


We now have all the pieces into place, the only bit of code we haven’t covered is initializing the game:

getLedger<Action>().then(async (ledger) => {
    const id = randomClientId();

    await store.dispatch(updateId(id));

    const queue = await upgradeTransport(2, id, ledger);
    await store.dispatch(updateQueue(queue));

    for (const action of ledger.getActions()) {
        if (action.clientId !== id) {

    await store.dispatch(updateGameStatus("Ready"));

The steps are:

The steps are pretty self-explanatory, maybe except getting the other player’s ID. The way that works is as follows: getActions() returns all actions posted on the ledger so far. We look for an action where the client ID is different than our client ID and store that as the other player’s ID. We are guaranteed to see at least one action from the other player, as we ran upgradeTransport(), which under the hood performs a public key exchange.

And that’s it - we have an end-to-end game of rock-paper-scissors.


We looked at implementing rock-paper-scissors using the Mental Poker toolkit. The full source code for the demo is under demos/rock-paper-scissors.

Note how easy it is to model a game if we rely on the toolkit’s primitives. We implement the game logic in the model, relying on the toolkit’s capabilities. We use Redux to store game state, which we can easily bind to a React view. That said, this was a very simple game. In the next post we’ll look at implementing a card game.