SECCON Beginners CTF (ctf4b) 作問者writeup

const keyUrl = "/enc.key";
class CustomLoader extends Hls.DefaultConfig.loader {
    constructor(config) {
        super(config);
        this.context = { url: keyUrl };
        const load = this.load.bind(this);
        this.load = function (context, config, callbacks) {
            if (context.type === "manifest") {
                const onSuccess = callbacks.onSuccess;
                callbacks.onSuccess = function (response, stats, context) {
                    response.data = response.data.replace(
                        /#EXT-X-KEY:METHOD=.*,URI=".*"/,
                        `#EXT-X-KEY:METHOD=AES-128,URI="${keyUrl}"`
                    );
                    onSuccess(response, stats, context);
                };
            } else {
                if (context.url.endsWith(keyUrl)) {
                    window.gContext = context
                    hlscotext.load(context);
                    context = window.gContext;
                }
                window.globalContext = null;
            }
            return load(context, config, callbacks);
        };
    }
}

function mediaPlayer() {
    const video = document.getElementById("video");
    if (!video) {
        return;
    }
    if (typeof Hls !== "undefined" && Hls.isSupported()) {
        const hls = new Hls({ loader: CustomLoader });
        const streamUrl = "/public/videos/video.m3u8";
        hls.loadSource(streamUrl);
        hls.on(Hls.Events.MANIFEST_PARSED, () => {
            hls.attachMedia(video);
            video.addEventListener("canplay", () => {
                console.info("The video can play!");
            });
        });
    } else {
        alert("sorry, your browser does not support.");
    }
}

const initWasm = async () => {
    console.log("wasm loading: start!");
    try {
        const go = new Go();
        const response = await fetch("/main.wasm");
        const buffer = await response.arrayBuffer();
        const result = await WebAssembly.instantiate(buffer, go.importObject);
        go.run(result.instance);
        console.log("wasm loading: finished!");
    } catch (e) {
        alert("sorry, your browser does not support wasm.");
    }
};
initWasm().then(() => {
    mediaPlayer();
});