ljm0/modules/e2ee/Worker.js

// Worker for E2EE/Insertable streams. Currently served as an inline blob.
const code = `
    // We use a ringbuffer of keys so we can change them and still decode packets that were
    // encrypted with an old key.
    // In the future when we dont rely on a globally shared key we will actually use it. For
    // now set the size to 1 which means there is only a single key. This causes some
    // glitches when changing the key but its ok.
    const keyRingSize = 1;

    // We use a 96 bit IV for AES GCM. This is signalled in plain together with the
    // packet. See https://developer.mozilla.org/en-US/docs/Web/API/AesGcmParams
    const ivLength = 12;

    // We copy the first bytes of the VP8 payload unencrypted.
    // For keyframes this is 10 bytes, for non-keyframes (delta) 3. See
    //   https://tools.ietf.org/html/rfc6386#section-9.1
    // This allows the bridge to continue detecting keyframes (only one byte needed in the JVB)
    // and is also a bit easier for the VP8 decoder (i.e. it generates funny garbage pictures
    // instead of being unable to decode).
    // This is a bit for show and we might want to reduce to 1 unconditionally in the final version.
    //
    // For audio (where frame.type is not set) we do not encrypt the opus TOC byte:
    //   https://tools.ietf.org/html/rfc6716#section-3.1
    const unencryptedBytes = {
        key: 10,
        delta: 3,
        undefined: 1 // frame.type is not set on audio
    };

    // Salt used in key derivation
    // FIXME: We currently use the MUC room name for this which has the same lifetime
    // as this worker. While not (pseudo)random as recommended in
    // https://developer.mozilla.org/en-US/docs/Web/API/Pbkdf2Params
    // this is easily available and the same for all participants.
    // We currently do not enforce a minimum length of 16 bytes either.
    let keySalt;

    // Raw keyBytes used to derive the key.
    let keyBytes;

    /**
     * Derives a AES-GCM key with 128 bits from the input using PBKDF2
     * @param {Uint8Array} keyBytes - Value to derive key from
     * @param {Uint8Array} salt - Salt used in key derivation
     */
    async function deriveKey(keyBytes, salt) {
        // https://developer.mozilla.org/en-US/docs/Web/API/SubtleCrypto/importKey
        const material = await crypto.subtle.importKey('raw', keyBytes,
            'PBKDF2', false, [ 'deriveBits', 'deriveKey' ]);

        // https://developer.mozilla.org/en-US/docs/Web/API/SubtleCrypto/deriveKey#PBKDF2
        return crypto.subtle.deriveKey({
            name: 'PBKDF2',
            salt,
            iterations: 100000,
            hash: 'SHA-256'
        }, material, {
            name: 'AES-GCM',
            length: 128
        }, false, [ 'encrypt', 'decrypt' ]);
    }


    /** Per-participant context holding the cryptographic keys and
     * encode/decode functions
     */
    class Context {
        /**
         * @param {string} id - local muc resourcepart
         */
        constructor(id) {
            // An array (ring) of keys that we use for sending and receiving.
            this._cryptoKeyRing = new Array(keyRingSize);

            // A pointer to the currently used key.
            this._currentKeyIndex = -1;

            // We keep track of how many frames we have sent per ssrc.
            // Starts with a random offset similar to the RTP sequence number.
            this._sendCounts = new Map();

            this._id = id;
        }

        /**
         * Sets a key and starts using it for encrypting.
         * @param {CryptoKey} key
         */
        setKey(key) {
            this._currentKeyIndex++;
            this._cryptoKeyRing[this._currentKeyIndex % this._cryptoKeyRing.length] = key;
        }

        /**
         * Construct the IV used for AES-GCM and sent (in plain) with the packet similar to
         * https://tools.ietf.org/html/rfc7714#section-8.1
         * It concatenates
         * - the 32 bit synchronization source (SSRC) given on the encoded frame,
         * - the 32 bit rtp timestamp given on the encoded frame,
         * - a send counter that is specific to the SSRC. Starts at a random number.
         * The send counter is essentially the pictureId but we currently have to implement this ourselves.
         * There is no XOR with a salt. Note that this IV leaks the SSRC to the receiver but since this is
         * randomly generated and SFUs may not rewrite this is considered acceptable.
         * The SSRC is used to allow demultiplexing multiple streams with the same key, as described in
         *   https://tools.ietf.org/html/rfc3711#section-4.1.1
         * The RTP timestamp is 32 bits and advances by the codec clock rate (90khz for video, 48khz for
         * opus audio) every second. For video it rolls over roughly every 13 hours.
         * The send counter will advance at the frame rate (30fps for video, 50fps for 20ms opus audio)
         * every second. It will take a long time to roll over.
         *
         * See also https://developer.mozilla.org/en-US/docs/Web/API/AesGcmParams
         */
        makeIV(synchronizationSource, timestamp) {
            const iv = new ArrayBuffer(ivLength);
            const ivView = new DataView(iv);

            // having to keep our own send count (similar to a picture id) is not ideal.
            if (!this._sendCounts.has(synchronizationSource)) {
                // Initialize with a random offset, similar to the RTP sequence number.
                this._sendCounts.set(synchronizationSource, Math.floor(Math.random() * 0xFFFF));
            }
            const sendCount = this._sendCounts.get(synchronizationSource);

            ivView.setUint32(0, synchronizationSource);
            ivView.setUint32(4, timestamp);
            ivView.setUint32(8, sendCount % 0xFFFF);

            this._sendCounts.set(synchronizationSource, sendCount + 1);

            return iv;
        }

        /**
         * Function that will be injected in a stream and will encrypt the given encoded frames.
         *
         * @param {RTCEncodedVideoFrame|RTCEncodedAudioFrame} encodedFrame - Encoded video frame.
         * @param {TransformStreamDefaultController} controller - TransportStreamController.
         *
         * The packet format is described below. One of the design goals was to not require
         * changes to the SFU which for video requires not encrypting the keyframe bit of VP8
         * as SFUs need to detect a keyframe (framemarking or the generic frame descriptor will
         * solve this eventually). This also "hides" that a client is using E2EE a bit.
         *
         * Note that this operates on the full frame, i.e. for VP8 the data described in
         *   https://tools.ietf.org/html/rfc6386#section-9.1
         *
         * The VP8 payload descriptor described in
         *   https://tools.ietf.org/html/rfc7741#section-4.2
         * is part of the RTP packet and not part of the frame and is not controllable by us.
         * This is fine as the SFU keeps having access to it for routing.
         *
         * The encrypted frame is formed as follows:
         * 1) Leave the first (10, 3, 1) bytes unencrypted, depending on the frame type and kind.
         * 2) Form the GCM IV for the frame as described above.
         * 3) Encrypt the rest of the frame using AES-GCM.
         * 4) Allocate space for the encrypted frame.
         * 5) Copy the unencrypted bytes to the start of the encrypted frame.
         * 6) Append the ciphertext to the encrypted frame.
         * 7) Append the IV.
         * 8) Append a single byte for the key identifier. TODO: we don't need all the bits.
         * 9) Enqueue the encrypted frame for sending.
         */
        encodeFunction(encodedFrame, controller) {
            const keyIndex = this._currentKeyIndex % this._cryptoKeyRing.length;

            if (this._cryptoKeyRing[keyIndex]) {
                const iv = this.makeIV(encodedFrame.synchronizationSource, encodedFrame.timestamp);

                return crypto.subtle.encrypt({
                    name: 'AES-GCM',
                    iv,
                    additionalData: new Uint8Array(encodedFrame.data, 0, unencryptedBytes[encodedFrame.type])
                }, this._cryptoKeyRing[keyIndex], new Uint8Array(encodedFrame.data,
                    unencryptedBytes[encodedFrame.type]))
                .then(cipherText => {
                    const newData = new ArrayBuffer(unencryptedBytes[encodedFrame.type] + cipherText.byteLength
                        + iv.byteLength + 1);
                    const newUint8 = new Uint8Array(newData);

                    newUint8.set(
                        new Uint8Array(encodedFrame.data, 0, unencryptedBytes[encodedFrame.type])); // copy first bytes.
                    newUint8.set(
                        new Uint8Array(cipherText), unencryptedBytes[encodedFrame.type]); // add ciphertext.
                    newUint8.set(
                        new Uint8Array(iv), unencryptedBytes[encodedFrame.type] + cipherText.byteLength); // append IV.
                    newUint8[unencryptedBytes[encodedFrame.type] + cipherText.byteLength + ivLength]
                        = keyIndex; // set key index.

                    encodedFrame.data = newData;

                    return controller.enqueue(encodedFrame);
                }, e => {
                    console.error(e);

                    // We are not enqueuing the frame here on purpose.
                });
            }

            /* NOTE WELL:
             * This will send unencrypted data (only protected by DTLS transport encryption) when no key is configured.
             * This is ok for demo purposes but should not be done once this becomes more relied upon.
             */
            controller.enqueue(encodedFrame);
        }

        /**
         * Function that will be injected in a stream and will decrypt the given encoded frames.
         *
         * @param {RTCEncodedVideoFrame|RTCEncodedAudioFrame} encodedFrame - Encoded video frame.
         * @param {TransformStreamDefaultController} controller - TransportStreamController.
         *
         * The decrypted frame is formed as follows:
         * 1) Extract the key index from the last byte of the encrypted frame.
         *    If there is no key associated with the key index, the frame is enqueued for decoding
         *    and these steps terminate.
         * 2) Determine the frame type in order to look up the number of unencrypted header bytes.
         * 2) Extract the 12-byte IV from its position near the end of the packet.
         *    Note: the IV is treated as opaque and not reconstructed from the input.
         * 3) Decrypt the encrypted frame content after the unencrypted bytes using AES-GCM.
         * 4) Allocate space for the decrypted frame.
         * 5) Copy the unencrypted bytes from the start of the encrypted frame.
         * 6) Append the plaintext to the decrypted frame.
         * 7) Enqueue the decrypted frame for decoding.
         */
        decodeFunction(encodedFrame, controller) {
            const data = new Uint8Array(encodedFrame.data);
            const keyIndex = data[encodedFrame.data.byteLength - 1];

            if (this._cryptoKeyRing[keyIndex]) {
                const iv = new Uint8Array(encodedFrame.data, encodedFrame.data.byteLength - ivLength - 1, ivLength);
                const cipherTextStart = unencryptedBytes[encodedFrame.type];
                const cipherTextLength = encodedFrame.data.byteLength - (unencryptedBytes[encodedFrame.type]
                    + ivLength + 1);

                return crypto.subtle.decrypt({
                    name: 'AES-GCM',
                    iv,
                    additionalData: new Uint8Array(encodedFrame.data, 0, unencryptedBytes[encodedFrame.type])
                }, this._cryptoKeyRing[keyIndex], new Uint8Array(encodedFrame.data, cipherTextStart, cipherTextLength))
                .then(plainText => {
                    const newData = new ArrayBuffer(unencryptedBytes[encodedFrame.type] + plainText.byteLength);
                    const newUint8 = new Uint8Array(newData);

                    newUint8.set(new Uint8Array(encodedFrame.data, 0, unencryptedBytes[encodedFrame.type]));
                    newUint8.set(new Uint8Array(plainText), unencryptedBytes[encodedFrame.type]);

                    encodedFrame.data = newData;

                    return controller.enqueue(encodedFrame);
                }, e => {
                    console.error(e);

                    // TODO: notify the application about error status.

                    // TODO: For video we need a better strategy since we do not want to based any
                    // non-error frames on a garbage keyframe.
                    if (encodedFrame.type === undefined) { // audio, replace with silence.
                        // audio, replace with silence.
                        const newData = new ArrayBuffer(3);
                        const newUint8 = new Uint8Array(newData);

                        newUint8.set([ 0xd8, 0xff, 0xfe ]); // opus silence frame.
                        encodedFrame.data = newData;
                        controller.enqueue(encodedFrame);
                    }
                });
            } else if (keyIndex >= this._cryptoKeyRing.length
                    && this._cryptoKeyRing[this._currentKeyIndex % this._cryptoKeyRing.length]) {
                // If we are encrypting but don't have a key for the remote drop the frame.
                // This is a heuristic since we don't know whether a packet is encrypted,
                // do not have a checksum and do not have signaling for whether a remote participant does
                // encrypt or not.
                return;
            }

            // TODO: this just passes through to the decoder. Is that ok? If we don't know the key yet
            // we might want to buffer a bit but it is still unclear how to do that (and for how long etc).
            controller.enqueue(encodedFrame);
        }
    }
    const contexts = new Map(); // Map participant id => context

    onmessage = async event => {
        const { operation } = event.data;

        if (operation === 'initialize') {
            keySalt = event.data.salt;
        } else if (operation === 'encode') {
            const { readableStream, writableStream, participantId } = event.data;

            if (!contexts.has(participantId)) {
                contexts.set(participantId, new Context(participantId));
            }
            const context = contexts.get(participantId);
            const transformStream = new TransformStream({
                transform: context.encodeFunction.bind(context)
            });

            readableStream
                .pipeThrough(transformStream)
                .pipeTo(writableStream);
            if (keyBytes) {
                context.setKey(await deriveKey(keyBytes, keySalt));
            }
        } else if (operation === 'decode') {
            const { readableStream, writableStream, participantId } = event.data;

            if (!contexts.has(participantId)) {
                contexts.set(participantId, new Context(participantId));
            }
            const context = contexts.get(participantId);
            const transformStream = new TransformStream({
                transform: context.decodeFunction.bind(context)
            });

            readableStream
                .pipeThrough(transformStream)
                .pipeTo(writableStream);
            if (keyBytes) {
                context.setKey(await deriveKey(keyBytes, keySalt));
            }
        } else if (operation === 'setKey') {
            keyBytes = event.data.key;
            contexts.forEach(async context => {
                if (keyBytes) {
                    context.setKey(await deriveKey(keyBytes, keySalt));
                } else {
                    context.setKey(false);
                }
            });
        } else {
            console.error('e2ee worker', operation);
        }
    };
`;

export const createWorkerScript = () => URL.createObjectURL(new Blob([ code ], { type: 'application/javascript' }));