e2ee: move to a worker (#1112)

Moveѕ e2ee operations to a worker that is included as text/blob for now
to simplify deployment.

doc/e2ee.md

 This allows the decoder to understand the frame a bit more and makes it generate the fun looking garbage we see in the video.
 This also means the SFU does not know (ideally) that the content is end-to-end encrypted and there are no changes in the SFU required at all.
 
-Decryption errors are currently handled by inserting silence or a black frame.
+## Using workers
+
+Insertable Streams are transferable and can be sent from the main javascript context to a Worker
+  https://developer.mozilla.org/en-US/docs/Web/API/Worker
+We are using a named worker (E2EEworker) which allows very easy inspection in Chrome Devtools.
+It also makes the keys very self-contained.

modules/e2ee/E2EEContext.js

-/* global __filename, TransformStream */
+/* global __filename */
 
+import { e2eeWorkerScript } from './Worker';
 import { getLogger } from 'jitsi-meet-logger';
 
 const logger = getLogger(__filename);
 
-// 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
-};
-
 // Flag to set on senders / receivers to avoid setting up the encryption transform
 // more than once.
 const kJitsiE2EE = Symbol('kJitsiE2EE');
     constructor(options) {
         this._options = options;
 
-        // 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();
+        // Initialize the E2EE worker.
+        this._worker = new Worker(e2eeWorkerScript, {
+            name: 'E2EE Worker'
+        });
+        this._worker.onerror = e => logger.onerror(e);
 
         // Initialize the salt and convert it once.
         const encoder = new TextEncoder();
 
-        this._salt = encoder.encode(options.salt);
+        // Send initial options to worker.
+        this._worker.postMessage({
+            operation: 'initialize',
+            salt: encoder.encode(options.salt)
+        });
     }
 
     /**
         if (receiver[kJitsiE2EE]) {
             return;
         }
+        receiver[kJitsiE2EE] = true;
 
         const receiverStreams
             = kind === 'video' ? receiver.createEncodedVideoStreams() : receiver.createEncodedAudioStreams();
-        const transform = new TransformStream({
-            transform: this._decodeFunction.bind(this)
-        });
-
-        receiverStreams.readableStream
-            .pipeThrough(transform)
-            .pipeTo(receiverStreams.writableStream);
 
-        receiver[kJitsiE2EE] = true;
+        this._worker.postMessage({
+            operation: 'decode',
+            readableStream: receiverStreams.readableStream,
+            writableStream: receiverStreams.writableStream
+        }, [ receiverStreams.readableStream, receiverStreams.writableStream ]);
     }
 
     /**
         if (sender[kJitsiE2EE]) {
             return;
         }
+        sender[kJitsiE2EE] = true;
 
         const senderStreams
             = kind === 'video' ? sender.createEncodedVideoStreams() : sender.createEncodedAudioStreams();
-        const transform = new TransformStream({
-            transform: this._encodeFunction.bind(this)
-        });
-
-        senderStreams.readableStream
-            .pipeThrough(transform)
-            .pipeTo(senderStreams.writableStream);
 
-        sender[kJitsiE2EE] = true;
+        this._worker.postMessage({
+            operation: 'encode',
+            readableStream: senderStreams.readableStream,
+            writableStream: senderStreams.writableStream
+        }, [ senderStreams.readableStream, senderStreams.writableStream ]);
     }
 
     /**
      *
      * @param {string} value - Value to be used as the new key. May be falsy to disable end-to-end encryption.
      */
-    async setKey(value) {
+    setKey(value) {
         let key;
 
         if (value) {
             const encoder = new TextEncoder();
 
-            key = await this._deriveKey(encoder.encode(value));
+            key = encoder.encode(value);
         } else {
             key = false;
         }
-        this._currentKeyIndex++;
-        this._cryptoKeyRing[this._currentKeyIndex % this._cryptoKeyRing.length] = key;
-    }
-
-    /**
-     * Derives a AES-GCM key with 128 bits from the input using PBKDF2
-     * The salt is configured in the constructor of this class.
-     * @param {Uint8Array} keyBytes - Value to derive key from
-     */
-    async _deriveKey(keyBytes) {
-        // 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: this._salt,
-            iterations: 100000,
-            hash: 'SHA-256'
-        }, material, {
-            name: 'AES-GCM',
-            length: 128
-        }, false, [ 'encrypt', 'decrypt' ]);
-    }
-
-    /**
-     * 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 => {
-                logger.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 => {
-                logger.error(e, encodedFrame.type);
-
-                // 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);
+        this._worker.postMessage({
+            operation: 'setKey',
+            key
+        });
     }
 }

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
+    };
+
+    // An array (ring) of keys that we use for sending and receiving.
+    const cryptoKeyRing = new Array(keyRingSize);
+
+        // A pointer to the currently used key.
+    let 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.
+    const sendCounts = new Map();
+
+    // 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 salt;
+
+    /**
+     * Derives a AES-GCM key with 128 bits from the input using PBKDF2
+     * The salt is configured in the constructor of this class.
+     * @param {Uint8Array} keyBytes - Value to derive key from
+     */
+    async function deriveKey(keyBytes) {
+        // 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' ]);
+    }
+
+    /**
+     * 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
+     */
+    function 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 (!sendCounts.has(synchronizationSource)) {
+            // Initialize with a random offset, similar to the RTP sequence number.
+            sendCounts.set(synchronizationSource, Math.floor(Math.random() * 0xFFFF));
+        }
+        const sendCount = sendCounts.get(synchronizationSource);
+
+        ivView.setUint32(0, synchronizationSource);
+        ivView.setUint32(4, timestamp);
+        ivView.setUint32(8, sendCount % 0xFFFF);
+
+        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.
+     */
+    function encodeFunction(encodedFrame, controller) {
+        const keyIndex = currentKeyIndex % cryptoKeyRing.length;
+
+        if (cryptoKeyRing[keyIndex]) {
+            const iv = makeIV(encodedFrame.synchronizationSource, encodedFrame.timestamp);
+
+            return crypto.subtle.encrypt({
+                name: 'AES-GCM',
+                iv,
+                additionalData: new Uint8Array(encodedFrame.data, 0, unencryptedBytes[encodedFrame.type])
+            }, 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.
+     */
+    function decodeFunction(encodedFrame, controller) {
+        const data = new Uint8Array(encodedFrame.data);
+        const keyIndex = data[encodedFrame.data.byteLength - 1];
+
+        if (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])
+            }, 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 => {
+                // 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 >= cryptoKeyRing.length && cryptoKeyRing[currentKeyIndex % 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);
+    }
+
+    onmessage = async (event) => {
+        const {operation} = event.data;
+        if (operation === 'initialize') {
+            salt = event.data.salt;
+        } else if (operation === 'encode') {
+            const {readableStream, writableStream} = event.data;
+            const transformStream = new TransformStream({
+                transform: encodeFunction,
+            });
+            readableStream
+                .pipeThrough(transformStream)
+                .pipeTo(writableStream);
+        } else if (operation === 'decode') {
+            const {readableStream, writableStream} = event.data;
+            const transformStream = new TransformStream({
+                transform: decodeFunction,
+            });
+            readableStream
+                .pipeThrough(transformStream)
+                .pipeTo(writableStream);
+        } else if (operation === 'setKey') {
+            const keyBytes = event.data.key;
+            let key;
+            if (keyBytes) {
+                key = await deriveKey(keyBytes);
+            } else {
+                key = false;
+            }
+            currentKeyIndex++;
+            cryptoKeyRing[currentKeyIndex % cryptoKeyRing.length] = key;
+        } else {
+            console.error('e2ee worker', operation);
+        }
+    };
+
+`;
+
+export const e2eeWorkerScript = URL.createObjectURL(new Blob([ code ], { type: 'application/javascript' }));