matrix-js-sdk/src/utils/internal/deriveKeys.ts

Matrix Client-Server SDK for Javascript

/*
 * Copyright 2024 The Matrix.org Foundation C.I.C.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

// salt for HKDF, with 8 bytes of zeros
const zeroSalt = new Uint8Array(8);

/**
 * Derive AES and HMAC keys from a master key.
 *
 * This is used for deriving secret storage keys: see https://spec.matrix.org/v1.11/client-server-api/#msecret_storagev1aes-hmac-sha2 (step 1).
 *
 * @param key
 * @param name
 */
export async function deriveKeys(key: Uint8Array<ArrayBuffer>, name: string): Promise<[CryptoKey, CryptoKey]> {
    const hkdfkey = await globalThis.crypto.subtle.importKey("raw", key, { name: "HKDF" }, false, ["deriveBits"]);
    const keybits = await globalThis.crypto.subtle.deriveBits(
        {
            name: "HKDF",
            salt: zeroSalt,
            // eslint-disable-next-line @typescript-eslint/ban-ts-comment
            // @ts-ignore: https://github.com/microsoft/TypeScript-DOM-lib-generator/pull/879
            info: new TextEncoder().encode(name),
            hash: "SHA-256",
        },
        hkdfkey,
        512,
    );

    const aesKey = keybits.slice(0, 32);
    const hmacKey = keybits.slice(32);

    const aesProm = globalThis.crypto.subtle.importKey("raw", aesKey, { name: "AES-CTR" }, false, [
        "encrypt",
        "decrypt",
    ]);

    const hmacProm = globalThis.crypto.subtle.importKey(
        "raw",
        hmacKey,
        {
            name: "HMAC",
            hash: { name: "SHA-256" },
        },
        false,
        ["sign", "verify"],
    );

    return Promise.all([aesProm, hmacProm]);
}