matrix-js-sdk/src/utils/encryptAESSecretStorageItem.ts

/*
 * 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.
 */

import { decodeBase64, encodeBase64 } from "../base64.ts";
import { deriveKeys } from "./internal/deriveKeys.ts";
import { AESEncryptedSecretStoragePayload } from "../@types/AESEncryptedSecretStoragePayload.ts";

/**
 * Encrypt a string as a secret storage item, using AES-CTR.
 *
 * @param data - the plaintext to encrypt
 * @param key - the encryption key to use as an input to the HKDF function which is used to derive the AES key for
 *    encryption. Obviously, the same key must be provided when decrypting.
 * @param name - the name of the secret. Used as an input to the HKDF operation which is used to derive the AES key,
 *    so again the same value must be provided when decrypting.
 * @param ivStr - the base64-encoded initialization vector to use. If not supplied, a random one will be generated.
 *
 * @returns The encrypted result, including the ciphertext itself, the initialization vector (as supplied in `ivStr`,
 *   or generated), and an HMAC on the ciphertext — all base64-encoded.
 */
export default async function encryptAESSecretStorageItem(
    data: string,
    key: Uint8Array,
    name: string,
    ivStr?: string,
): Promise<AESEncryptedSecretStoragePayload> {
    let iv: Uint8Array;
    if (ivStr) {
        iv = decodeBase64(ivStr);
    } else {
        iv = new Uint8Array(16);
        globalThis.crypto.getRandomValues(iv);

        // clear bit 63 of the IV to stop us hitting the 64-bit counter boundary
        // (which would mean we wouldn't be able to decrypt on Android). The loss
        // of a single bit of iv is a price we have to pay.
        iv[8] &= 0x7f;
    }

    const [aesKey, hmacKey] = await deriveKeys(key, name);
    const encodedData = new TextEncoder().encode(data);

    const ciphertext = await globalThis.crypto.subtle.encrypt(
        {
            name: "AES-CTR",
            counter: iv,
            length: 64,
        },
        aesKey,
        encodedData,
    );

    const hmac = await globalThis.crypto.subtle.sign({ name: "HMAC" }, hmacKey, ciphertext);

    return {
        iv: encodeBase64(iv),
        ciphertext: encodeBase64(new Uint8Array(ciphertext)),
        mac: encodeBase64(new Uint8Array(hmac)),
    };
}