MegolmExportEncryption.ts

/*
Copyright 2024 New Vector Ltd.
Copyright 2020 The Matrix.org Foundation C.I.C.
Copyright 2017 Vector Creations Ltd

SPDX-License-Identifier: AGPL-3.0-only OR GPL-3.0-only OR LicenseRef-Element-Commercial
Please see LICENSE files in the repository root for full details.
*/

import { logger } from "matrix-js-sdk/src/logger";

import { _t } from "../languageHandler";
import SdkConfig from "../SdkConfig";

const subtleCrypto = window.crypto.subtle;

/**
 * Make an Error object which has a friendlyText property which is already
 * translated and suitable for showing to the user.
 *
 * @param {string} message message for the exception
 * @param {string} friendlyText
 * @returns {{message: string, friendlyText: string}}
 */
function friendlyError(message: string, friendlyText: string): { message: string; friendlyText: string } {
    return { message, friendlyText };
}

function cryptoFailMsg(): string {
    return _t("encryption|export_unsupported");
}

/**
 * Decrypt a megolm key file
 *
 * @param {ArrayBuffer} data file to decrypt
 * @param {String} password
 * @return {Promise<String>} promise for decrypted output
 *
 *
 */
export async function decryptMegolmKeyFile(data: ArrayBuffer, password: string): Promise<string> {
    const body = unpackMegolmKeyFile(data);
    const brand = SdkConfig.get().brand;

    // check we have a version byte
    if (body.length < 1) {
        throw friendlyError("Invalid file: too short", _t("encryption|import_invalid_keyfile", { brand }));
    }

    const version = body[0];
    if (version !== 1) {
        throw friendlyError("Unsupported version", _t("encryption|import_invalid_keyfile", { brand }));
    }

    const ciphertextLength = body.length - (1 + 16 + 16 + 4 + 32);
    if (ciphertextLength < 0) {
        throw friendlyError("Invalid file: too short", _t("encryption|import_invalid_keyfile", { brand }));
    }

    const salt = body.subarray(1, 1 + 16);
    const iv = body.subarray(17, 17 + 16);
    const iterations = (body[33] << 24) | (body[34] << 16) | (body[35] << 8) | body[36];
    const ciphertext = body.subarray(37, 37 + ciphertextLength);
    const hmac = body.subarray(-32);

    const [aesKey, hmacKey] = await deriveKeys(salt, iterations, password);
    const toVerify = body.subarray(0, -32);

    let isValid;
    try {
        isValid = await subtleCrypto.verify({ name: "HMAC" }, hmacKey, hmac, toVerify);
    } catch (e) {
        throw friendlyError("subtleCrypto.verify failed: " + e, cryptoFailMsg());
    }
    if (!isValid) {
        throw friendlyError("hmac mismatch", _t("encryption|import_invalid_passphrase"));
    }

    let plaintext;
    try {
        plaintext = await subtleCrypto.decrypt(
            {
                name: "AES-CTR",
                counter: iv,
                length: 64,
            },
            aesKey,
            ciphertext,
        );
    } catch (e) {
        throw friendlyError("subtleCrypto.decrypt failed: " + e, cryptoFailMsg());
    }

    return new TextDecoder().decode(new Uint8Array(plaintext));
}

/**
 * Encrypt a megolm key file
 *
 * @param {String} data
 * @param {String} password
 * @param {Object=} options
 * @param {Number=} options.kdf_rounds Number of iterations to perform of the
 *    key-derivation function.
 * @return {Promise<ArrayBuffer>} promise for encrypted output
 */
export async function encryptMegolmKeyFile(
    data: string,
    password: string,
    options?: { kdf_rounds?: number }, // eslint-disable-line camelcase
): Promise<ArrayBuffer> {
    options = options || {};
    const kdfRounds = options.kdf_rounds || 500000;

    const salt = new Uint8Array(16);
    window.crypto.getRandomValues(salt);

    const iv = new Uint8Array(16);
    window.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(salt, kdfRounds, password);
    const encodedData = new TextEncoder().encode(data);

    let ciphertext;
    try {
        ciphertext = await subtleCrypto.encrypt(
            {
                name: "AES-CTR",
                counter: iv,
                length: 64,
            },
            aesKey,
            encodedData,
        );
    } catch (e) {
        throw friendlyError("subtleCrypto.encrypt failed: " + e, cryptoFailMsg());
    }

    const cipherArray = new Uint8Array(ciphertext);
    const bodyLength = 1 + salt.length + iv.length + 4 + cipherArray.length + 32;
    const resultBuffer = new Uint8Array(bodyLength);
    let idx = 0;
    resultBuffer[idx++] = 1; // version
    resultBuffer.set(salt, idx);
    idx += salt.length;
    resultBuffer.set(iv, idx);
    idx += iv.length;
    resultBuffer[idx++] = kdfRounds >> 24;
    resultBuffer[idx++] = (kdfRounds >> 16) & 0xff;
    resultBuffer[idx++] = (kdfRounds >> 8) & 0xff;
    resultBuffer[idx++] = kdfRounds & 0xff;
    resultBuffer.set(cipherArray, idx);
    idx += cipherArray.length;

    const toSign = resultBuffer.subarray(0, idx);

    let hmac;
    try {
        hmac = await subtleCrypto.sign({ name: "HMAC" }, hmacKey, toSign);
    } catch (e) {
        throw friendlyError("subtleCrypto.sign failed: " + e, cryptoFailMsg());
    }

    const hmacArray = new Uint8Array(hmac);
    resultBuffer.set(hmacArray, idx);
    return packMegolmKeyFile(resultBuffer);
}

/**
 * Derive the AES and HMAC-SHA-256 keys for the file
 *
 * @param {Unit8Array} salt  salt for pbkdf
 * @param {Number} iterations number of pbkdf iterations
 * @param {String} password  password
 * @return {Promise<[CryptoKey, CryptoKey]>} promise for [aes key, hmac key]
 */
async function deriveKeys(salt: Uint8Array, iterations: number, password: string): Promise<[CryptoKey, CryptoKey]> {
    const start = new Date();

    let key;
    try {
        key = await subtleCrypto.importKey("raw", new TextEncoder().encode(password), { name: "PBKDF2" }, false, [
            "deriveBits",
        ]);
    } catch (e) {
        throw friendlyError("subtleCrypto.importKey failed: " + e, cryptoFailMsg());
    }

    let keybits;
    try {
        keybits = await subtleCrypto.deriveBits(
            {
                name: "PBKDF2",
                salt: salt,
                iterations: iterations,
                hash: "SHA-512",
            },
            key,
            512,
        );
    } catch (e) {
        throw friendlyError("subtleCrypto.deriveBits failed: " + e, cryptoFailMsg());
    }

    const now = new Date();
    logger.log("E2e import/export: deriveKeys took " + (now.getTime() - start.getTime()) + "ms");

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

    const aesProm = subtleCrypto
        .importKey("raw", aesKey, { name: "AES-CTR" }, false, ["encrypt", "decrypt"])
        .catch((e) => {
            throw friendlyError("subtleCrypto.importKey failed for AES key: " + e, cryptoFailMsg());
        });

    const hmacProm = subtleCrypto
        .importKey(
            "raw",
            hmacKey,
            {
                name: "HMAC",
                hash: { name: "SHA-256" },
            },
            false,
            ["sign", "verify"],
        )
        .catch((e) => {
            throw friendlyError("subtleCrypto.importKey failed for HMAC key: " + e, cryptoFailMsg());
        });

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

const HEADER_LINE = "-----BEGIN MEGOLM SESSION DATA-----";
const TRAILER_LINE = "-----END MEGOLM SESSION DATA-----";

/**
 * Unbase64 an ascii-armoured megolm key file
 *
 * Strips the header and trailer lines, and unbase64s the content
 *
 * @param {ArrayBuffer} data  input file
 * @return {Uint8Array} unbase64ed content
 */
function unpackMegolmKeyFile(data: ArrayBuffer): Uint8Array {
    // parse the file as a great big String. This should be safe, because there
    // should be no non-ASCII characters, and it means that we can do string
    // comparisons to find the header and footer, and feed it into window.atob.
    const fileStr = new TextDecoder().decode(new Uint8Array(data));

    // look for the start line
    let lineStart = 0;
    // eslint-disable-next-line no-constant-condition
    while (1) {
        const lineEnd = fileStr.indexOf("\n", lineStart);
        if (lineEnd < 0) {
            throw new Error("Header line not found");
        }
        const line = fileStr.slice(lineStart, lineEnd).trim();

        // start the next line after the newline
        lineStart = lineEnd + 1;

        if (line === HEADER_LINE) {
            break;
        }
    }

    const dataStart = lineStart;

    // look for the end line
    // eslint-disable-next-line no-constant-condition
    while (1) {
        const lineEnd = fileStr.indexOf("\n", lineStart);
        const line = fileStr.slice(lineStart, lineEnd < 0 ? undefined : lineEnd).trim();
        if (line === TRAILER_LINE) {
            break;
        }

        if (lineEnd < 0) {
            throw new Error("Trailer line not found");
        }

        // start the next line after the newline
        lineStart = lineEnd + 1;
    }

    const dataEnd = lineStart;
    return decodeBase64(fileStr.slice(dataStart, dataEnd));
}

/**
 * ascii-armour a  megolm key file
 *
 * base64s the content, and adds header and trailer lines
 *
 * @param {Uint8Array} data  raw data
 * @return {ArrayBuffer} formatted file
 */
function packMegolmKeyFile(data: Uint8Array): ArrayBuffer {
    // we split into lines before base64ing, because encodeBase64 doesn't deal
    // terribly well with large arrays.
    const LINE_LENGTH = (72 * 4) / 3;
    const nLines = Math.ceil(data.length / LINE_LENGTH);
    const lines = new Array(nLines + 3);
    lines[0] = HEADER_LINE;
    let o = 0;
    let i;
    for (i = 1; i <= nLines; i++) {
        lines[i] = encodeBase64(data.subarray(o, o + LINE_LENGTH));
        o += LINE_LENGTH;
    }
    lines[i++] = TRAILER_LINE;
    lines[i] = "";
    return new TextEncoder().encode(lines.join("\n")).buffer;
}

/**
 * Encode a typed array of uint8 as base64.
 * @param {Uint8Array} uint8Array The data to encode.
 * @return {string} The base64.
 */
function encodeBase64(uint8Array: Uint8Array): string {
    // Misinterpt the Uint8Array as Latin-1.
    // window.btoa expects a unicode string with codepoints in the range 0-255.
    const latin1String = String.fromCharCode.apply(null, Array.from(uint8Array));
    // Use the builtin base64 encoder.
    return window.btoa(latin1String);
}

/**
 * Decode a base64 string to a typed array of uint8.
 * @param {string} base64 The base64 to decode.
 * @return {Uint8Array} The decoded data.
 */
function decodeBase64(base64: string): Uint8Array {
    // window.atob returns a unicode string with codepoints in the range 0-255.
    const latin1String = window.atob(base64);
    // Encode the string as a Uint8Array
    const uint8Array = new Uint8Array(latin1String.length);
    for (let i = 0; i < latin1String.length; i++) {
        uint8Array[i] = latin1String.charCodeAt(i);
    }
    return uint8Array;
}