Encryption and decryption for megolm backups
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Richard van der Hoff
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3 changed files with 544 additions and 0 deletions
312
src/utils/MegolmExportEncryption.js
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312
src/utils/MegolmExportEncryption.js
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/*
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Copyright 2017 Vector Creations Ltd
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Licensed under the Apache License, Version 2.0 (the "License");
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you may not use this file except in compliance with the License.
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You may obtain a copy of the License at
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http://www.apache.org/licenses/LICENSE-2.0
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Unless required by applicable law or agreed to in writing, software
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distributed under the License is distributed on an "AS IS" BASIS,
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WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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See the License for the specific language governing permissions and
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limitations under the License.
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*/
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"use strict";
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// polyfill textencoder if necessary
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let TextEncoder = window.TextEncoder;
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if (!TextEncoder) {
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TextEncoder = require('./TextEncoderPolyfill');
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}
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let TextDecoder = window.TextDecoder;
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if (TextDecoder) {
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TextDecoder = require('./TextDecoderPolyfill');
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}
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const subtleCrypto = window.crypto.subtle || window.crypto.webkitSubtle;
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/**
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* Decrypt a megolm key file
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*
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* @param {ArrayBuffer} file
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* @param {String} password
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* @return {Promise<String>} promise for decrypted output
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*/
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export function decryptMegolmKeyFile(data, password) {
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const body = unpackMegolmKeyFile(data);
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// check we have a version byte
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if (body.length < 1) {
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throw new Error('Invalid file: too short');
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}
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const version = body[0];
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if (version !== 1) {
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throw new Error('Unsupported version');
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}
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const ciphertextLength = body.length-(1+16+16+4+32);
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if (body.length < 0) {
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throw new Error('Invalid file: too short');
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}
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const salt = body.subarray(1, 1+16);
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const iv = body.subarray(17, 17+16);
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const iterations = body[33] << 24 | body[34] << 16 | body[35] << 8 | body[36];
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const ciphertext = body.subarray(37, 37+ciphertextLength);
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const hmac = body.subarray(-32);
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return deriveKeys(salt, iterations, password).then((keys) => {
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const [aes_key, sha_key] = keys;
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const toVerify = body.subarray(0, -32);
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return subtleCrypto.verify(
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{name: 'HMAC'},
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sha_key,
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hmac,
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toVerify,
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).then((isValid) => {
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if (!isValid) {
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throw new Error('Authentication check failed: incorrect password?')
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}
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return subtleCrypto.decrypt(
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{
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name: "AES-CTR",
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counter: iv,
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length: 64,
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},
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aes_key,
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ciphertext,
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);
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});
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}).then((plaintext) => {
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return new TextDecoder().decode(new Uint8Array(plaintext));
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});
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}
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/**
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* Encrypt a megolm key file
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*
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* @param {String} data
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* @param {String} password
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* @param {Object=} options
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* @param {Nunber=} options.kdf_rounds Number of iterations to perform of the
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* key-derivation function.
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* @return {Promise<ArrayBuffer>} promise for encrypted output
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*/
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export function encryptMegolmKeyFile(data, password, options) {
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options = options || {};
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const kdf_rounds = options.kdf_rounds || 100000;
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const salt = new Uint8Array(16);
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window.crypto.getRandomValues(salt);
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const iv = new Uint8Array(16);
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window.crypto.getRandomValues(iv);
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return deriveKeys(salt, kdf_rounds, password).then((keys) => {
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const [aes_key, sha_key] = keys;
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return subtleCrypto.encrypt(
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{
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name: "AES-CTR",
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counter: iv,
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length: 64,
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},
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aes_key,
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new TextEncoder().encode(data),
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).then((ciphertext) => {
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const cipherArray = new Uint8Array(ciphertext);
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const bodyLength = (1+salt.length+iv.length+4+cipherArray.length+32);
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const resultBuffer = new Uint8Array(bodyLength);
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let idx = 0;
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resultBuffer[idx++] = 1; // version
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resultBuffer.set(salt, idx); idx += salt.length;
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resultBuffer.set(iv, idx); idx += iv.length;
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resultBuffer[idx++] = kdf_rounds >> 24;
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resultBuffer[idx++] = (kdf_rounds >> 16) & 0xff;
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resultBuffer[idx++] = (kdf_rounds >> 8) & 0xff;
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resultBuffer[idx++] = kdf_rounds & 0xff;
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resultBuffer.set(cipherArray, idx); idx += cipherArray.length;
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const toSign = resultBuffer.subarray(0, idx);
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return subtleCrypto.sign(
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{name: 'HMAC'},
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sha_key,
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toSign,
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).then((hmac) => {
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hmac = new Uint8Array(hmac);
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resultBuffer.set(hmac, idx);
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return packMegolmKeyFile(resultBuffer);
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});
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});
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});
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}
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/**
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* Derive the AES and SHA keys for the file
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*
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* @param {Unit8Array} salt salt for pbkdf
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* @param {Number} iterations number of pbkdf iterations
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* @param {String} password password
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* @return {Promise<[CryptoKey, CryptoKey]>} promise for [aes key, sha key]
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*/
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function deriveKeys(salt, iterations, password) {
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return subtleCrypto.importKey(
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'raw',
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new TextEncoder().encode(password),
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{name: 'PBKDF2'},
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false,
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['deriveBits']
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).then((key) => {
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return subtleCrypto.deriveBits(
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{
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name: 'PBKDF2',
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salt: salt,
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iterations: iterations,
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hash: 'SHA-512',
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},
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key,
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512
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);
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}).then((keybits) => {
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const aes_key = keybits.slice(0, 32);
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const sha_key = keybits.slice(32);
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const aes_prom = subtleCrypto.importKey(
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'raw',
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aes_key,
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{name: 'AES-CTR'},
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false,
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['encrypt', 'decrypt']
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);
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const sha_prom = subtleCrypto.importKey(
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'raw',
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sha_key,
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{
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name: 'HMAC',
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hash: {name: 'SHA-256'},
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},
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false,
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['sign', 'verify']
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);
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return Promise.all([aes_prom, sha_prom]);
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});
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}
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const HEADER_LINE = '-----BEGIN MEGOLM SESSION DATA-----';
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const TRAILER_LINE = '-----END MEGOLM SESSION DATA-----';
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/**
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* Unbase64 an ascii-armoured megolm key file
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*
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* Strips the header and trailer lines, and unbase64s the content
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*
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* @param {ArrayBuffer} data input file
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* @return {Uint8Array} unbase64ed content
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*/
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function unpackMegolmKeyFile(data) {
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// parse the file as a great big String. This should be safe, because there
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// should be no non-ASCII characters, and it means that we can do string
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// comparisons to find the header and footer, and feed it into window.atob.
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const fileStr = new TextDecoder().decode(new Uint8Array(data));
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// look for the start line
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let lineStart = 0;
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while (1) {
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const lineEnd = fileStr.indexOf('\n', lineStart);
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if (lineEnd < 0) {
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throw new Error('Header line not found');
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}
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const line = fileStr.slice(lineStart, lineEnd).trim();
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// start the next line after the newline
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lineStart = lineEnd+1;
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if (line === HEADER_LINE) {
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break;
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}
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}
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const dataStart = lineStart;
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// look for the end line
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while (1) {
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const lineEnd = fileStr.indexOf('\n', lineStart);
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const line = fileStr.slice(lineStart, lineEnd < 0 ? undefined : lineEnd)
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.trim();
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if (line === TRAILER_LINE) {
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break;
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}
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if (lineEnd < 0) {
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throw new Error('Trailer line not found');
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}
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// start the next line after the newline
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lineStart = lineEnd+1;
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}
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const dataEnd = lineStart;
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return decodeBase64(fileStr.slice(dataStart, dataEnd));
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}
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/**
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* ascii-armour a megolm key file
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*
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* base64s the content, and adds header and trailer lines
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*
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* @param {Uint8Array} data raw data
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* @return {ArrayBuffer} formatted file
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*/
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function packMegolmKeyFile(data) {
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// we split into lines before base64ing, because encodeBase64 doesn't deal
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// terribly well with large arrays.
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const LINE_LENGTH = (72 * 4 / 3);
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const nLines = Math.ceil(data.length / LINE_LENGTH);
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const lines = new Array(nLines + 3);
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lines[0] = HEADER_LINE;
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let o = 0;
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let i;
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for (i = 1; i <= nLines; i++) {
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lines[i] = encodeBase64(data.subarray(o, o+LINE_LENGTH));
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o += LINE_LENGTH;
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}
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lines[i++] = TRAILER_LINE;
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lines[i] = '';
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return (new TextEncoder().encode(lines.join('\n'))).buffer;
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}
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/**
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* Encode a typed array of uint8 as base64.
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* @param {Uint8Array} uint8Array The data to encode.
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* @return {string} The base64.
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*/
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function encodeBase64(uint8Array) {
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// Misinterpt the Uint8Array as Latin-1.
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// window.btoa expects a unicode string with codepoints in the range 0-255.
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var latin1String = String.fromCharCode.apply(null, uint8Array);
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// Use the builtin base64 encoder.
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return window.btoa(latin1String);
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}
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/**
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* Decode a base64 string to a typed array of uint8.
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* @param {string} base64 The base64 to decode.
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* @return {Uint8Array} The decoded data.
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*/
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function decodeBase64(base64) {
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// window.atob returns a unicode string with codepoints in the range 0-255.
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var latin1String = window.atob(base64);
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// Encode the string as a Uint8Array
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var uint8Array = new Uint8Array(latin1String.length);
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for (var i = 0; i < latin1String.length; i++) {
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uint8Array[i] = latin1String.charCodeAt(i);
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}
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return uint8Array;
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}
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