/**
 * This file is part of Haketilo.
 *
 * Functions: Operations related to "signing" of data.
 *
 * Copyright (C) 2021 Wojtek Kosior
 *
 * This program is free software: you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation, either version 3 of the License, or
 * (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * As additional permission under GNU GPL version 3 section 7, you
 * may distribute forms of that code without the copy of the GNU
 * GPL normally required by section 4, provided you include this
 * license notice and, in case of non-source distribution, a URL
 * through which recipients can access the Corresponding Source.
 * If you modify file(s) with this exception, you may extend this
 * exception to your version of the file(s), but you are not
 * obligated to do so. If you do not wish to do so, delete this
 * exception statement from your version.
 *
 * As a special exception to the GPL, any HTML file which merely
 * makes function calls to this code, and for that purpose
 * includes it by reference shall be deemed a separate work for
 * copyright law purposes. If you modify this code, you may extend
 * this exception to your version of the code, but you are not
 * obligated to do so. If you do not wish to do so, delete this
 * exception statement from your version.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program.  If not, see <https://www.gnu.org/licenses/>.
 *
 * I, Wojtek Kosior, thereby promise not to sue for violation of this file's
 * license. Although I request that you do not make use this code in a
 * proprietary program, I am not going to enforce this in court.
 */

/*
 * IMPORTS_START
 * IMPORT sha256
 * IMPORT browser
 * IMPORT is_mozilla
 * IMPORTS_END
 */

/*
 * In order to make certain data synchronously accessible in certain contexts,
 * Haketilo smuggles it in string form in places like cookies, URLs and headers.
 * When using the smuggled data, we first need to make sure it isn't spoofed.
 * For that, we use this pseudo-signing mechanism.
 *
 * Despite what name suggests, no assymetric cryptography is involved, as it
 * would bring no additional benefits and would incur bigger performance
 * overhead. Instead, we hash the string data together with some secret value
 * that is supposed to be known only by this browser instance. Resulting hash
 * sum plays the role of the signature. In the hash we also include current
 * time. This way, even if signed data leaks (which shouldn't happen in the
 * first place), an attacker won't be able to re-use it indefinitely.
 *
 * The secret shared between execution contexts has to be available
 * synchronously. Under Mozilla, this is the extension's per-session id. Under
 * Chromium, this is a dummy web-accessible-resource name that resides in the
 * manifest and is supposed to be constructed by each user using a unique value
 * (this is done automatically by `build.sh').
 */

function get_secret()
{
    if (is_mozilla)
	return browser.runtime.getURL("dummy");

    return chrome.runtime.getManifest().web_accessible_resources
	.map(r => /^chromium-key-dummy-file-(.*)/.exec(r)).filter(r => r)[0][1];
}

function extract_signed(signature, signed_data)
{
    const match = /^([1-9][0-9]{12}|0)_(.*)$/.exec(signed_data);
    if (!match)
	return {fail: "bad format"};

    const result = {time: parseInt(match[1]), data: match[2]};
    if (sign_data(result.data, result.time)[0] !== signature)
	result.fail = "bad signature";

    return result;
}

/*
 * Sign a given string for a given time. Time should be either 0 or in the range
 * 10^12 <= time < 10^13.
 */
function sign_data(data, time) {
    return [sha256(get_secret() + time + data), `${time}_${data}`];
}

/*
 * EXPORTS_START
 * EXPORT extract_signed
 * EXPORT sign_data
 * EXPORTS_END
 */