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/**
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* This file is part of Haketilo.
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*
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* Functions: Operations related to "signing" of data.
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*
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* Copyright (C) 2021 Wojtek Kosior
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*
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* This program is free software: you can redistribute it and/or modify
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* it under the terms of the GNU General Public License as published by
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* the Free Software Foundation, either version 3 of the License, or
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* (at your option) any later version.
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*
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* This program is distributed in the hope that it will be useful,
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* but WITHOUT ANY WARRANTY; without even the implied warranty of
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* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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* GNU General Public License for more details.
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*
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* As additional permission under GNU GPL version 3 section 7, you
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* may distribute forms of that code without the copy of the GNU
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* GPL normally required by section 4, provided you include this
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* license notice and, in case of non-source distribution, a URL
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* through which recipients can access the Corresponding Source.
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* If you modify file(s) with this exception, you may extend this
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* exception to your version of the file(s), but you are not
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* obligated to do so. If you do not wish to do so, delete this
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* exception statement from your version.
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*
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* As a special exception to the GPL, any HTML file which merely
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* makes function calls to this code, and for that purpose
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* includes it by reference shall be deemed a separate work for
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* copyright law purposes. If you modify this code, you may extend
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* this exception to your version of the code, but you are not
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* obligated to do so. If you do not wish to do so, delete this
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* exception statement from your version.
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*
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* You should have received a copy of the GNU General Public License
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* along with this program. If not, see <https://www.gnu.org/licenses/>.
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*
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* I, Wojtek Kosior, thereby promise not to sue for violation of this file's
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* license. Although I request that you do not make use this code in a
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* proprietary program, I am not going to enforce this in court.
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*/
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/*
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* IMPORTS_START
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* IMPORT sha256
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* IMPORT browser
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* IMPORT is_mozilla
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* IMPORTS_END
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*/
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/*
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* In order to make certain data synchronously accessible in certain contexts,
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* Haketilo smuggles it in string form in places like cookies, URLs and headers.
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* When using the smuggled data, we first need to make sure it isn't spoofed.
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* For that, we use this pseudo-signing mechanism.
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*
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* Despite what name suggests, no assymetric cryptography is involved, as it
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* would bring no additional benefits and would incur bigger performance
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* overhead. Instead, we hash the string data together with some secret value
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* that is supposed to be known only by this browser instance. Resulting hash
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* sum plays the role of the signature. In the hash we also include current
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* time. This way, even if signed data leaks (which shouldn't happen in the
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* first place), an attacker won't be able to re-use it indefinitely.
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*
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* The secret shared between execution contexts has to be available
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* synchronously. Under Mozilla, this is the extension's per-session id. Under
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* Chromium, this is a dummy web-accessible-resource name that resides in the
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* manifest and is supposed to be constructed by each user using a unique value
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* (this is done automatically by `build.sh').
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*/
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function get_secret()
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{
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if (is_mozilla)
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return browser.runtime.getURL("dummy");
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return chrome.runtime.getManifest().web_accessible_resources
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.map(r => /^chromium-key-dummy-file-(.*)/.exec(r)).filter(r => r)[0][1];
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}
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function extract_signed(signature, signed_data)
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{
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const match = /^([1-9][0-9]{12}|0)_(.*)$/.exec(signed_data);
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if (!match)
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return {fail: "bad format"};
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const result = {time: parseInt(match[1]), data: match[2]};
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if (sign_data(result.data, result.time)[0] !== signature)
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result.fail = "bad signature";
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return result;
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}
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/*
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* Sign a given string for a given time. Time should be either 0 or in the range
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* 10^12 <= time < 10^13.
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*/
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function sign_data(data, time) {
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return [sha256(get_secret() + time + data), `${time}_${data}`];
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}
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/*
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* EXPORTS_START
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* EXPORT extract_signed
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* EXPORT sign_data
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* EXPORTS_END
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*/
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