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/**
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* part of Hachette
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* Functions related to "signing" of data, refactored to a separate file.
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*
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* Copyright (C) 2021 Wojtek Kosior
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* Redistribution terms are gathered in the `copyright' file.
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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_chrome
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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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* hachette 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 the key that resides in the manifest.
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*
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* An idea to (under Chromium) instead store the secret in a file fetched
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* synchronously using XMLHttpRequest is being considered.
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*/
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function get_secret()
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{
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if (is_chrome)
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return browser.runtime.getManifest().key.substring(0, 50);
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else
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return browser.runtime.getURL("dummy");
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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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