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/**
* part of Hachette
* Functions related to "signing" of data, refactored to a separate file.
*
* Copyright (C) 2021 Wojtek Kosior
* Redistribution terms are gathered in the `copyright' file.
*/
/*
* IMPORTS_START
* IMPORT sha256
* IMPORT browser
* IMPORT is_mozilla
* IMPORTS_END
*/
/*
* In order to make certain data synchronously accessible in certain contexts,
* hachette 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
*/
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