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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_chrome
+ * 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 the key that resides in the manifest.
+ *
+ * An idea to (under Chromium) instead store the secret in a file fetched
+ * synchronously using XMLHttpRequest is being considered.
+ */
+
+function get_secret()
+{
+    if (is_chrome)
+	return browser.runtime.getManifest().key.substring(0, 50);
+    else
+	return browser.runtime.getURL("dummy");
+}
+
+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
+ */