View Full Version : Internet security: Duly notarised

11th September 2011, 03:41 PM
FOR all its decentralised charm, the internet remains a top-down affair when it comes to security. Every time you connect to a secure website it is parties anointed with authority from on high that tell you whether or not the site should be trusted. Such dogma has been in place since the dawn of secured web communications. But heretics are becoming increasingly vocal. "It is insane to me that we can pick an organisation or set of organisations that we can trust not just now, but for ever, whether they continue to behave appropriately or not," laments Moxie Marlinspike, the man behind one of four related reformation movements which are beginning to challenge the old order.


As Babbage discussed in an earlier post, part of the bedrock of internet security are digital documents called certificates. These are bundles of cryptographic information issued by third parties known as certificate authorities (CAs). A cryptographic watermark ensures that the certificate was in fact issued by the CA featuring on it. Certificates themselves do not guarantee that the website has been well set up and secured. (That falls to other parties like TRUSTe that perform certain forms of routine audits and offers seals of approval.) Rather, they are bound together cryptographically to particular internet domains to assure the user that he is in fact connected to the desired site, and not a malicious one pretending to be it. The point is to prevent so-called man-in-the-middle attacks, in which an interloper sneaks in between two parties and relays their mutual messages. Browsers and operating systems use built-in lists of trusted CAs. However, if a CA's reliability is called into question, the lists, which contain hundreds of names, cannot be easily updated.

On August 29th news broke that a Dutch CA, DigiNotar, had improperly issued a certificate for all Google domains to a party other than the search giant. It ought not have issued such a certificate at all, and certainly not to a different firm. The company says its systems were subverted and independent security observers say as many as 250 certificates for an unknown number of domains were released. The Google document apparently remained valid for five weeks.

When a purported Iranian user spotted the certificate in his country pundits noted that repressive governments like Iran's could use it to spy on their citizens. It would be especially useful to eavesdroppers capable of re-routing internet traffic and poisoning domain-name lookups, where a domain name typed in by a user is turned into an numeric machine address, but not the one the user intended. A user in such a subverted system might click on https://mail.google.com, the secure address for Gmail, say, only to be redirected to a computer operated by someone else. Normally, if the fake site then presented a certificate from an unknown or untrusted source, the browser would flag this up as a security threat. If, however, the fake certificate came with the imprimatur of a trusted CA, the user would be none the wiser.

DigiNotar's illegitimate certificates have now been revoked and blacklisted. This alone does not, however, make the problem go away. Although browsers are programmed to consult such blacklists, whenever access to a list is blocked—something governments, for instance, can do all too easily—a browser will not signal this to the user. This is because the blacklist services are often unreliable in returning results: browsers would constantly warn that they could not assure the integrity of any secure site, not just those which have had their certificates revoked. And in any case, certificates are rarely revoked so the whole exercise is bereft of sense.

For their part, browser makers rushed to release full updates to their software that remove DigiNotar from the built-in list of trusted CAs; some browsers now also have an explicit list that blocks these and other certificates permanently. Users who do not download these updates (or fail to make tedious manual tweaks to their existing browsers) run the risk of accepting the dodgy certificates, thereby compromising their communications.

The kerfuffle has led to renewed calls to overhaul the internet's existing security architecture by dispensing with CAs' anointed role altogether, or at least making it a complement to a suite of validation methods. The idea was first mooted in 2008 by researchers at Carnegie-Mellon University in Pittsburgh. That led to their Perspectives Project. The three other efforts, by Mr Marlinspike (Convergence), the Electronic Frontier Foundation (SSL Observatory) and Google (Certificate Catalog), all acknowledge Perspectives' leading role.

In the Perspectives system CAs could still issue certificates. However, there would be no requirement to have a list of permanently trusted issuers baked into browser software. Instead, every time a browser retrieved a certificate from a secure website, it would consult an array of "notary" servers. The job of these would be to perform regular and exhaustive scans of all the secure servers on the internet from different parts of the globe, retrieving all their current certificates and recording the data in an abstracted shorthand called a fingerprint. If over time a certificate remains identical that gives some confidence that it is legitimately associated with a given domain. When a user's browser connects to a secure server it retrieves the associated certificate and compares it to the notary records. If the two match, the site is probably kosher. If not, the browser sounds an alarm.

Crucially, no one is being forced to use Perspectives' notary servers. In principle, any party may run such servers, and the code is available to install or develop notary servers. The Perspectives group expects trusted not-for-profit organisations and corporations might one day offer notary services to the public. Some companies and governments may, of course, wish to set up their own notaries and trust only them.

Mr Marlinspike has taken a slightly different tack. His notary software allows for multifarious forms of validation. Besides a system like Perspectives' users or groups may, for instance, wish to examine the route data took from a server to a browser and compare that against a constantly updated list that is publicly distributed to internet routers. If the server is located on a different network than where it is expected to be found, it's suspicious. Another technique would rely on checking certificate fingerprints that have been inserted into domain-name-system records (described in the earlier post). Both Perspectives and Convergence offer a Firefox add-on.

Google, meanwhile, has incorporated the Certificate Catalog into its Chrome browser, the first such direct integration. (It was Chrome that alerted the ostensible Iranian user to DigiNotar's suspect certificate.) As for the Electronic Frontier Foundation, whose mission is to support digital privacy rights and transparency on the internet, its goal is purely informative for now.

The technical advantage of notary approaches is that no website or other secure server needs to amend their configuration or install upgraded software. Makers of browsers and operating-systems could add support for dynamic certificate validation to replace the frozen CA lists that are at the root of the current troubles. In the meantime, users can install security-enhancing plug-ins.

Credit: The Economist