On July 15th, a small but significant internet event occurred. On that day, years of planning culminated in the deployment of a cryptographic signature on the root DNS zone. To simplify greatly, this means that internet users will soon be able to have a much higher degree of trust in the hierarchical Domain Name System by utilizing the powers of recursion and cryptography. When a user’s computer is told that the IP address for “gmail.com” is 220.127.116.11, the user can be sure that this answer is true. This is important if you are someone such as a Chinese dissident who wants to reliably and securely reach gmail.com in order to communicate with your peers. The rollout of this throughout all domains, DNS resolvers, and client applications will take a little while, but the basic infrastructure is now in place.
This mitigates a certain class of vulnerabilities that web users used to face. Although it forecloses attacks at the domain name-to-IP address stage of requesting a web page, it does not necessarily foreclose attacks at other stages. For instance, an attacker that gets between you and the server you are trying to reach can simply claim that he is the server at 18.104.22.168. Our traditional way of protecting against this style of attack has been to rely on Certificate Authorities — trusted third-parties who certify digital key-pairs only for the true owners of a given domain name. Thus, even if an attacker tries to execute one of these “man-in-the-middle” attacks, he won’t possess the secret portion of the digital key-pair that is required to prove that his communications come from the true gmail.com. Your browser checks for a certified corresponding public key in the process of setting up a secure SSL/TLS connection to https://gmail.com.
Unfortunately, there are several technical, operational, and jurisdictional shortcomings of the Certificate Authority model. As I discussed in an earlier post, many of these problems are not present in the hierarchical and delegated model of DNS. However, DNS does not inherently provide the ability to store domain name-to-key-pair information. But could it? At one of the recent DNSSEC deployment ceremonies, Vint Cerf noted:
More has happened here today than meets the eye. An infrastructure has been created for a hierarchical security system, which can be purposed and re-purposed in a number of different ways. And so I would predict that although we started out putting this system together to assure that the domain name lookups return valid internet addresses, that in the long run this hierarchical structure of trust will be applied to a number of other functions that require strong authentication. And so you will have seen a new major milestone in the internet story.
I believe that storing SSL/TLS keys in DNSSEC-secured DNS records will be the first significant “other function” that will emerge. An alternative to Certificate Authorities for domain-to-key mapping is sorely needed. There are two major practical hurdles to getting there: 1) We must define a standard for placing keys in DNS and 2) We must secure the “last mile” from the service provder’s DNS resolver to the end-user’s computer.
The first hurdle involves the type of standard-setting that the internet community is quite familiar with. On a technical level, it means that we need to collectively decide what these DNS records look like. The second hurdle involves building more functionality into end users’ software so that it can do cryptographic validation of DNS results rather than blindly trusting its upstream DNS resolver. There may be temporary ways to do this within web browser code, but ultimately it will probably have to be built into what is called the “stub resolver” — a local service running on your computer that usually just asks for the results from the upstream resolver.
It is important to note that none of his makes Certificate Authorities obsolete. Although the DNS-based approach replaces the most basic type of SSL certificates, the Certificate Authorities will continue to be the only entities that can offer validation of real-world identity of site owners. The DNS-based approach and basic “domain validated” Certificate Authority certificates both verify only that whoever controls the domain name is the entity that your computer is communicating with, without saying who that is. In recent years, “Extended Validation” certificates (the ones that make your browser bar glow green) have begun to be offered by all major certificate authorities. These certificates require more rigorous validation of the identity of the owner, so that for example you know that the person who controls bankofamerica.com is really Bank of America Corporation.
At this year’s Black Hat and Defcon, Dan Kaminsky demonstrated some new software he is releasing that could make deploying DNSSEC more easy in general, and that could also address the two main hurdles to placing keys in DNS. He readily admits that his particular implementation is not perfect, and has encouraged critiques and changes. [Update: His slides are available here.]
Hopefully, with the input of the many smart folks in the security, internet standards, and software development communities, we will see a production-quality DNSSEC-secured solution to domain-to-key authentication in the near future.