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Ben Laurie blathering


What Is DNSSEC Good For?

A lot of solutions to all our problems begin with “first find a public key for the server”, for example, signing XRD files. But where can we get a public key for a server? Currently the only even slightly sane way is by using an X.509 certificate for the server. However, there are some problems with this approach

  1. If you are going to trust the key, then the certificate must come from a trusted CA, and hence costs money.
  2. Because the certificate is a standard X.509 certificate, it can be used (with the corresponding private key, of course) to validate an HTTPS server – but you may not want to trust the server with that power.
  3. The more we (ab)use X.509 certificates for this purpose, the more services anyone with a certificate can masquerade as (for the certified domain, of course).

One obvious way to fix these is to add extensions to the certificates that prevent their use for inappropriate services. Of course, then we would have to get the CAs to support these extensions and figure out how to validate certificate requests that used them.

But I have to wonder why we’re involving CAs in this process at all? All the CA does is to establish that the person requesting the certificate is the owner of the corresponding domain. But why do we need that service? Why could the owner of the domain not simply include the certificate in the DNS – after all, only the owner of the domain can do that, so what further proof is required?

Obviously the answer is: DNS is not secure! This would allow anyone to easily spoof certificates for any domain. Well, yes – that’s why you need DNSSEC. Forgetting the details of DNSSEC, the interesting feature is that the owner of a domain also owns a private key that can sign entries in that domain (and no-one else does, if the owner is diligent). So, the domain owner can include any data they want in their zone and the consumer of the data can be sure, using DNSSEC, that the data is valid.

So, when the question “what is the public key for service X on server Y?” arises, the answer should be “look it up in the DNS with DNSSEC enabled”. The answer is every bit as secure as current CA-based certificates, and, what’s more, once the domain owner has set up his domain, there is no further cost to him – any new keys he needs he can just add to his zone and he’s done.

Does DNSSEC have any other uses? OK, it would be nice to know that the A record you just got back corresponds to the server you were looking for, but if you trust a connection just on the basis that you used the right address, you are dead meat – you’ll need some key checking on top of it (for example, by using TLS) to avoid attacks by evil proxies (such as rogue wifi hotspots) or routing attacks and so forth. For me, the real value in DNSSEC is cryptographic key distribution.

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