Monday, October 8, 2012

Some thoughts on the DANE protocol

A while ago I was writing on why we need an alternative authentication method in TLS. Then I described the SSH-style authentication and how it was implemented it GnuTLS. Another approach is the DANE protocol. It uses the hierarchic model of DNSSEC to provide an alternative authentication method in TLS.

DNSSEC uses a hierarchical model similar to a single root CA that is delegating CA responsibilities to each individual domain holder for its domain. In DANE a DNSSEC-enabled domain may sign entries for each TLS server in the domain. The entries contain information, such as the hash, of the TLS server's certificate. That's a nice idea and can be used instead, or in addition to the existing commercial CA verification. Let's see two examples that demonstrate DANE. Let's suppose we have an HTTPS server called and a CA.

A typical web client after it successfully verifies's certificate using the CA's certificate, will check the DANE entries. If they match it's assured that a CA compromise does not affect its current connection. That is, in this scenario, an attacker needs not only to compromise the's CA, but also to compromise the server's DNS domain service.

Another significant example is when there is no CA at all. The server is a low-budget one and wants to avoid paying any commercial CA. If it has DNSSEC set up then it just advertises its key on the DNS and clients can verify its certificate using the DNSSEC signature. That is the trust is moved from the commercial CAs to the DNS administrators. Whether they can cope with that, will be seen in time.

Even though the whole idea is attractive, the actual protocol is (IMO) quite bloated. On a recent post in the DANE mailing list I summarized several issues (note that I was wrong on the first). The most important I believe is the fact that DANE separates the certificates to CA signed certificates and to self-signed certificates. That is if you have a CA signed certificate you mark it in the DNS entry as 1, while if you have a self signed one you mark it as 3. The reasoning behind this is unclear but it's effect is that it is harder to move from the CA signed to non-CA signed world or vice-versa. That is if you have a CA signed certificate and it expires, the DANE entry automatically becomes invalid as well. This may be considered a good property for some (especially the CAs), but I see no much point in that. It is also impossible for the server administrator to know whether a client trusts its CA, thus this certificate distinction doesn't always make sense. A work-around is to always have your certificates marked as self-signed and CA signed in two different DNS entries.

Despite this and few other shortcomings that I worry of, this is the best protocol we have so far to divide the trust for TLS certificates verification to more entities than the commercial CAs. For this reason, a companion library implementing DANE will be included in the upcoming GnuTLS 3.1.3 release.