Still not computationally expensive (06 Feb 2011)

F5 have put up a blog post explaining why my previous statements about SSL's computational costs are a myth. I'm glad that the idea is getting out there!

Keep in mind, however, that F5 want to sell you SSL hardware and that the blog post is a marketing piece. (I've never used F5 products but, from the client side, I've never found any problems with them either.)

Some assertions in the text are simple and wrong, so I'll just point these out:

  • “All commercial certificate authorities now issue only 2048-bit keys”: clearly wrong, as anyone can confirm.
  • They link to a nearly 5-year old version of the NIST guidelines rather than the guidelines issued last month.
  • Their numbers RSA operations per second are both slow (it matches my 2.3Ghz, Core2 laptop) and per core. An eight-core server is over eight times faster than they suggest!
  • “Obviously the more servers you have, the more certificates you need to deploy”: bizarrely, they assume that you're putting a different certificate on each server.

Certificate lengths

It's true that NIST have recommended that 1024-bit certificates should be phased out by 2013. CA certificates should be 2048-bit now and we need to work to remove the 1024-bit ones.

But even in 2013 it's still going to take tens of millions of dollars of computer hardware a year to factor a 1024-bit RSA key. If you're the sort of organisation which is considering deploying HTTPS do you think that your attackers are going to do that, or are they going to bribe someone to steal the private key from the servers?

Likewise, SSL hardware will probably make your key harder to steal via a software exploit, but keys can be revoked the problem dealt with. There are some organisations where hardware protected keys make sense, but for 99% of sites, key material isn't what's important. It would be far more damaging for customer data to be dumped on the web and SSL hardware doesn't save you there.

As we go towards 2013, CAs will try to issue fewer 1024-bit certificates and 2048-bit certificates are 5x slower. But it only takes one CA to start issuing ECDSA certificates along with a 2048-bit RSA one and the problem is much less vexing. Browsers largely support it and you can serve the ECDSA certificate to the ones which do and the RSA to the rest. P256-ECDSA is about as fast as 1024-bit RSA. The only problem is that people don't know to demand it from their CAs so the CAs don't do it yet.


RC4 has good key agility, it's a stream cipher (which saves bytes on the wire), it's quick and very well analysed. It's not the strongest cipher in the world, but it's significantly stronger than some other parts of the SSL ecosystem. With overwhelming probability, you are not the sort of organisation that needs to worry about attackers brute-forcing your cipher.

All together now...

SSL is just not that computationally expensive any more. Here are the real costs of HTTPS deployment these days:

  • Virtual hosting still doesn't work in the real world because Microsoft never put support into Windows XP.
  • Sorting out mixed content issues on your website.

The F5 article does mention the first of these, but SSL hardware doesn't help with either of them.

All sites should deploy HTTPS because attacks like Firesheep are too easy to do. Even sites where you don't login should deploy HTTPS (imagine the effect of spoofing news websites at a major financial conference to headline “Market crashes”). You should use HSTS to stop sslstrip. But you are probably not the sort of organisation which needs to worry about multi-million dollar attacks aimed at factoring your key.