Last year (May 2013), I ran an experiment by contacting SSL servers randomly: I was connecting to port 443 of random IPv4 addresses and, if I received a response, my client engaged in a series of aborted handshakes in order to work out what cipher suites were actually supported by the server. I tried a few millions of addresses, and found about 13000 servers speaking SSL/TLS over port 443.
Of these, about 7.5% supported an ECDHE cipher suite.
Mind the details:
- This was one year ago. Things may have changed since.
- My client did not send any extension claiming support for any specific elliptic curve. It was only claiming "generic" ECDHE support. This may have prevented some servers from trying ECDHE cipher suites. In that sense, my figure may be an underestimate.
- A corollary is that "ECDHE support" cannot be stated really generically. Many ECC implementations are limited to some curve types, most of them being able to process only a couple specific curves (the P-256 and P-384 standard NIST curves, so-called "suite B").
- There are several ways to make sampling measures. My code was using random IPv4 addresses, thus finding not only Web sites but also many systems like home modems/routers. Existing, published SSL surveys like that one use the "top 1 million Web servers" from Alexa; this favours a notion of "economical significance" which may or may not map to what you want. Concentrating on "top sites" weeds out unmaintained SSL servers and, indeed, home modems and routers, which should increase ECDHE support. Indeed, that survey (from early 2014) finds that a whopping 21.6% of these "top sites" support ECDHE cipher suites.
As a side note, I must say that in most cases, performance increase implied by ECDHE (over DHE) is unlikely to be relevant. It takes an awful lot of connections per second to a basic Web server to actually see the difference (we are talking hundreds per second here). Another salient point is that you get the performance increase by supporting ECDHE cipher suites, and configuring your client or server to prefer such cipher suites when possible; you don't need to stop supporting non-ECC cipher suites to get that purported speed bonus. In that sense, decision to enable ECDHE cipher suites does not need to be backed by figures on who supports them and who does not; you can just activate them and use them opportunistically.