rrinc wrote
I'm running my site with a 1024bit RSA key (I assume the key length would matter in this case). Obviously everyone wants to have a fast website. I don't handle transactions or terribly sensitive data or anything so I wouldn't mind going to say a 512bit key to add some speed. How much does SSL affect the transfer size? I would assume SSL would add overhead. I'm guessing its insignificant, but I'm still curious.
The certificate/key exchange/negiciation occurs only when the connection is established. After that, there is almost overhead since data is encrypted using a 1 to 1 scheme and is sent inside packets with ~ 80 bytes of extra information.
The initial exchange is not even mandatory if the client and the server have already communicated. Servers usually maintain a cache of the result of negociations with clients and will reuse that information for subsequent connections with them. This speeds up the process a lot.
For the gory details, we recommend reading
http://en.wikipedia.org/wiki/Transport_Layer_Security#How_it_works .