Where do 4096 bit RSA keys for SSL certs currently stand in terms of things like CA support, browser support, etc? In the overall scheme of things is the increased security worth the risk of 4096 bit keys not having the widespread support and compatibility as 2048 bit keys do, not to mention the increased CPU load required to process the key exchange? Are things slowly turning in favor of 4096?
Advisories recommend 2048 for now. Security experts are projecting that 2048 bits will be sufficient for commercial use until around the year 2030.
The main downside to using a large cert, such as 3072 or 4096, is that the algorithm is slightly slower (still fractions of a second, though).
Current browsers should all support certs upto 4096.
Some CAs won't issue a cert that large, so if you want a 4096 bit cert, you might have to shop around for a CA that will issue it.
I always generate 4096 bit keys since the downside is minimal (slightly lower performance) and security is slightly higher (although not as high as one would like). Larger keys like 8192 bit or even larger take forever to generate and require specially patched sw to use so are impractical. Luckily there are other algorithms slowly replacing rsa...
Since 2019, OpenSSH defaults to 3072 bit keys for RSA:
- ssh-keygen(1): Increase the default RSA key size to 3072 bits, following NIST Special Publication 800-57's guidance for a 128-bit equivalent symmetric security level.