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?

  • Is there actually "widespread support and compatibility" for 2096bit keys? – user49075 Aug 12 '14 at 1:33
  • 2
    I'd rather use 2048 bit RSA for signing together with ECDH for key exchange instead of larger RSA for encryption. Unlike encryption/key-exchange keys, signing keys don't need long term security so 2048 is fine. – CodesInChaos Aug 12 '14 at 11:13
  • Unless quantum computing progresses significantly, tho is just number porn. 64 bits is enough to prevent brute force. – Robert Fraser Feb 23 '16 at 8:02
  • @RobertFraser , I think you're confused about asymmetric vs symmetric keys. For factoring a 64-bit number, you only need to check every number from 2 to 2^32, which is quite easily done in a day on any recent desktop processor. – rsaxvc Mar 7 '16 at 14:46
up vote 32 down vote accepted

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...

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