Q: Maximum keysizes supported for CA certificates? (RSA and EC)

[JPMfg]

Hi,

a customer plans to build a new PKI to last for the next decade (at least), and they plan to create CA certificates (root and intermediates) with very long RSA keys (8192 and 16384 bit!).

Since these CAs are planned to be used to issue VPN Server certificates as well as client (identity) Certificates, the FortiGate and FortiManager here will have to be able to use those CA certificates (to be more specific: their 8192 and 16384 bit long RSA public keys) to verifiy that Client and Server certificates are signed by those CAs.

Is that supported? What are the limits for RSA keys and EC curves for certificate validations (in user authentication, VPN IKE authentication etc.)

Please note that we do not plan to generate certificate requests with FortiGates or FortiManager that have more than 4096 bit RSA keys, but they will have to be able to verify signatures made by those CAs.

At the moment there is nothing implemented, so we cannot test it out.

[emnoc]

Good questions, I 've  built both private and  subCa for public-uses and by  using 8K but ( never heard of anyone doing more and almost  ALL public CAs are  at 4k bit keys  ).

You can load a  8K bit into foriiOS and almost all other  browsers and|or OSes. o that hurdle should be overcome with ease. BTW: you can create a CSR with a large keysize so use  openssl  for the CSR generation & priv-key.

Do you really need a a need for 16K bit keypair or  intermediate keysizes ? (probably not ). Will all other applications support  8+ k  keys ? ( maybe )  Will the need come to have  8+ keysizes  ? ( yes but probably  not until a  few decades and I'm sure TLS would be totally revamp ( SSL/TLS is over 25+  years old  technologies )

Keep in mind  all public CA are using 4 or 8k  root or intermediates for keysizes, and for ECC  384 keysizes.

FWIW 7 IMHO: I think the  added overhead for 8/16k bit keys   is not warrant , and you gain nothing from  user/server/web-certificate as far as that  goes and if  the server-cert is still a 2K bit key-pair.

Ken