Diffie-Hellman "p" length 1024/2048 bits

[rafaelbn]

Hey folks! Spoiler: very tricky question ahead!

On diffie-hellman negotiation (TLSv1.2 and TLS_DHE_RSA_WITH_AES_256_GCM_SHA384 cipher-suite to be more specific), the length of p (aka the size 1024/2048 bits) is dependent of just configuration or could the certificate issued influece it?

I ask this because I have two VS that share the same cipher-suites on the client-ssl profile but negotiate different sizes: one is 1024 and the other is 2048. And I read this K82014843 that BIG-IP is not supposed to use 2048 (as in it's not implemented) and to my surprised I'm getting 2048bits DH on my tests.

Any tips for me?

Thanks!

[jaikumar_f5 · Accepted solution]

When it comes to handshake, the ciphers alone play role in negotiation. The certificate has no play here.

The DHE suites are 1024 alone in F5, if you had seen a 2048 bit, It should have been ECDHE.

Can you put a logging rule to confirm if it indeed was DHE suite and not ECDHE ?

[rafaelbn]

Hello Jaikumar! Thanks for the reply.

I will investigate it further. But will let you know.

Do you recommend any article/training that explain this? I wish to understand this type of thing better.

Thanks!

[jaikumar_f5]

Here mate, hope this helps.

SSL Profiles Part 1: Handshakes

Troubleshooting SSL/TLS handshake failures

Troubleshooting Handshake by Capturing Traffic

Qualsys SSL lab test reports a result of DH 1024 bits WEAK

[rafaelbn]

Thanks Jaikumar!