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rafaelbn_176840's avatar
rafaelbn_176840
Icon for Altocumulus rankAltocumulus
Apr 24, 2019

SSL/TLS Ciphers and PFS

Hello devs!

 

I'm trying to wrap my head around all things crypto and thus I have some questions for you guys:

 

My understanding is that most TLS1.2 ciphers suites uses Diffie-Hellman or RSA for the key exchange and so, the certificate/key that you configure under the client-ssl profile has nothing to do with the key exchange itself. Correct?

 

Which key exchange protocol uses the certificate/key under the client-ssl profile to cipher the secret and does not have FS?

 

I found K16700 that have an iRule that takes notes of sessionids+master-keys. Is it correct to assume that with this iRule, I can capture most modern TLS1.1 and TLS1.2 cipher suites and decrypts client traffic? I ask this because since I first read about PFS I was very worried that I would never be able to capture and see what was going on at L7, but to me, it seems that if you capture session-id+master-key you can pretty much decrypt those PFS ciphers, meaning PFS means that if you don't capture session-id+master-key you're in trouble because the client-ssl private-key won't help.

 

Thanks!

 

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  • Dario_Garrido's avatar
    Dario_Garrido
    Icon for Noctilucent rankNoctilucent
    Apr 26, 2019

    Hello rafaelbn

    The PFS depends of the Key Exchange (Kx) which is part of the cipher suite

     openssl ciphers -v
ECDHE-RSA-AES256-GCM-SHA384  TLSv1.2  Kx=ECDH  Au=RSA  Enc=AESGCM(256)  Mac=AEAD

    A good explanation of the cipher suite here -> https://www.youtube.com/watch?v=ZM3tXhPV8v0

    The term 'ephemeral' refers to protocols which implements PFS

    • ECDHE - Elliptic-curve Diffie-Hellman Ephemeral
    • DHE - Diffie-Hellman Ephemeral

    ECDH, DH, RSA have not PFS.

    The TMM uses their own set of cipher suites:

     tmm --clientciphers NATIVE

    You can custom your clientssl profile to use any specific set of cipher suites.

    More info here -> https://support.f5.com/csp/article/K17370

    Regarding to decrypting, to decrypt a PFS Key Exchange you should have the random number of the exchange and the private key. In case of RSA, you only need the private key.

    KR, Dario.