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rafaelbn_176840's avatar
rafaelbn_176840
Icon for Altocumulus rankAltocumulus
Apr 23, 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 f...
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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.