Assuming you have a PCAP file with HTTPS traffic and having the key pair (privat

Question

Assuming you have a PCAP file with HTTPS traffic and having the key pair (private & public), it is possible to decrypt the traffic if it uses Diffie Hellman Ephemeral? Using openSSL lib for example.

As far as I know the answer is no because DHE generates a new pair for each connection, so it do not uses the key that we put in our apache configuration, isn't it?

My task is, knowing the private and public keys, decrypt the traffic. Of course there are many CipherSuites. I'm studying the limitations of this objective.

Explanation / Answer

When TLS/SSL handshake starts, the first step is, server is authenticated using a private key associated to the server's certificate. In a 2nd step, client and server exchange a session key used to encrypt the payload of the connection. The session key is used for one session means when the session is closed the same key can't be used to encrypt the traffic of another session. Now the problem is SSL/TLS often use RSA cipher suites in which the session key is derived from private key. So the session key can be calculated in future if the underlying private is known.

DHE and ECDHE provides Perfect Forward Secrecy(PFS), means session keys are not derived from private key. So the attacker can not decrypt the traffic even when he has the private key used in the session handshake.

In DHE(not DH) the session keys are calculated using the Random Numbers.

Conclusion

You can't decrypt the traffic using private key when DHE or ECDHE is used.

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