![]() ![]() When an application’s logs come up empty, Wireshark is often the best way to figure out what’s going with software. When troubleshooting issues with SSL/TLS, Wireshark is invaluable. Have you ever gotten an error message complaining about secure negotiation? Most Sysadmins have. Where is that failure occurring? Do the client and server have a version of TLS in common which they both support? Is there at least one cipher they can agree on? By looking at the SSL/TLS handshake taking place, you can see exactly where communication is breaking down. The SSL/TLS handshake by necessity happens in the clear – you can’t send encrypted communication until that channel has been forged. What about messages sent later, encrypted over that secure tunnel? By providing Wireshark with the server’s private key, most of the time we can decrypt this traffic as well, right from within the Wireshark interface. ![]() The exception to this, is if the cipher agreed upon between client and server leverages Diffie-Hellman. Perfect forward secrecy (PFS) thwarts Wireshark’s ability to decrypt the data after the fact, even with access to the server’s private key. As PFS is mandated by TLS 1.3, it’s time for those of us who are used to temporarily disabling DH ciphers to learn a new technique. ![]() Today, we will walk through the steps necessary to instruct Google Chrome to write a special logfile containing the DH Pre-Master key which will allow Wireshark to decrypt the conversation from the client’s perspective. From the System tab, select Advanced System Settings. Make sure all parent directories of this path exist! We will be creating an environment variable that will instruct Chrome to write out the logfile we need.Īt the bottom, select Environment Variables.Ĭreate a System Variable named SSLKeyLogFile with a path where you want the file to be written. Restart your machine for this configuration to take effect.ĭownload and install Wireshark (Which will install a library called Npcap) to your system. You will need to restart your machine again before you can use Wireshark. When you open Wireshark, you will be met with this interface.
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