Decrypting SSL sessions in Wireshark

Debugging secure communication programs is no fun. One thing I learned during all these years developing VoIP applications is to never, ever trust the logs to tell the truth, and especially the one I put myself in the code. The truth is on the wire, so capturing the packets and been able to analyze them, for example with Wireshark, is one of the most important tool that a communication developer can use. But the need for secure communications makes this tool difficult to use.

As said before, the first solution would be to display the packets directly in the logs before they are encrypted or after they are decrypted but as this is probably the same idiot who wrote the code to be debugged that will write the code to generate the logs, there is little chance to have something useful as a result.

Another solution is to have a switch that permits to use the software under test in a debug mode which does not require to encrypt and decrypt the communications. A first problem is that it increases the probability of forgetting to turn the switch off after debugging or to use the wrong setting in production. Another problem is that the secure and unsecure modes may very well use different code paths, and so may behave differently. And lastly some communication protocols, especially the modern one, do not have an unsecure setting (for example RELOAD and RTCWEB), and for very good reasons.

A slightly better solution that can reduce the difference between code paths and work with secure-only protocols is to use a null cipher but for security reasons both sides must agree beforehand to use a null cipher. That in fact probably increases the probability that someone forget to switch off the null cipher after a test.

So the only remaining solution is to somehow decrypt the packets in Wireshark. The standard way of doing that is to install the private RSA in Wireshark, which immediately creates even more problems:

  • This cannot be used to debug programs in production, because the sysadmin will never accept to give the private key.
  • This does not work if the private key is stored in an hardware token, like a smartcard as, by design, it is impossible to read the private key from these devices.
  • Even with the private key, the SSL modes that can be used are limited. For example a Diffie–Hellman key exchange cannot be decrypted by Wireshark.

Fortunately since version 1.6, Wireshark can use SSL session keys instead of the private key. Session keys are keys that are created from the private key but that are used only for a specific session, and disclosing them does not disclose the private key. This solves most of the problems listed above:

  • Sysadmins can disclose only the session key related to a specific session.
  • Session keys are available even if the private key is stored in an hardware token.
  • Sessions keys are the result of, e.g., a Diffie–Hellman key exchange, so there is no need to restrict the SSL modes for debugging.

Now we just need to have the program under test storing the session keys somewhere so Wireshark can use them. For example the next version of NSS (the security module used by Firefox, Thunderbird and Chrome) will have an environment variable that can be used to generate a file that can be directly used by Wireshark (see this link for more details).

Adding the support for this format in Java requires to maintain a modified build of Java, which can be inconvenient. A simpler solution is to process the output of the,keygen debug option. The just uploaded Debian package named keygen2keylog contains a program that does this for you. After installation, start Wireshark in capture mode with the name of the SSL session key file that will be generated as parameter, something like this:

$ wireshark -o ssl.keylog_file:mykeys.log -k -i any -f "host"

(Remember that you do not need to run Wireshark under root to capture packets if you run the following command after each update of the package: sudo setcap ‘CAP_NET_RAW+eip CAP_NET_ADMIN+eip’ /usr/bin/dumpcap)

Then you just need to pipe the debug output of your Java program to keygen2keylog to see the packets been decrypted in Wireshark, e.g.:

$ java,keygen -jar mycode.jar | keygen2keylog mykeys.log

And the beauty of this technique is that the packets are decrypted as they are captured.

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