XMPP uses SASL (Simple Authentication and Security Layer) for user authentication which supports a variety of different authentication mechanisms. One of them, "EXTERNAL", allows authenticating with a TLS client certificate.
Add support for providing a TLS client certificate and using it for SASL EXTERNAL authentication. You may need to setup a test server to test this.
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When you've done, commit your work and post the resulting changeset url to this task.