The wolfSSL library includes a useful tool for sniffing TLS traffic. This can be used to capture and decrypt live or recorded PCAP traces when at least one of the keys is known. Typically a static RSA ciphersuite would be used, however with TLS v1.3 only Perfect Forward Secrecy (PFS) ciphers are allowed. For TLS v1.3 all cipher suites use a new ephemeral key for each new session.
In order to solve this we added a “static ephemeral” feature, which allows setting a known key that is used for deriving a shared secret. The key can be rolled periodically and synchronized with the sniffer tool to decrypt traffic. This feature is disabled by default and is only recommended for internal or test environments.
As a proof of concept we added this support to Apache httpd to demonstrate real-time decryption of web traffic. We are also working on a key manager to assist with key rolling and synchronization.
A use case that might be interesting is a company internal web server that requires auditing.
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