Understanding TLS 1.2 Handshakes, Mutual Authentication, and Certificate Formats

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SSL vs TLS vs mTLS

SSL and TLS are both protocols: sets of rules for how a client and server agree on encryption. SSL came first; later TLS replaced it. People still refer to “SSL” out of habit, but the real SSL versions (SSL 2.0, SSL 3.0) are broken and no longer used. Any connection called “SSL” today almost always runs TLS.

You may then wonder what OpenSSL command we use in the terminal? Well, it’s not a protocol. It is a library and command-line tool that implements TLS (and, in its early years, SSL). It kept the name “OpenSSL” from the 1990s, before TLS existed. Modern OpenSSL negotiates TLS 1.2 or TLS 1.3 by default and no longer supports SSL 2.0 at all.

TLS has several versions. TLS 1.2 is still common. TLS 1.3is newer and faster. Both agree on a shared secret first, then encrypt all traffic...

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