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I'm trying to create some test cases for a TLS library for the Key Derivation outlined in RFC 8446 7.1.

I tried to get this information from openssl using the following:

# Generate a ECDH private key

# generate the key to pem format
openssl genpkey -algorithm X25519 -out x25519.pem

# Connect using the private key and log the traffic secrets and display handshake messages
openssl s_client -connect google.com:443 -tls1_3 -keylogfile keys.log -key x25519.pem -msg -msgfile messages.log -state -ciphersuites TLS_AES_128_GCM_SHA256 -debug -keymatexport key_material -trace -nocommands -tlsextdebug

I was able to get many of the secrets in the keylogfile, but I can't find the initial ephemeral secret used for ECDHE. I noticed that the public key from the -key I passed in does not match the key share in the handshakes, so I don't think it used my generated private key for ECDHE. I'm having trouble finding info on what that -key parameter is.

Is there a way to get openssl to log the ephemeral private key used for the initial ECDHE? Or is there a better way to generate test cases for TLS 1.3 key derivation?

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    $\begingroup$ -key is the key matching the cert for client authentication -- if server requests that, which google.com (and most other servers) does not, so here it is unused. This is sort of implicit in the man page statement "If [-key is] not specified the certificate file [i.e. -cert] will be used to read also the key" although it could be clearer. The last E in ECDHE (or DHE aka FFDHE) means 'ephemeral' which means NOT stored (with keylogfile being a deliberate violation of the security design of SSL/TLS). $\endgroup$ Apr 7, 2023 at 1:47
  • $\begingroup$ Thanks for the info @dave_thompson_085. $\endgroup$
    – JBaczuk
    Apr 10, 2023 at 15:05

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After some more searching, I came across RFC 8448, which was actually referenced in RFC 8446, and which contains test vectors for the key derivation and handshake messages.

Another item of interest, I came across ACVP, which seems helpful, but is a hosted service that requires explicit permission to use.

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