[erlang-questions] SSL: Getting master_secret and client_random (or premaster_secret)

Roger Lipscombe roger@REDACTED
Thu Jan 5 15:24:07 CET 2017

Further searching shows that Ingela rejected something related in
but I'm not sure whether that was about exposing the key material or
the way that that patch should have been implemented as ssl:prf (which
it eventually was).

On 5 January 2017 at 14:20, Roger Lipscombe <roger@REDACTED> wrote:
> We're using ECDHE and DHE ciphers for our SSL connections. This
> provides perfect forward secrecy, which is good, but it makes it
> impossible to decipher packet captures in wireshark, which is
> expected, and also good, almost all of the time.
> Sometimes, however, we *do* need to decipher the traffic.
> Note that we own both the client (which is embedded) and the server
> (which uses Erlang -- otherwise I wouldn't be asking here -- and
> ranch). We *could* offer a different cipher suite on the server, which
> would disable PFS, but would do it for all connections. I'd prefer
> something a bit more fine-grained.
> You can feed a key log to Wireshark, as documented at
> https://developer.mozilla.org/en-US/docs/Mozilla/Projects/NSS/Key_Log_Format,
> and it'll correctly decipher the traffic for that connection.
> I'd like to find a way to generate a key log file. This requires
> either (client_random, master_secret) or (encrypted_premaster_secret,
> premaster_secret).
> Note that I'm looking at the OTP 17.5 source, because that's what we're using.
> It would seem that premaster_secret is not stored past the initial
> negotiation, but the client_random and master_secret values are in the
> #security_parameters record in the #connection_state record in the
> #connection_states record, which is in the #state record of the SSL
> connection pid.
> But I can't see any (clean) way to retrieve these values, in order to
> generate a key log suitable for Wireshark.
> Is there any clean way to do this in OTP 17.5, or is there a supported
> way to do this in OTP 18.x or 19.x?
> Regards,
> Roger.

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