[openssl-users] Server-side visibility of signature algorithm and key exchange properties?

Matt Caswell matt at openssl.org
Mon Nov 12 09:27:15 UTC 2018

On 09/11/2018 19:42, Viktor Dukhovni wrote:
> On Fri, Nov 09, 2018 at 06:42:28PM +0000, Matt Caswell wrote:
>>> I am having a bit of trouble finding the equivalent information for
>>> the 3rd line on the server side.  Anyone know how, in TLS 1.3 where
>>> these are not implied by the ciphersuite, to determine the signature
>>> algorithm (and curve for ECDSA), the hash algorithm and key exchange
>>> public key (with bit count for DH or curve name for ECDSA)?
>> I don't believe we currently expose the signature algorithm selected on
>> the server side. It's held in s->s3->tmp.sigalg, but AFAICT that is only
>> ever used internally.
> Thanks for confirming, it is then not surprising I failed to find
> the relevant interfaces. :-)
>> Similarly the key exchange public key is held in s->s3->peer_tmp. We do
>> expose that via SSL_get_server_tmp_key(), but its a client side only
>> function. We explicitly check that and return 0 if called on the server
>> side.
> Quick question about that, since the client may also sign the key
> exchange when a client certificate is requested and returned, what
> is held on the server in s->s3->tmp.sigalg?  [ I expect still the
> signature the server used in its CertificateVerify. And on the
> client side, I would expect this to hold the algorithm used by the
> client to sign its ClientVerify if a client cert was used. ]

Right - exactly that. s->s3->tmp.sigalg always holds the sigalg the
endpoint selected to sign its own CertificateVerify (whether that be
client or server).

> And it seems that on the server side s->s3->peer_tmp does actually
> hold the client's key exchange public key, which is necessarily for
> the same group as the server, so all we'd need to do is remove that
> 'explicit check' and that key be visible on the server side, right?

Right - although we might want to think about the name of the function.


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