[gnutls-help] gnutls_protocol_get_name() and session resumption

[Nikos Mavrogiannopoulos]

On Sun, Apr 14, 2019 at 9:19 PM Jeremy Harris <jgh at wizmail.org> wrote:
>
> On 14/04/2019 15:09, Nikos Mavrogiannopoulos wrote:
> > On Sat, Apr 13, 2019 at 8:20 PM Jeremy Harris <jgh at wizmail.org> wrote:
> >> GnuTLS 3.6.7
> >>
> >> On resuming a TLS1.3:ECDHE_RSA_AES_256_GCM_SHA384:256 session
> >> I'm getting a reported ciphersuite TLS1.3:NULL:256
> >>
> >> The "NULL" derives from gnutls_cipher_suite_get_name() and
> >> the difference from the original is that the kx has changed
> >> from 12 (GNUTLS_KX_ECDHE_RSA) to 14 (GNUTLS_KX_ECDHE_PSK).
> >>
> >>
> >> Should I be using gnutls_kx_get_name() (&c for cipher and mac)
> >> separately, rather than gnutls_cipher_suite_get_name() ?
> >
> > There are no key exchange methods under TLS1.3, or they are kind of
> > implied, that's why you see null there. I'd recommend to use
> > gnutls_session_get_desc() which gives a description applicable for
> > gnutls but uniform across versions.
>
> Using that, the original connection gets
>  (TLS1.3)-(ECDHE-SECP256R1)-(RSA-PSS-RSAE-SHA256)-(AES-256-GCM)
> and the resumed session gets
>  (TLS1.3)-(ECDHE-PSK-SECP256R1)-(AES-256-GCM)
>
> Assuming the ECDHE is "implied by the TLS1.3" and the PSK part
> is saying the key was shared by the initial connection...
> what has happened to the cipher?

The cipher is in both cases AES-256-GCM. What has changed is the key
exchange and authentication method. Under TLS1.3 the resumption is a
new key exchange authenticated using a pre-shared key (PSK) with the
server rather than a separate resumption mechanism as in TLS1.2.
That's why on tls1.3 resumed sessions you see the new key exchange
rather than the previous one.

regards,
Nikos