[gnutls-help] gnutls_protocol_get_name() and session resumption
nmav at gnutls.org
Mon Apr 15 08:33:08 CEST 2019
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
> and the resumed session gets
> 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.
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