On Wed, Aug 08, 2018 at 02:52:27PM +0100, Matt Caswell wrote: > > > On 08/08/18 14:45, Eric Rescorla wrote: > > > > > > On Wed, Aug 8, 2018 at 6:26 AM, Matt Caswell <m...@openssl.org > > <mailto:m...@openssl.org>> wrote: > > > > > > > > On 08/08/18 14:21, Benjamin Kaduk wrote: > > > On Wed, Aug 08, 2018 at 02:05:00PM +0100, Matt Caswell wrote: > > >> Draft 28 defines the inappropriate_fallback alert as follows: > > >> > > >> inappropriate_fallback Sent by a server in response to an invalid > > >> connection retry attempt from a client > > >> > > >> With the introduction of the downgrade protection sentinels it now > > seems > > >> that an inappropriate fallback could also be detected by the client. > > >> Should this wording be changed? > > > > > > Well, *fallback* specifically is inherently client-driven; the things > > the > > > client could detect would be more of an incorrectly negotiated version > > > (presumably due to an active attack). > > > > Consider the scenario where a server supports TLSv1.3/TLSv1.2 but does > > not support RFC7507 (TLS Fallback Signalling Cipher Suite Value). > > > > If the client attempts a TLSv1.3 connection first and fails (e.g. an > > active attacker prevented it) and then falls back to TLSv1.2 it would be > > able to detect that its fallback attempt was inappropriate when it sees > > the downgrade protection sentinels. In that case inappropriate_fallback > > seems reasonable. > > > > > > I don't think that is the alert I would choose in this circumstance. > > Probably "illegal_parameter" > > illegal_parameter suggests to me that the peer is misbehaving in some > way - which isn't the case here? Also it seems strange that we would > have a more explicit alert than the generic illegal_parameter, that > seems to precisely describe the scenario (a fallback occurred, and it > turns out it was inappropriate) but not be able to use it.
Aren't the semantics here "whoops, I made a new connection attempt that I shouldn't have; let me go back out of that"? In which case one could argue even for close_notify... -Ben _______________________________________________ TLS mailing list TLS@ietf.org https://www.ietf.org/mailman/listinfo/tls
