Re: [TLS] xLS 1.3: cookie
Eric Rescorla <ekr@rtfm.com> Wed, 29 March 2017 14:30 UTC
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From: Eric Rescorla <ekr@rtfm.com>
Date: Wed, 29 Mar 2017 09:29:58 -0500
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To: Mark Dunn <mark.dunn@objectiveintegration.uk>
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Subject: Re: [TLS] xLS 1.3: cookie
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Hi Mark, Thanks for your note. Some comments below... On Wed, Mar 29, 2017 at 8:10 AM, Mark Dunn < mark.dunn@objectiveintegration.uk> wrote: > I am trying to implement cookie and finding it a little underspecified. > > I am using the TLS 1.3 specified in github and > draft-rescorla-tls-dtls13-01 > > > 1a) Should a client expect to respond to a cookie during session > resumption? > Yes, it has to be ready to. As you say, it kills 0-RTT, but that's how HRR always behaves. > > 1b) If (1a) is false then do you agree that the "cookie" extension MUST > be accompanied by the "key_share" extension? Otherwisewe might be faced with > > ClientHello > > +key_share > > +... -----> > > HelloRetryRequest > > <-----+cookie > > ClientHello > > +cookie > > +key_share > > +... -----> > > HelloRetryRequest > > <----- +key_share > > ClientHello > > +key_share > > +... -----> > > ServerHello > > +key_share > > +... > If I understand you correctly, then I think this is wrong. You only send key_share to correct the client's key_share, so if the client sent a key_share but you send cookie to force a round-trip, then you don't send key_share, just cookie. > 2) I understand this is too late for TLS(only DTLS SHOULD use cookies) > but is there a better solution than a cookie? > This seems like it would be a big restructure of the handshake, and given the relatively modest use of client auth in many contexts, I don't think it would probably be worth it. Best, -Ekr > For authenticatedclients a botnet DOS attack could be mitigated by > authenticating the clientfirst. > > In the worst case where the client provided an unacceptable key_share the > Server would have to provide > > ClientHello // flight 1 > > +key_share > > +signature_algorithms > > +supported_groups > > +... -----> > > HelloRetryRequest // flight 2 > > +cookie // from DTLS 1.2: HMAC(secret, > Client-IP, Client-Parameters) > > +key_share(server_share) > > +signature_algorithms // not required as > supplied by client > > +supported_groups // not required as > supplied by client > > <----- {CertificateRequest} // implied when > key_share(server_share) is used > > ClientHello // flight 3 > > +key_share > > +cookie // this may provide state information to the server > > +... > > {Cetrificate} > > {CertificateVerify} > > {finished} // provides reachability for the client first and is > protected by server_handshake_traffic_secret > > [application data] -----> // protected using keys derived from > traffic_secret_N > > ServerHello // flight 4 > > {EncryptedExtensions} > > {Certificate} > > {CertificateVerify} > > {Finished} // provides reachability for > the server > > <----- [application data] > > > > For authenticating clients, this would provide better defense against DOS, > DOS amplification and require fewer round trips. > > > > > > _______________________________________________ > TLS mailing list > TLS@ietf.org > https://www.ietf.org/mailman/listinfo/tls >
- Re: [TLS] [xLS 1.3: cookie] - DTLS queries Mark Dunn
- Re: [TLS] [xLS 1.3: cookie] - DTLS queries Hannes Tschofenig
- Re: [TLS] [xLS 1.3: cookie] - DTLS queries Benjamin Kaduk
- Re: [TLS] [xLS 1.3: cookie] - DTLS queries Hannes Tschofenig
- Re: [TLS] xLS 1.3: cookie Eric Rescorla
- [TLS] xLS 1.3: cookie Mark Dunn