Re: [TLS] Publication of draft-rhrd-tls-tls13-visibility-00

Stephen Farrell <stephen.farrell@cs.tcd.ie> Fri, 13 October 2017 12:46 UTC

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To: Hubert Kario <hkario@redhat.com>
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From: Stephen Farrell <stephen.farrell@cs.tcd.ie>
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Subject: Re: [TLS] Publication of draft-rhrd-tls-tls13-visibility-00
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On 13/10/17 12:05, Hubert Kario wrote:
> On Thursday, 12 October 2017 15:16:08 CEST Stephen Farrell wrote:
>> (With the obvious caveat that I hate the whole
>> idea... :-)
> 
> to be clear: me too

IMO the more we hear of that the better

> 1. Alice sends a share to Bob: g^a
> 2. Bob sends Alice's and his share to Carol: g^a, g^b, g^ab
> 3. Carol replies to Bob with her share added to Alice's and his: g^ac, g^bc
> 4. Bob sends the Carol's reply to Alice as a Server Key Share: g^bc
> 5. Alice calculates the shared secret g^bca
> 6. Bob calculates the shared secret: g^acb
> 7. Carol calculates the shared secret: g^abc
> 
> so it doesn't look to me like it requires a lot of chamfer to fit that square 
> peg in the round hole, only the 2 and 3 need to happen out-of band.
> 
> of course, I haven't analysed how Carol would be authenticated in that 
> communication (if signing just the SKS by Carol is enough, transferred in the 
> encrypted extensions, with server signature of the handshake in certificate 
> verify being sufficient for integrity)

So the problems with that are numerous but include:

- there can be >1 carol, (and maybe all the carols also need to
  "approve" of one another), if we were crazy enough to try do
  this we'd have at least:
      - corporate outbound snooper
      - data-centre snooper (if you buy those supposed use-cases)
      - government snooper(s) in places where they don't care about
        doing that openly
  ...port 80 would suddenly be quicker than 443 again;-(
- carol is quite likely to only have a name like: 2001:db8::bad:1dea
  or your.friendly-listener.bigcdn.example.net and authenticating
  those is essentially meaningless to the endpoints in most TLS contexts
  whether or not those endpoints have humans associated with them
- the TLS endpoints can't handle the semantics of allowing in Carol(s)
  as those endpoints were designed to use TLS and not bolloxed-TLS (be
  that mcTLS or draft-rehired)

So I think this ends up as bad as the design in draft-rehired. Which
of them is more obviously bad is another question.

Cheers,
S.