[Doh] DoH discovery security goals

[nusenu <[e-mail]>]

> If you want to update RFC 7858, that's great, but this would all be
> work for the DPRIVE WG, which is the one that standardized DoT.
 
Yes I'm aware, I just replied to this question in Ben's email since he asked here.

The rest of the email was related to the current 
Resolver-Associated DoH draft:

>> An active adversary "in the middle" can fully substitute itself for a Do53
>> resolver without alerting the user.  Do you see an achievable defense
>> against downgrade attacks, or is this not part of your threat model?
> 
> Good point, lets define a threat model first:
> 
> Consider an adversary that has the following capabilities:
> 
> - can manipulate and read any network traffic from the user (including DHCP)
> 
> - the adversary's capabilities are limited to some networks
> (i.e. public wifis) but he has no global capabilities
> 
> - the attacker can _not_ issue trusted certificates for arbitrary IP addresses
> - the adversary has _no_ access to the client system
>  
> security goals:
> if the victim uses DHCP for Do53 discovery:
>  - prevent attacks that impact the victim's DoH server URI
> beyond the visit in an adversary-controlled network (i.e. public wifis)
> 
> if the client is configured to use a static Do53 server (no DHCP for DNS server discovery used):
>  - prevent the adversary from feeding the victim with his DoH URI
>  - prevent a silent downgrade attack that makes the victim believe
> a previously discovered DoH server does no longer speak DoH (to force the victim to plain Do53)
> 
> As outlined in [1] I believe these goals are currently not met yet and an adversary can feed a victim
> his DoH server URI even beyond the victim's visit in the adversary controlled network.
> It would be great if we could improve this (I think we can).

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