[freenet-dev] Short refs was Re: alternative to #freenet-refs

[Matthew Toseland]

On Sunday 18 November 2007 12:10, Michael Rogers wrote:
> Matthew Toseland wrote:
> > We're assuming different attack models here. Both are valid for certain 
> > assumptions.
> 
> Fair point - I accept that if you assume the exchange is unobservable, a
> one-way invite can provide mutual authentication. But IMHO we shouldn't
> make that assumption - eavesdropping is known to be widespread, whereas
> at least we're still in the dark about the extent of active MITM. ;-)

We cannot assume there is no MITM either! My point is at least that one side 
must be untamperable, and we cannot assume this if both are sent by instant 
message. The only way to be sure is to do *something* out of band (encrypted 
mail, face to face exchange, phone call on certain assumptions, etc), whether 
that be a post connect out-of-band password or fingerprint exchange, or 
sending the invite out of band in the first place. *Something* must occur out 
of band, because MITM is trivial if you work at the ISP, or are otherwise 
closer to Alice than Bob is. Our only other option is to assume that the 
attacker will not be able to identify the noderef quickly enough to 
substitute it or connect to it.

Protocol questions:

Can we make a two-way exchange safe when only one side is out of band? Even if 
we don't know which side it is? Do we need to specify it? Hoping that one 
side is OOB is probably a false sense of security...

How easy can we make out of band post connect verification? E.g. exchange 
invites, then each node generates a short password which is exchanged out of 
band e.g. over the phone with a small maximum number of attempts? (Making 
certain assumptions about the state of voice recognition...)

Is it reasonable to offer (advanced?) users the choice of indicating that an 
invite is unobservable because they sent it by encrypted email, delivered it 
by hand etc?

> Cheers,
> Michael
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