end-to-end encryption question

[Scott Bennett]

     On Thu, 13 Sep 2007 11:59:25 -0400 Nick Mathewson <nickm at freehaven.net>
wrote:
>  [Lines re-wrapped.]
>
>On Thu, Sep 13, 2007 at 05:06:54AM -0500, Scott Bennett wrote:
>>      On Thu, 13 Sep 2007 11:46:33 +0200 Peter Palfrader <peter at palfrader.=
>org>
>> wrote:
>> >On Thu, 13 Sep 2007, Scott Bennett wrote:
>> >
>> >>      For obvious reasons, tor should not be getting directory informat=
>ion over
>> >> a connection that is not encrypted from end to end, even if everyone k=
>nows
>> >> exactly what the content of the directory information happens to be at=
> any
>> >> given moment.
>> >
>> >What are those reasons?  I'm sorry they are not apparent to me.
>> >
>
>>      Well, when I wrote that, one thing that was worrying me was a
>> MITM attack.  After sending it, I remembered that the directory
>> information would be signed by an authority, which should
>> infinitesimalize any chance of a MITM corruption of the information
>> going undetected.  But that still leaves open a somewhat less
>> dangerous situation in which the MITM always damages the information
>> or interferes with the connection somehow, creating a form of DoS,
>> so that the recipient cannot obtain valid directory information.
>> That would be a type of DoS that would not, for example, trigger any
>> alarms in a router or other typical network monitor.
>
>Any MITM that can alter unencrypted data in order to make it unusable
>can also alter encrypted data in order to make it unusable, surely?
>
     Yes.  However, in the case of end-to-end encryption, the attacker
would first have to have a way to know with which packets to tamper,
which would mean the attacker would have to have cracked the tor system
entirely.  If the data in question travel all the way to their
destination through a tor circuit, then they are encrypted, mingled
with other tor traffic, and not obviously distinguishable from any
other packets traversing the tor network.


                                  Scott Bennett, Comm. ASMELG, CFIAG
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