[bittorrent] Friend-machines-assisted approach to boosting one's BT download

Nikitas Liogkas nikitas at CS.UCLA.EDU
Sun Jun 11 20:32:58 EDT 2006

I would agree with Harold on this matter. The described attack is not 
viable, since most modern clients will ban peers sending corrupt data. 
Even when subpieces of the same piece are downloaded from different 
peers, the culprit can still be identified by using statistical 
measures, e.g. i got this corrupt piece from peers foo and bar, and 
this corrupt piece from peers foo and baz, so peer foo is probably 
bad. The damage done will not be much, since the malicious peer will 
be banned pretty quickly and for a long time.
For more details, and an experimental validation of the 
non-effectiveness of this attack, see:

As for the non-ability to verify a block, it is not IMHO a weakness, 
but rather a smart engineering trade-off between performance (due to 
pipelining) and security.


----- Original Message ----- 
From: "Harold Feit" <dwknight at depthstrike.com>
To: "Elliott Mitchell" <ehem at m5p.com>; <bittorrent at lists.ibiblio.org>
Sent: Sunday, June 11, 2006 3:31 PM
Subject: Re: [bittorrent] Friend-machines-assisted approach to 
boosting one's BT download

>> One method is to advertise posession of all pieces, but ensure no 
>> one
>> ever downloads more than one block from you. By corrupting one 
>> block of
>> every piece, the clients will have to discard 224KB of data merely 
>> for
>> 32KB of bandwidth on your part. Oh, make that discard 240KB of data 
>> for
>> merely 16KB of bandwidth, the smaller block makes this attack a lot 
>> more
>> efficient. Magnifying your effecting corrupting bandwidth by a 
>> factor of
>> 15 is pretty powerful. The problem here being that you cannot 
>> immediately
>> verify every block you download.

> I've seen this attack.
> Most implementations of this attack come from a single /16 or 
> smaller
> subnet that can be trivially blocked with an ip blocker application.
> Additionally, the long-term effectiveness of such an attack against 
> a
> given swarm is almost nonexistent due to the automatic banning
> algorithms already in place in modern mainstream clients.

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