[tbb-dev] Documenting best practices for P2P Sybil avoidance over Tor

[Jeremy Rand]

Hi Applications Team!

I would like to propose the following addendum to the SOCKS username 
section of the Tor-Friendly Applications Best Practices:

"If your application needs to open a small number of connections (e.g. 
10 long-lived connections) to a P2P network, and you want to prevent 
Sybil attacks, you should seriously consider using a unique SOCKS5 
username per connection (e.g. by including a new randomly generated 
string in the username each time a connection is opened), which will 
minimize the chance of a malicious exit relay interfering with your view 
of the P2P network.  For example, Bitcoin Core does this.  On the other 
hand, if your application intends to open a very large number of 
connections, you should probably not do this, as it will put too much 
load on the Tor network.  For example, Bitcoin DNS seeders should not do 
this while spidering P2P nodes."

I think this is probably uncontroversial advice within the Tor community 
(I think the Tor devs are aware of Bitcoin Core's behavior and haven't 
asked the Bitcoin Core team to change it), but it is not necessarily 
obvious to application developers who may be unfamiliar with Tor, so I 
think it's worth documenting.  Please let me know if this text is okay 
to add (or if there's anything that can be improved); I don't want to 
step on toes by adding this without consulting anyone.

Cheers,
-- 
-Jeremy Rand
Lead Application Engineer at Namecoin
Mobile email: jeremyrandmobile at airmail.cc
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My business email jeremy at veclabs.net is having technical issues at the 
moment.

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