Subject: Re: [bitcoin-dev] Making Electrum more anonymous

From: Eric Voskuil <[email-masked]>
Date: Thu, 23 Jul 2015 19:26:28 -0700

I've looked at IT-PIR for Libbitcoin. It's certainly more elegant than
onion routing for query privacy, but doesn't improve on the collusion
problem. As a result the related directory problem would also remain.

"This protocol sacrifices some level of privacy to gain robustness.
Because of this  we need to assume that there is no collusion between
some number of servers. In some  settings, it is unclear how this
requirement can be enforced or detected. This uncertainty  may make this
protocol less desirable than others with different privacy guarantees."

http://www.cypherpunks.ca/~iang/pubs/hybridpir-pets.pdf

From the same source, regarding aforementioned options:

"An assumption used in many PETs, including mix networks, secret
sharing, onion routing and some voting protocols, is that no more than
some threshold of agents are colluding against the user to discover the
private information."

Another option is computationally-bounded CPIR, but from the same source:

"The main problem with the CPIR protocols already  discussed is that
they do not generally perform queries faster than the trivial protocol."

Where the "trivial protocol" in our problem is full blockchain download.

WRT an alternative CPIR proposed in 2007:

"The idea behind their protocol is to add noise to the query in a way
that the server  cannot discover which record the client is interested
in, but with the secret information  that the client has, she can remove
the noise from the server’s response."

This is the idea behind stealth prefix queries:

https://github.com/libbitcoin/libbitcoin-explorer/wiki/bx-fetch-stealth

From our perspective, another important objective of query privacy is
allowing the caller make the trade-off between the relative levels of
privacy and performance - from absolute to non-existent. In some cases
privacy is neither required nor desired.

Prefix filtering accomplishes the client-tuning objective. It also does
not suffer server collusion attacks nor is it dependent on computational
bounds. The primary trade-off becomes result set (download) size against
privacy.

e

On 07/23/2015 05:23 AM, Stefan Richter wrote:
> This looks like a prime application for this PIR
> library: http://percy.sourceforge.net/apidocs/index.html
> 
> Eric Voskuil via bitcoin-dev <[email-masked]
> <mailto:[email-masked]>> schrieb am Do., 23. Juli
> 2015 um 02:07 Uhr:
> 
>     This is a good point. I didn't delve into the specifics of
>     implementation due to the larger issues that I raised. Libbitcoin Server
>     uses CurveZMQ, an implementation of CurveCP.
> 
>     http://curvecp.org
>     http://curvezmq.org
>     https://en.bitcoin.it/wiki/Libbitcoin_Server
> 
>     e
> 
>     On 07/22/2015 04:11 PM, gb via bitcoin-dev wrote:
>     > Why RSA?
>     >
>     >>
>     >> Here is an idea, inspired by TOR, on which I would like to have some
>     >> feedback: We create an anonymous routing layer between Electrum
>     servers
>     >> and clients.
>     >>
>     >> * Each server S publishes a RSA public key, KS
>     >> * Each client receives a list of available servers and their pubkeys
>     >> * For each wallet address, addr_i, a client chooses a server S_i,
>     and a
>     >> RSA keypair (K_addr_i, k_addr_i)
>     >> * The client creates a list of encrypted requests. Each request
>     contains
>     >> addr_i and K_addr_i, and is encrypted with the pubkey KS_i of S_i
>     >> * The client chooses a main server M, and sends the list of encrypted
>     >> requests to M
>     >> * M dispatches the client's requests to the corresponding servers S_i
>     >> (without the client's IP address.)
>     >> * Each server decrypts the requests it receives, performs the
>     request,
>     >> and encrypts the result with K_addr_i
>     >> * M receives encrypted responses, and forwards them to the client.
>     >> * The client decrypts the encrypted response with k_addr_i
> 
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