[bitcoin-dev] Making Electrum more anonymous

[Joseph Gleason ⑈]

I've thought about this as well, (in addition to making a java
implementation of the electrum server).

One solution would be for the client to combine all the addresses they are
interested in into a single bloom filter and send that to the server.

It would be pretty expensive for the server to check every address against
the bloom filter, but maybe for recent blocks (client can send how behind
they are) and for new transactions it wouldn't be bad at all.

The client could end up receiving a bunch of transactions they weren't
interested in but it would likely be manageable.

The client would also need to be fairly clever, including a set of static
ruse addresses and dynamic ruse addresses that they include in the filter.
That way a server would have a hard time using the bloom filter as a
fingerprint and figuring out which addresses are the real ones.

An alternative would be the server sending a bloom filter of addresses in
each block and then the client requesting entire blocks.  This would use
more bandwidth, but it seems like it would be pretty simple to implement
and give good anonymity.

Basically the idea is to spend more bandwidth and CPU to keep the server in
the dark on what the client really wants.

On Wed, Jul 22, 2015 at 8:51 AM Thomas Voegtlin via bitcoin-dev <
bitcoin-dev at lists.linuxfoundation.org> wrote:

> Hello,
>
> Although Electrum clients connect to several servers in order to fetch
> block headers, they typically request address balances and address
> histories from a single server. This means that the chosen server knows
> that a given set of addresses belong to the same wallet. That is true
> even if Electrum is used over TOR.
>
> There have been various proposals to improve on that, but none of them
> really convinced me so far. One recurrent proposal has been to create
> subsets of wallet addresses, and to send them to separate servers. In my
> opinion, this does not really improve anonymity, because it requires
> trusting more servers.
>
> 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
>
> What do you think? What are the costs and benefits of such an approach?
>
> (Note: this will not work if all servers, or a large fraction of them,
> are controlled by the same entity that controls M)
>
>
> Thomas
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