>Perfect privacy without extraordinary computational overhead today means downloading everything. But we could provide better tools to *shift* bandwidth requirements rather than try to reduce them.
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> I think we need to provide users with better options than that.
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I've been thinking about a setup where user runs a UTXO only, and maybe even outbound-connect only (like bitcoinj), full node at home. Then using Tor, mostly for tunneling, they host a hidden service they can connect back to from their smartphone to see balances, manage receive addresses, send funds, etc.
The smartphone is not doing SPV, it's like a web client for the wallet running at home. The initial connection between the smartphone and home wallet has the phone learn two codes, one is the hidden service name, another is an access token which is revocable. You may require further authentication from that point.
With fast bootstrapping / checkpointing of the UTXO I think usability could be as good as SPV, and you would get push-notification of relevant transactions with zero privacy trade-off.
I wonder if people would want to run such an app, if they would run it on their desktop, a dedicated machine, or an old smartphone or other cheap ARM device.