Subject: Re: [bitcoin-dev] Improving JoinMarket's resistance to sybil attacks using fidelity bonds

From: "David A. Harding" <[email-masked]>
Date: Sat, 27 Jul 2019 09:34:17 -1000

On Thu, Jul 25, 2019 at 12:47:54PM +0100, Chris Belcher via bitcoin-dev wrote:
> A way to create a fidelity bond is to burn an amount of bitcoins by
> sending to a OP_RETURN output. Another kind is time-locked addresses
> created using OP_CHECKLOCKTIMEVERIFY where the valuable thing being
> sacrificed is time rather than money, but the two are related because of
> the time-value-of-money.

Timelocking bitcoins, especially for long periods, carries some special
risks in Bitcoin:

1. Inability to sell fork coins, also creating an inability to influence
the price signals that help determine the outcome of chainsplits.

2. Possible inability to transition to new security mechanisms if
a major weakness is discovered in ECC or a hash function.

An alternative to timelocks might be coin age---the value of a UTXO
multiplied by the time since that UTXO was confirmed.  Coin age may be
even harder for an attacker to acquire given that it is a measure of
past patience rather than future sacrifice.  It also doesn't require
using any particular script and so is flexible no matter what policy the
coin owner wants to use (especially if proof-of-funds signatures are
generated using something like BIP322).

Any full node (archival or pruned) can verify coin age using the UTXO
set.[1]  Unlike script-based timelock (CLTV or CSV), there is no current
SPV-level secure way to prove to lite clients that an output is still
unspent, however such verification may be possible within each lite
client's own security model related to transaction withholding attacks:

- Electrum-style clients can poll their server to see if a particular
  UTXO is unspent.

- BIP158 users who have saved their past filters to disk can use them to
  determine which blocks subsequent to the one including the UTXO may
  contain a spend from it.  However, since a UTXO can be spent in the
  same block, they'd always need to download the block containing the
  UTXO (alternatively, the script could contain a 1-block CSV delay
  ensuring any spend occurred in a later block).  If BIP158 filters
  become committed at some point, this mechanism is upgraded to SPV-level
  security.

> Note that a long-term holder (or hodler) of bitcoins can buy time-locked
> fidelity bonds essentially for free, assuming they never intended to
> transact with their coins much anyway.

This is the thing I most like about the proposal.  I suspect most
honest makers are likely to have only a small portion of their funds
under JoinMarket control, with the rest sitting idle in a cold wallet.
Giving makers a way to communicate that they fit that user template
would indeed seem to provide significant sybil resistance.

-Dave

[1] See, bitcoin-cli help gettxout