wallet: Avoid leaking nLockTime fingerprint when anti-fee-sniping #15039

2612 | @@ -2599,8 +2613,15 @@ bool CWallet::CreateTransaction(interfaces::Chain::Lock& locked_chain, const std
2613 |      if (GetRandInt(10) == 0)
2614 |          txNew.nLockTime = std::max(0, (int)txNew.nLockTime - GetRandInt(100));
2615 |  
2616 | +    // If our chain is lagging behind, we can't discourage fee sniping nor help
2617 | +    // the privacy of high-latency transactions. To avoid leaking a potentially
2618 | +    // unique "nLockTime fingerprint", set nLockTime to a constant.
2619 | +    if (!IsCurrentForAntiFeeSniping()) txNew.nLockTime = 0;

luke-jr commented at 10:37 PM on December 26, 2018:

Why not just skip the above setting it?

2548 | +{
2549 | +    if (IsInitialBlockDownload()) {
2550 | +        return false;
2551 | +    }
2552 | +    constexpr int64_t MAX_ANTI_FEE_SNIPING_TIP_AGE = 8 * 60 * 60; // in seconds
2553 | +    if (chainActive.Tip()->GetBlockTime() < (GetTime() - MAX_ANTI_FEE_SNIPING_TIP_AGE)) {

Empact commented at 12:27 AM on December 27, 2018:

nit: could return this directly (inverted)

2515 | @@ -2516,6 +2516,65 @@ bool CWallet::FundTransaction(CMutableTransaction& tx, CAmount& nFeeRet, int& nC
2516 |      return true;
2517 |  }
2518 |  
2519 | +static bool IsCurrentForAntiFeeSniping(interfaces::Chain::Lock& locked_chain)
2520 | +{
2521 | +    if (IsInitialBlockDownload()) {
2522 | +        return false;
2523 | +    }
2524 | +    constexpr int64_t MAX_ANTI_FEE_SNIPING_TIP_AGE = 8 * 60 * 60; // in seconds

meshcollider commented at 10:45 AM on January 3, 2019:

Any rationale on why 8 hours was chosen? Seems sane though

2560 | +
2561 | +        // Secondly occasionally randomly pick a nLockTime even further back, so
2562 | +        // that transactions that are delayed after signing for whatever reason,
2563 | +        // e.g. high-latency mix networks and some CoinJoin implementations, have
2564 | +        // better privacy.
2565 | +        if (GetRandInt(10) == 0)

laanwj commented at 8:30 AM on January 4, 2019:

This if needs {} but I understand if you prefer to keep this move-only.