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Merge pull request #7822

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99bee1c Apply gamma distr from chain tip when selecting decoys (j-berman)
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luigi1111 2021-08-26 21:08:56 -04:00
commit ca1b1b7332
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src/wallet/wallet2.cpp
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@ -142,6 +142,9 @@ using namespace cryptonote;
#define IGNORE_LONG_PAYMENT_ID_FROM_BLOCK_VERSION 12
#define DEFAULT_UNLOCK_TIME (CRYPTONOTE_DEFAULT_TX_SPENDABLE_AGE * DIFFICULTY_TARGET_V2)
#define RECENT_SPEND_WINDOW (50 * DIFFICULTY_TARGET_V2)
static const std::string MULTISIG_SIGNATURE_MAGIC = "SigMultisigPkV1";
static const std::string MULTISIG_EXTRA_INFO_MAGIC = "MultisigxV1";
@ -1037,6 +1040,34 @@ uint64_t gamma_picker::pick()
{
double x = gamma(engine);
x = exp(x);
if (x > DEFAULT_UNLOCK_TIME)
{
// We are trying to select an output from the chain that appeared 'x' seconds before the
// current chain tip, where 'x' is selected from the gamma distribution recommended in Miller et al.
// (https://arxiv.org/pdf/1704.04299/).
// Our method is to get the average time delta between outputs in the recent past, estimate the number of
// outputs 'n' that would have appeared between 'chain_tip - x' and 'chain_tip', select the real output at
// 'current_num_outputs - n', then randomly select an output from the block where that output appears.
// Source code to paper: https://github.com/maltemoeser/moneropaper
//
// Due to the 'default spendable age' mechanic in Monero, 'current_num_outputs' only contains
// currently *unlocked* outputs, which means the earliest output that can be selected is not at the chain tip!
// Therefore, we must offset 'x' so it matches up with the timing of the outputs being considered. We do
// this by saying if 'x` equals the expected age of the first unlocked output (compared to the current
// chain tip - i.e. DEFAULT_UNLOCK_TIME), then select the first unlocked output.
x -= DEFAULT_UNLOCK_TIME;
}
else
{
// If the spent time suggested by the gamma is less than the unlock time, that means the gamma is suggesting an output
// that is no longer feasible to be spent (possible since the gamma was constructed when consensus rules did not enforce the
// lock time). The assumption made in this code is that an output expected spent quicker than the unlock time would likely
// be spent within RECENT_SPEND_WINDOW after allowed. So it returns an output that falls between 0 and the RECENT_SPEND_WINDOW.
// The RECENT_SPEND_WINDOW was determined with empirical analysis of observed data.
x = crypto::rand_idx(static_cast<uint64_t>(RECENT_SPEND_WINDOW));
}
uint64_t output_index = x / average_output_time;
if (output_index >= num_rct_outputs)
return std::numeric_limits<uint64_t>::max(); // bad pick