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Clean decoy selection

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This commit is contained in:
Luke Parker 2024-07-08 02:38:01 -04:00
parent d7f7f69738
commit b744ac9a76
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2 changed files with 114 additions and 163 deletions
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5
coins/monero/rpc/src/lib.rs
View file

@ -910,7 +910,10 @@ pub trait Rpc: Sync + Clone + Debug {
}
let res: SendRawResponse = self
.rpc_call("send_raw_transaction", Some(json!({ "tx_as_hex": hex::encode(tx.serialize()) })))
.rpc_call(
"send_raw_transaction",
Some(json!({ "tx_as_hex": hex::encode(tx.serialize()), "do_sanity_checks": false })),
)
.await?;
if res.status != "OK" {

270
coins/monero/wallet/src/decoys.rs
View file

@ -1,5 +1,3 @@
// TODO: Clean this
use std_shims::{io, vec::Vec, collections::HashSet};
use zeroize::{Zeroize, ZeroizeOnDrop};
@ -13,7 +11,7 @@ use curve25519_dalek::{Scalar, EdwardsPoint};
use crate::{
DEFAULT_LOCK_WINDOW, COINBASE_LOCK_WINDOW, BLOCK_TIME,
primitives::Commitment,
primitives::{Commitment, Decoys},
rpc::{RpcError, Rpc},
output::OutputData,
WalletOutput,
@ -24,45 +22,78 @@ const BLOCKS_PER_YEAR: usize = 365 * 24 * 60 * 60 / BLOCK_TIME;
#[allow(clippy::cast_precision_loss)]
const TIP_APPLICATION: f64 = (DEFAULT_LOCK_WINDOW * BLOCK_TIME) as f64;
#[allow(clippy::too_many_arguments)]
async fn select_n<'a, R: RngCore + CryptoRng>(
rng: &mut R,
async fn select_n(
rng: &mut (impl RngCore + CryptoRng),
rpc: &impl Rpc,
distribution: &[u64],
height: usize,
high: u64,
per_second: f64,
real: &[u64],
used: &mut HashSet<u64>,
count: usize,
real_output: u64,
ring_len: usize,
fingerprintable_canonical: bool,
) -> Result<Vec<(u64, [EdwardsPoint; 2])>, RpcError> {
// TODO: consider removing this extra RPC and expect the caller to handle it
if fingerprintable_canonical && (height > rpc.get_height().await?) {
// TODO: Don't use InternalError for the caller's failure
Err(RpcError::InternalError("decoys being requested from too young blocks".to_string()))?;
if height < DEFAULT_LOCK_WINDOW {
Err(RpcError::InternalError("not enough blocks to select decoys".to_string()))?;
}
if height > rpc.get_height().await? {
Err(RpcError::InternalError(
"decoys being requested from blocks this node doesn't have".to_string(),
))?;
}
#[cfg(test)]
let decoy_count = ring_len - 1;
// Get the distribution
let distribution = rpc.get_output_distribution(.. height).await?;
let highest_output_exclusive_bound = distribution[distribution.len() - DEFAULT_LOCK_WINDOW];
// This assumes that each miner TX had one output (as sane) and checks we have sufficient
// outputs even when excluding them (due to their own timelock requirements)
// Considering this a temporal error for very new chains, it's sufficiently sane to have
if highest_output_exclusive_bound.saturating_sub(u64::try_from(COINBASE_LOCK_WINDOW).unwrap()) <
u64::try_from(decoy_count).unwrap()
{
Err(RpcError::InternalError("not enough decoy candidates".to_string()))?;
}
// Determine the outputs per second
#[allow(clippy::cast_precision_loss)]
let per_second = {
let blocks = distribution.len().min(BLOCKS_PER_YEAR);
let initial = distribution[distribution.len().saturating_sub(blocks + 1)];
let outputs = distribution[distribution.len() - 1].saturating_sub(initial);
(outputs as f64) / ((blocks * BLOCK_TIME) as f64)
};
// Don't select the real output
let mut do_not_select = HashSet::new();
do_not_select.insert(real_output);
let decoy_count = ring_len - 1;
let mut res = Vec::with_capacity(decoy_count);
let mut iters = 0;
let mut confirmed = Vec::with_capacity(count);
// Retries on failure. Retries are obvious as decoys, yet should be minimal
while confirmed.len() != count {
let remaining = count - confirmed.len();
// TODO: over-request candidates in case some are locked to avoid needing
// round trips to the daemon (and revealing obvious decoys to the daemon)
// Iterates until we have enough decoys
// If an iteration only returns a partial set of decoys, the remainder will be obvious as decoys
// to the RPC
// The length of that remainder is expected to be minimal
while res.len() != decoy_count {
let remaining = decoy_count - res.len();
let mut candidates = Vec::with_capacity(remaining);
while candidates.len() != remaining {
#[cfg(test)]
{
// Ensure this isn't infinitely looping
iters += 1;
// This is cheap and on fresh chains, a lot of rounds may be needed
if iters == 100 {
#[cfg(not(test))]
const MAX_ITERS: usize = 10;
// When testing on fresh chains, increased iterations can be useful and we don't necessitate
// reasonable performance
#[cfg(test)]
const MAX_ITERS: usize = 100;
if iters == MAX_ITERS {
Err(RpcError::InternalError("hit decoy selection round limit".to_string()))?;
}
}
// Use a gamma distribution
// Use a gamma distribution, as Monero does
// TODO: Cite these constants
let mut age = Gamma::<f64>::new(19.28, 1.0 / 1.61).unwrap().sample(rng).exp();
#[allow(clippy::cast_precision_loss)]
if age > TIP_APPLICATION {
@ -74,16 +105,19 @@ async fn select_n<'a, R: RngCore + CryptoRng>(
#[allow(clippy::cast_sign_loss, clippy::cast_possible_truncation)]
let o = (age * per_second) as u64;
if o < high {
let i = distribution.partition_point(|s| *s < (high - 1 - o));
if o < highest_output_exclusive_bound {
// Find which block this points to
let i = distribution.partition_point(|s| *s < (highest_output_exclusive_bound - 1 - o));
let prev = i.saturating_sub(1);
let n = distribution[i] - distribution[prev];
if n != 0 {
// Select an output from within this block
let o = distribution[prev] + (rng.next_u64() % n);
if !used.contains(&o) {
// It will either actually be used, or is unusable and this prevents trying it again
used.insert(o);
if !do_not_select.contains(&o) {
candidates.push(o);
// This output will either be used or is unusable
// In either case, we should not try it again
do_not_select.insert(o);
}
}
}
@ -92,47 +126,39 @@ async fn select_n<'a, R: RngCore + CryptoRng>(
// If this is the first time we're requesting these outputs, include the real one as well
// Prevents the node we're connected to from having a list of known decoys and then seeing a
// TX which uses all of them, with one additional output (the true spend)
let mut real_indexes = HashSet::with_capacity(real.len());
if confirmed.is_empty() {
for real in real {
candidates.push(*real);
}
let real_index = if iters == 0 {
candidates.push(real_output);
// Sort candidates so the real spends aren't the ones at the end
candidates.sort();
for real in real {
real_indexes.insert(candidates.binary_search(real).unwrap());
}
}
Some(candidates.binary_search(&real_output).unwrap())
} else {
None
};
// TODO: make sure that the real output is included in the response, and
// that mask and key are equal to expected
for (i, output) in rpc
.get_unlocked_outputs(&candidates, height, fingerprintable_canonical)
.await?
.iter_mut()
.enumerate()
{
// Don't include the real spend as a decoy, despite requesting it
if real_indexes.contains(&i) {
// We could check the returned info is equivalent to our expectations, yet that'd allow the
// node to malleate the returned info to see if they can cause this error (allowing them to
// figure out the output being spent)
//
// Some degree of this attack (forcing resampling/trying to observe errors) is likely
// always possible
if real_index == Some(i) {
continue;
}
// If this is an unlocked output, push it to the result
if let Some(output) = output.take() {
confirmed.push((candidates[i], output));
res.push((candidates[i], output));
}
}
}
Ok(confirmed)
}
fn offset(ring: &[u64]) -> Vec<u64> {
let mut res = vec![ring[0]];
res.resize(ring.len(), 0);
for m in (1 .. ring.len()).rev() {
res[m] = ring[m] - ring[m - 1];
}
res
Ok(res)
}
async fn select_decoys<R: RngCore + CryptoRng>(
@ -141,135 +167,57 @@ async fn select_decoys<R: RngCore + CryptoRng>(
ring_len: usize,
height: usize,
input: &WalletOutput,
// TODO: Decide "canonical" or "deterministic" (updating RPC terminology accordingly)
fingerprintable_canonical: bool,
) -> Result<Decoys, RpcError> {
let mut distribution = vec![];
let decoy_count = ring_len - 1;
// Convert the inputs in question to the raw output data
let mut real = vec![input.relative_id.index_on_blockchain];
let output = (real[0], [input.key(), input.commitment().calculate()]);
if distribution.len() < height {
// TODO: verify distribution elems are strictly increasing
let extension = rpc.get_output_distribution(distribution.len() .. height).await?;
distribution.extend(extension);
}
// If asked to use an older height than previously asked, truncate to ensure accuracy
// Should never happen, yet risks desyncing if it did
distribution.truncate(height);
if distribution.len() < DEFAULT_LOCK_WINDOW {
Err(RpcError::InternalError("not enough blocks to select decoys".to_string()))?;
}
#[allow(clippy::cast_precision_loss)]
let per_second = {
let blocks = distribution.len().min(BLOCKS_PER_YEAR);
let initial = distribution[distribution.len().saturating_sub(blocks + 1)];
let outputs = distribution[distribution.len() - 1].saturating_sub(initial);
(outputs as f64) / ((blocks * BLOCK_TIME) as f64)
};
let mut used = HashSet::<u64>::new();
used.insert(output.0);
// TODO: Create a TX with less than the target amount, as allowed by the protocol
let high = distribution[distribution.len() - DEFAULT_LOCK_WINDOW];
// This assumes that each miner TX had one output (as sane) and checks we have sufficient
// outputs even when excluding them (due to their own timelock requirements)
if high.saturating_sub(u64::try_from(COINBASE_LOCK_WINDOW).unwrap()) <
u64::try_from(ring_len).unwrap()
{
Err(RpcError::InternalError("not enough decoy candidates".to_string()))?;
}
// Select all decoys for this transaction, assuming we generate a sane transaction
// We should almost never naturally generate an insane transaction, hence why this doesn't
// bother with an overage
let mut decoys = select_n(
let decoys = select_n(
rng,
rpc,
&distribution,
height,
high,
per_second,
&real,
&mut used,
decoy_count,
input.relative_id.index_on_blockchain,
ring_len,
fingerprintable_canonical,
)
.await?;
real.zeroize();
// Grab the decoys for this specific output
let mut ring = decoys.drain((decoys.len() - decoy_count) ..).collect::<Vec<_>>();
ring.push(output);
// Form the complete ring
let mut ring = decoys;
ring.push((input.relative_id.index_on_blockchain, [input.key(), input.commitment().calculate()]));
ring.sort_by(|a, b| a.0.cmp(&b.0));
// Sanity checks are only run when 1000 outputs are available in Monero
// We run this check whenever the highest output index, which we acknowledge, is > 500
// This means we assume (for presumably test blockchains) the height being used has not had
// 500 outputs since while itself not being a sufficiently mature blockchain
// Considering Monero's p2p layer doesn't actually check transaction sanity, it should be
// fine for us to not have perfectly matching rules, especially since this code will infinite
// loop if it can't determine sanity, which is possible with sufficient inputs on
// sufficiently small chains
if high > 500 {
// Make sure the TX passes the sanity check that the median output is within the last 40%
let target_median = high * 3 / 5;
while ring[ring_len / 2].0 < target_median {
// If it's not, update the bottom half with new values to ensure the median only moves up
for removed in ring.drain(0 .. (ring_len / 2)).collect::<Vec<_>>() {
// If we removed the real spend, add it back
if removed.0 == output.0 {
ring.push(output);
} else {
// We could not remove this, saving CPU time and removing low values as
// possibilities, yet it'd increase the amount of decoys required to create this
// transaction and some removed outputs may be the best option (as we drop the first
// half, not just the bottom n)
used.remove(&removed.0);
}
}
/*
Monero does have sanity checks which it applies to the selected ring.
// Select new outputs until we have a full sized ring again
ring.extend(
select_n(
rng,
rpc,
&distribution,
height,
high,
per_second,
&[],
&mut used,
ring_len - ring.len(),
fingerprintable_canonical,
)
.await?,
);
ring.sort_by(|a, b| a.0.cmp(&b.0));
}
They're statistically unlikely to be hit and only occur when the transaction is published over
the RPC (so they are not a relay rule). The RPC allows disabling them, which monero-rpc does to
ensure they don't pose a problem.
// The other sanity check rule is about duplicates, yet we already enforce unique ring
// members
They aren't worth the complexity to implement here, especially since they're non-deterministic.
*/
// We need to convert our positional indexes to offset indexes
let mut offsets = Vec::with_capacity(ring.len());
{
offsets.push(ring[0].0);
for m in 1 .. ring.len() {
offsets.push(ring[m].0 - ring[m - 1].0);
}
}
Ok(
Decoys::new(
offset(&ring.iter().map(|output| output.0).collect::<Vec<_>>()),
offsets,
// Binary searches for the real spend since we don't know where it sorted to
u8::try_from(ring.partition_point(|x| x.0 < output.0)).unwrap(),
ring.iter().map(|output| output.1).collect(),
u8::try_from(ring.partition_point(|x| x.0 < input.relative_id.index_on_blockchain)).unwrap(),
ring.into_iter().map(|output| output.1).collect(),
)
.unwrap(),
)
}
pub use monero_serai::primitives::Decoys;
/// An output with decoys selected.
#[derive(Clone, PartialEq, Eq, Debug, Zeroize, ZeroizeOnDrop)]
pub struct OutputWithDecoys {