serai/coins/monero/src/wallet/decoys.rs
Justin Berman 228e36a12d
monero-serai: fee calculation parity with Monero's wallet2 (#301)
* monero-serai: fee calculation parity with Monero's wallet2

* Minor lint

---------

Co-authored-by: Luke Parker <lukeparker5132@gmail.com>
2023-07-19 15:06:05 -04:00

284 lines
9.7 KiB
Rust

use std_shims::{sync::OnceLock, vec::Vec, collections::HashSet};
#[cfg(not(feature = "std"))]
use std_shims::sync::Mutex;
#[cfg(feature = "std")]
use futures::lock::Mutex;
use zeroize::{Zeroize, ZeroizeOnDrop};
use rand_core::{RngCore, CryptoRng};
use rand_distr::{Distribution, Gamma};
#[cfg(not(feature = "std"))]
use rand_distr::num_traits::Float;
use curve25519_dalek::edwards::EdwardsPoint;
use crate::{
serialize::varint_len,
wallet::SpendableOutput,
rpc::{RpcError, RpcConnection, Rpc},
};
const LOCK_WINDOW: usize = 10;
const MATURITY: u64 = 60;
const RECENT_WINDOW: usize = 15;
const BLOCK_TIME: usize = 120;
const BLOCKS_PER_YEAR: usize = 365 * 24 * 60 * 60 / BLOCK_TIME;
const TIP_APPLICATION: f64 = (LOCK_WINDOW * BLOCK_TIME) as f64;
// TODO: Expose an API to reset this in case a reorg occurs/the RPC fails/returns garbage
// TODO: Update this when scanning a block, as possible
static DISTRIBUTION_CELL: OnceLock<Mutex<Vec<u64>>> = OnceLock::new();
#[allow(non_snake_case)]
fn DISTRIBUTION() -> &'static Mutex<Vec<u64>> {
DISTRIBUTION_CELL.get_or_init(|| Mutex::new(Vec::with_capacity(3000000)))
}
#[allow(clippy::too_many_arguments)]
async fn select_n<'a, R: RngCore + CryptoRng, RPC: RpcConnection>(
rng: &mut R,
rpc: &Rpc<RPC>,
distribution: &[u64],
height: usize,
high: u64,
per_second: f64,
real: &[u64],
used: &mut HashSet<u64>,
count: usize,
) -> Result<Vec<(u64, [EdwardsPoint; 2])>, RpcError> {
if 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"))?;
}
#[cfg(test)]
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();
let mut candidates = Vec::with_capacity(remaining);
while candidates.len() != remaining {
#[cfg(test)]
{
iters += 1;
// This is cheap and on fresh chains, a lot of rounds may be needed
if iters == 100 {
Err(RpcError::InternalError("hit decoy selection round limit"))?;
}
}
// Use a gamma distribution
let mut age = Gamma::<f64>::new(19.28, 1.0 / 1.61).unwrap().sample(rng).exp();
if age > TIP_APPLICATION {
age -= TIP_APPLICATION;
} else {
// f64 does not have try_from available, which is why these are written with `as`
age = (rng.next_u64() % u64::try_from(RECENT_WINDOW * BLOCK_TIME).unwrap()) as f64;
}
let o = (age * per_second) as u64;
if o < high {
let i = distribution.partition_point(|s| *s < (high - 1 - o));
let prev = i.saturating_sub(1);
let n = distribution[i] - distribution[prev];
if n != 0 {
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);
candidates.push(o);
}
}
}
}
// 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);
}
// 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());
}
}
for (i, output) in rpc.get_unlocked_outputs(&candidates, height).await?.iter_mut().enumerate() {
// Don't include the real spend as a decoy, despite requesting it
if real_indexes.contains(&i) {
continue;
}
if let Some(output) = output.take() {
confirmed.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
}
/// Decoy data, containing the actual member as well (at index `i`).
#[derive(Clone, PartialEq, Eq, Debug, Zeroize, ZeroizeOnDrop)]
pub struct Decoys {
pub(crate) i: u8,
pub(crate) offsets: Vec<u64>,
pub(crate) ring: Vec<[EdwardsPoint; 2]>,
}
#[allow(clippy::len_without_is_empty)]
impl Decoys {
pub fn fee_weight(offsets: &[u64]) -> usize {
varint_len(offsets.len()) + offsets.iter().map(|offset| varint_len(*offset)).sum::<usize>()
}
pub fn len(&self) -> usize {
self.offsets.len()
}
/// Select decoys using the same distribution as Monero.
pub async fn select<R: RngCore + CryptoRng, RPC: RpcConnection>(
rng: &mut R,
rpc: &Rpc<RPC>,
ring_len: usize,
height: usize,
inputs: &[SpendableOutput],
) -> Result<Vec<Decoys>, RpcError> {
#[cfg(not(feature = "std"))]
let mut distribution = DISTRIBUTION().lock();
#[cfg(feature = "std")]
let mut distribution = DISTRIBUTION().lock().await;
let decoy_count = ring_len - 1;
// Convert the inputs in question to the raw output data
let mut real = Vec::with_capacity(inputs.len());
let mut outputs = Vec::with_capacity(inputs.len());
for input in inputs {
real.push(input.global_index);
outputs.push((real[real.len() - 1], [input.key(), input.commitment().calculate()]));
}
if distribution.len() <= height {
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 + 1); // height is inclusive, and 0 is a valid height
let high = distribution[distribution.len() - 1];
let per_second = {
let blocks = distribution.len().min(BLOCKS_PER_YEAR);
let outputs = high - distribution[distribution.len().saturating_sub(blocks + 1)];
(outputs as f64) / ((blocks * BLOCK_TIME) as f64)
};
let mut used = HashSet::<u64>::new();
for o in &outputs {
used.insert(o.0);
}
// TODO: Create a TX with less than the target amount, as allowed by the protocol
if (high - MATURITY) < u64::try_from(inputs.len() * ring_len).unwrap() {
Err(RpcError::InternalError("not enough decoy candidates"))?;
}
// 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(
rng,
rpc,
&distribution,
height,
high,
per_second,
&real,
&mut used,
inputs.len() * decoy_count,
)
.await?;
real.zeroize();
let mut res = Vec::with_capacity(inputs.len());
for o in outputs {
// Grab the decoys for this specific output
let mut ring = decoys.drain((decoys.len() - decoy_count) ..).collect::<Vec<_>>();
ring.push(o);
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 == o.0 {
ring.push(o);
} 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);
}
}
// 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(),
)
.await?,
);
ring.sort_by(|a, b| a.0.cmp(&b.0));
}
// The other sanity check rule is about duplicates, yet we already enforce unique ring
// members
}
res.push(Decoys {
// Binary searches for the real spend since we don't know where it sorted to
i: u8::try_from(ring.partition_point(|x| x.0 < o.0)).unwrap(),
offsets: offset(&ring.iter().map(|output| output.0).collect::<Vec<_>>()),
ring: ring.iter().map(|output| output.1).collect(),
});
}
Ok(res)
}
}