Implement simple random mixin selection which passes sanity

coins/monero/src/transaction/mixins.rs

@@ -1,15 +1,113 @@
-// TODO
-pub(crate) fn select(o: u64) -> (u8, Vec<u64>) {
-  let mut mixins: Vec<u64> = (o .. o + 11).into_iter().collect();
-  mixins.sort();
-  (0, mixins)
+use std::collections::HashSet;
+
+use rand_core::{RngCore, CryptoRng};
+
+use curve25519_dalek::edwards::EdwardsPoint;
+
+use monero::VarInt;
+
+use crate::{transaction::SpendableOutput, rpc::{RpcError, Rpc}};
+
+const MIXINS: usize = 11;
+
+async fn select_single<R: RngCore + CryptoRng>(
+  rng: &mut R,
+  rpc: &Rpc,
+  height: usize,
+  high: u64,
+  used: &mut HashSet<u64>
+) -> Result<(u64, [EdwardsPoint; 2]), RpcError> {
+  let mut o;
+  let mut output = None;
+  while {
+    o = rng.next_u64() % u64::try_from(high).unwrap();
+    used.contains(&o) || {
+      output = rpc.get_outputs(&[o], height).await?[0];
+      output.is_none()
+    }
+  } {}
+  used.insert(o);
+  Ok((o, output.unwrap()))
 }
 
-pub(crate) fn offset(mixins: &[u64]) -> Vec<u64> {
-  let mut res = vec![mixins[0]];
-  res.resize(11, 0);
+// Uses VarInt as this is solely used for key_offsets which is serialized by monero-rs
+fn offset(mixins: &[u64]) -> Vec<VarInt> {
+  let mut res = vec![VarInt(mixins[0])];
+  res.resize(mixins.len(), VarInt(0));
   for m in (1 .. mixins.len()).rev() {
-    res[m] = mixins[m] - mixins[m - 1];
+    res[m] = VarInt(mixins[m] - mixins[m - 1]);
   }
   res
 }
+
+pub(crate) async fn select<R: RngCore + CryptoRng>(
+  rng: &mut R,
+  rpc: &Rpc,
+  height: usize,
+  inputs: &[SpendableOutput]
+) -> Result<Vec<(Vec<VarInt>, u8, Vec<[EdwardsPoint; 2]>)>, RpcError> {
+  // Convert the inputs in question to the raw output data
+  let mut outputs = Vec::with_capacity(inputs.len());
+  for input in inputs {
+    outputs.push((
+      rpc.get_o_indexes(input.tx).await?[input.o],
+      [input.key, input.commitment.calculate()]
+    ));
+  }
+
+  let high = rpc.get_high_output(height - 1).await?;
+  let high_f = high as f64;
+  if (high_f as u64) != high {
+    panic!("Transaction output index exceeds f64");
+  }
+
+  let mut used = HashSet::<u64>::new();
+  for o in &outputs {
+    used.insert(o.0);
+  }
+
+  let mut res = Vec::with_capacity(inputs.len());
+  for (i, o) in outputs.iter().enumerate() {
+    let mut mixins = Vec::with_capacity(MIXINS);
+    for _ in 0 .. MIXINS {
+      mixins.push(select_single(rng, rpc, height, high, &mut used).await?);
+    }
+    mixins.sort_by(|a, b| a.0.cmp(&b.0));
+
+    // Make sure the TX passes the sanity check that the median output is within the last 40%
+    // This actually checks the median is within the last third, a slightly more aggressive boundary,
+    // as the height used in this calculation will be slightly under the height this is sanity
+    // checked against
+    while mixins[MIXINS / 2].0 < (high * 2 / 3) {
+      // If it's not, update the bottom half with new values to ensure the median only moves up
+      for m in 0 .. MIXINS / 2 {
+        // We could not remove this, saving CPU time and removing low values as possibilities, yet
+        // it'd increase the amount of mixins required to create this transaction and some banned
+        // outputs may be the best options
+        used.remove(&mixins[m].0);
+        mixins[m] = select_single(rng, rpc, height, high, &mut used).await?;
+      }
+      mixins.sort_by(|a, b| a.0.cmp(&b.0));
+    }
+
+    // Replace the closest selected decoy with the actual
+    let mut replace = 0;
+    let mut distance = u64::MAX;
+    for m in 0 .. mixins.len() {
+      let diff = mixins[m].0.abs_diff(o.0);
+      if diff < distance {
+        replace = m;
+        distance = diff;
+      }
+    }
+
+    mixins[replace] = outputs[i];
+    res.push((
+      offset(&mixins.iter().map(|output| output.0).collect::<Vec<_>>()),
+      u8::try_from(replace).unwrap(),
+      mixins.iter().map(|output| output.1).collect()
+    ));
+  }
+
+  Ok(res)
+}

coins/monero/src/transaction/mod.rs

@@ -71,6 +71,7 @@ pub enum TransactionError {
 pub struct SpendableOutput {
   pub tx: Hash,
   pub o: usize,
+  pub key: EdwardsPoint,
   pub key_offset: Scalar,
   pub commitment: Commitment
 }
@@ -126,7 +127,7 @@ pub fn scan(tx: &Transaction, view: Scalar, spend: EdwardsPoint) -> Vec<Spendabl
           }
         }
 
-        res.push(SpendableOutput { tx: tx.hash(), o, key_offset, commitment });
+        res.push(SpendableOutput { tx: tx.hash(), o, key: output_key, key_offset, commitment });
         break;
       }
     }
@@ -191,26 +192,31 @@ impl Output {
   }
 }
 
-async fn prepare_inputs(
+async fn prepare_inputs<R: RngCore + CryptoRng>(
+  rng: &mut R,
   rpc: &Rpc,
-  spend: &Scalar,
   inputs: &[SpendableOutput],
+  spend: &Scalar,
   tx: &mut Transaction
 ) -> Result<Vec<(Scalar, clsag::Input, EdwardsPoint)>, TransactionError> {
   // TODO sort inputs
 
   let mut signable = Vec::with_capacity(inputs.len());
-  for (i, input) in inputs.iter().enumerate() {
-    // Select mixins
-    let (m, mixins) = mixins::select(
-      rpc.get_o_indexes(input.tx).await.map_err(|e| TransactionError::RpcError(e))?[input.o]
-    );
 
+  // Select mixins
+  let mixins = mixins::select(
+    rng,
+    rpc,
+    rpc.get_height().await.map_err(|e| TransactionError::RpcError(e))?,
+    inputs
+  ).await.map_err(|e| TransactionError::RpcError(e))?;
+
+  for (i, input) in inputs.iter().enumerate() {
     signable.push((
       spend + input.key_offset,
       clsag::Input::new(
-        rpc.get_ring(&mixins).await.map_err(|e| TransactionError::RpcError(e))?,
-        m,
+        mixins[i].2.clone(),
+        mixins[i].1,
         input.commitment
       ).map_err(|e| TransactionError::ClsagError(e))?,
       key_image::generate(&(spend + input.key_offset))
@@ -218,7 +224,7 @@ async fn prepare_inputs(
 
     tx.prefix.inputs.push(TxIn::ToKey {
       amount: VarInt(0),
-      key_offsets: mixins::offset(&mixins).iter().map(|x| VarInt(*x)).collect(),
+      key_offsets: mixins[i].0.clone(),
       k_image: KeyImage { image: Hash(signable[i].2.compress().to_bytes()) }
     });
   }
@@ -360,7 +366,7 @@ impl SignableTransaction {
     let (commitments, mask_sum) = self.prepare_outputs(rng)?;
     let mut tx = self.prepare_transaction(&commitments, bulletproofs::generate(&commitments)?);
 
-    let signable = prepare_inputs(rpc, spend, &self.inputs, &mut tx).await?;
+    let signable = prepare_inputs(rng, rpc, &self.inputs, spend, &mut tx).await?;
 
     let clsags = clsag::sign(
       rng,

coins/monero/src/transaction/multisig.rs

@@ -25,11 +25,11 @@ pub struct TransactionMachine {
   leader: bool,
   signable: SignableTransaction,
   our_images: Vec<EdwardsPoint>,
-  inputs: Vec<TxIn>,
-  tx: Option<Transaction>,
   mask_sum: Rc<RefCell<Scalar>>,
   msg: Rc<RefCell<[u8; 32]>>,
-  clsags: Vec<AlgorithmMachine<Ed25519, clsag::Multisig>>
+  clsags: Vec<AlgorithmMachine<Ed25519, clsag::Multisig>>,
+  inputs: Vec<TxIn>,
+  tx: Option<Transaction>,
 }
 
 impl SignableTransaction {
@@ -41,28 +41,58 @@ impl SignableTransaction {
     included: &[usize]
   ) -> Result<TransactionMachine, TransactionError> {
     let mut our_images = vec![];
-    let mut inputs = vec![];
+
     let mask_sum = Rc::new(RefCell::new(Scalar::zero()));
     let msg = Rc::new(RefCell::new([0; 32]));
     let mut clsags = vec![];
-    for input in &self.inputs {
-      // Select mixins
-      let (m, mixins) = mixins::select(
-        rpc.get_o_indexes(input.tx).await.map_err(|e| TransactionError::RpcError(e))?[input.o]
-      );
 
+    let mut inputs = vec![];
+
+    // Create a RNG out of the input shared keys, which either requires the view key or being every
+    // sender, and the payments (address and amount), which a passive adversary may be able to know
+    // The use of input shared keys technically makes this one time given a competent wallet which
+    // can withstand the burning attack
+    // The lack of dedicated entropy here is frustrating. We can probably provide entropy inclusion
+    // if we move CLSAG ring to a Rc RefCell like msg and mask? TODO
+    let mut transcript = Transcript::new(b"InputMixins");
+    let mut shared_keys = Vec::with_capacity(self.inputs.len() * 32);
+    for input in &self.inputs {
+      shared_keys.extend(&input.key_offset.to_bytes());
+    }
+    transcript.append_message(b"input_shared_keys", &shared_keys);
+    let mut payments = Vec::with_capacity(self.payments.len() * ((2 * 32) + 8));
+    for payment in &self.payments {
+      // Network byte and spend/view key
+      // Doesn't use the full address as monero-rs may provide a payment ID which adds bytes
+      // By simply cutting this short, we get the relevant data without length differences nor the
+      // need to prefix
+      payments.extend(&payment.0.as_bytes()[0 .. 65]);
+      payments.extend(payment.1.to_le_bytes());
+    }
+    transcript.append_message(b"payments", &payments);
+
+    // Select mixins
+    let mixins = mixins::select(
+      &mut transcript.seeded_rng(b"mixins", None),
+      rpc,
+      rpc.get_height().await.map_err(|e| TransactionError::RpcError(e))?,
+      &self.inputs
+    ).await.map_err(|e| TransactionError::RpcError(e))?;
+
+    for (i, input) in self.inputs.iter().enumerate() {
       let keys = keys.offset(dalek_ff_group::Scalar(input.key_offset));
       let (image, _) = key_image::generate_share(
         rng,
         &keys.view(included).map_err(|e| TransactionError::FrostError(e))?
       );
       our_images.push(image);
+
       clsags.push(
         AlgorithmMachine::new(
           clsag::Multisig::new(
             clsag::Input::new(
-              rpc.get_ring(&mixins).await.map_err(|e| TransactionError::RpcError(e))?,
-              m,
+              mixins[i].2.clone(),
+              mixins[i].1,
               input.commitment
             ).map_err(|e| TransactionError::ClsagError(e))?,
             msg.clone(),
@@ -75,7 +105,7 @@ impl SignableTransaction {
 
       inputs.push(TxIn::ToKey {
         amount: VarInt(0),
-        key_offsets: mixins::offset(&mixins).iter().map(|x| VarInt(*x)).collect(),
+        key_offsets: mixins[i].0.clone(),
         k_image: KeyImage { image: Hash([0; 32]) }
       });
     }
@@ -87,11 +117,11 @@ impl SignableTransaction {
       leader: keys.params().i() == included[0],
       signable: self,
       our_images,
-      inputs,
-      tx: None,
       mask_sum,
       msg,
-      clsags
+      clsags,
+      inputs,
+      tx: None
     })
   }
 }