Use a global transcript

coins/monero/Cargo.toml

@@ -12,6 +12,7 @@ thiserror = "1"
 
 rand_core = "0.6"
 rand_distr = "0.4"
+rand_chacha = { version = "0.3", optional = true }
 
 tiny-keccak = { version = "2.0", features = ["keccak"] }
 blake2 = "0.10"
@@ -34,7 +35,7 @@ monero-epee-bin-serde = "1.0"
 reqwest = { version = "0.11", features = ["json"] }
 
 [features]
-multisig = ["ff", "group", "transcript", "frost", "dalek-ff-group"]
+multisig = ["ff", "group", "rand_chacha", "transcript", "frost", "dalek-ff-group"]
 
 [dev-dependencies]
 rand = "0.8"

coins/monero/src/clsag/multisig.rs

@@ -1,7 +1,8 @@
 use core::fmt::Debug;
 use std::{rc::Rc, cell::RefCell};
 
-use rand_core::{RngCore, CryptoRng};
+use rand_core::{RngCore, CryptoRng, SeedableRng};
+use rand_chacha::ChaCha12Rng;
 
 use curve25519_dalek::{
   constants::ED25519_BASEPOINT_TABLE,
@@ -27,7 +28,7 @@ use crate::{
 
 impl Input {
   fn transcript<T: TranscriptTrait>(&self, transcript: &mut T) {
-    // Doesn't dom-sep as this is considered part of the larger input signing proof
+    // Doesn't domain separate as this is considered part of the larger CLSAG proof
 
     // Ring index
     transcript.append_message(b"ring_index", &[self.i]);
@@ -61,12 +62,13 @@ struct ClsagSignInterim {
 #[allow(non_snake_case)]
 #[derive(Clone, Debug)]
 pub struct Multisig {
-  commitments_H: Vec<u8>,
-  image: EdwardsPoint,
-  AH: (dfg::EdwardsPoint, dfg::EdwardsPoint),
-
+  transcript: Transcript,
   input: Input,
 
+  image: EdwardsPoint,
+  commitments_H: Vec<u8>,
+  AH: (dfg::EdwardsPoint, dfg::EdwardsPoint),
+
   msg: Rc<RefCell<[u8; 32]>>,
   mask: Rc<RefCell<Scalar>>,
 
@@ -75,18 +77,20 @@ pub struct Multisig {
 
 impl Multisig {
   pub fn new(
+    transcript: Transcript,
     input: Input,
     msg: Rc<RefCell<[u8; 32]>>,
     mask: Rc<RefCell<Scalar>>,
   ) -> Result<Multisig, MultisigError> {
     Ok(
       Multisig {
-        commitments_H: vec![],
-        image: EdwardsPoint::identity(),
-        AH: (dfg::EdwardsPoint::identity(), dfg::EdwardsPoint::identity()),
-
+        transcript,
         input,
 
+        image: EdwardsPoint::identity(),
+        commitments_H: vec![],
+        AH: (dfg::EdwardsPoint::identity(), dfg::EdwardsPoint::identity()),
+
         msg,
         mask,
 
@@ -138,14 +142,28 @@ impl Algorithm<Ed25519> for Multisig {
       Err(FrostError::InvalidCommitmentQuantity(l, 9, serialized.len() / 32))?;
     }
 
+    if self.commitments_H.len() == 0 {
+      self.transcript.domain_separate(b"CLSAG");
+      self.input.transcript(&mut self.transcript);
+      self.transcript.append_message(b"message", &*self.msg.borrow());
+      self.transcript.append_message(b"mask", &self.mask.borrow().to_bytes());
+    }
+
     let (share, serialized) = key_image::verify_share(view, l, serialized).map_err(|_| FrostError::InvalidShare(l))?;
+    // Given the fact there's only ever one possible value for this, this may technically not need
+    // to be committed to. If signing a TX, it'll be double committed to thanks to the message
+    // It doesn't hurt to have though and ensures security boundaries are well formed
+    self.transcript.append_message(b"image_share", &share.compress().to_bytes());
     self.image += share;
 
     let alt = &hash_to_point(&self.input.ring[usize::from(self.input.i)][0]);
 
     // Uses the same format FROST does for the expected commitments (nonce * G where this is nonce * H)
-    self.commitments_H.extend(&u64::try_from(l).unwrap().to_le_bytes());
-    self.commitments_H.extend(&serialized[0 .. 64]);
+    // Given this is guaranteed to match commitments, which FROST commits to, this also technically
+    // doesn't need to be committed to if a canonical serialization is guaranteed
+    // It, again, doesn't hurt to include and ensures security boundaries are well formed
+    self.transcript.append_message(b"participant", &u64::try_from(l).unwrap().to_le_bytes());
+    self.transcript.append_message(b"commitments_H", &serialized[0 .. 64]);
 
     #[allow(non_snake_case)]
     let H = (
@@ -171,21 +189,8 @@ impl Algorithm<Ed25519> for Multisig {
     Ok(())
   }
 
-  fn transcript(&self) -> Option<Self::Transcript> {
-    let mut transcript = Self::Transcript::new(b"Monero Multisig");
-    self.input.transcript(&mut transcript);
-    transcript.append_message(b"dom-sep", b"CLSAG");
-    // Given the fact there's only ever one possible value for this, this may technically not need
-    // to be committed to. If signing a TX, it's be double committed to thanks to the message
-    // It doesn't hurt to have though and ensures security boundaries are well formed
-    transcript.append_message(b"image", &self.image.compress().to_bytes());
-    // Given this is guaranteed to match commitments, which FROST commits to, this also technically
-    // doesn't need to be committed to if a canonical serialization is guaranteed
-    // It, again, doesn't hurt to include and ensures security boundaries are well formed
-    transcript.append_message(b"commitments_H", &self.commitments_H);
-    transcript.append_message(b"message", &*self.msg.borrow());
-    transcript.append_message(b"mask", &self.mask.borrow().to_bytes());
-    Some(transcript)
+  fn transcript(&mut self) -> &mut Self::Transcript {
+    &mut self.transcript
   }
 
   fn sign_share(
@@ -203,8 +208,8 @@ impl Algorithm<Ed25519> for Multisig {
     // The transcript contains private data, preventing passive adversaries from recreating this
     // process even if they have access to commitments (specifically, the ring index being signed
     // for, along with the mask which should not only require knowing the shared keys yet also the
-    // input commitment mask)
-    let mut rng = self.transcript().unwrap().seeded_rng(b"decoy_responses", None);
+    // input commitment masks)
+    let mut rng = ChaCha12Rng::from_seed(self.transcript.rng_seed(b"decoy_responses", None));
 
     #[allow(non_snake_case)]
     let (clsag, c, mu_C, z, mu_P, C_out) = sign_core(

coins/monero/src/frost.rs

@@ -22,7 +22,7 @@ use dalek_ff_group as dfg;
 
 use crate::random_scalar;
 
-pub(crate) type Transcript = DigestTranscript::<blake2::Blake2b512>;
+pub type Transcript = DigestTranscript::<blake2::Blake2b512>;
 
 #[derive(Error, Debug)]
 pub enum MultisigError {

coins/monero/src/transaction/multisig.rs

@@ -1,6 +1,7 @@
 use std::{rc::Rc, cell::RefCell};
 
-use rand_core::{RngCore, CryptoRng};
+use rand_core::{RngCore, CryptoRng, SeedableRng};
+use rand_chacha::ChaCha12Rng;
 
 use curve25519_dalek::{scalar::Scalar, edwards::{EdwardsPoint, CompressedEdwardsY}};
 
@@ -24,6 +25,8 @@ use crate::{
 pub struct TransactionMachine {
   leader: bool,
   signable: SignableTransaction,
+  transcript: Transcript,
+
   our_images: Vec<EdwardsPoint>,
   mask_sum: Rc<RefCell<Scalar>>,
   msg: Rc<RefCell<[u8; 32]>>,
@@ -35,6 +38,7 @@ pub struct TransactionMachine {
 impl SignableTransaction {
   pub async fn multisig<R: RngCore + CryptoRng>(
     mut self,
+    label: Vec<u8>,
     rng: &mut R,
     rpc: &Rpc,
     keys: Rc<MultisigKeys<Ed25519>>,
@@ -51,25 +55,30 @@ impl SignableTransaction {
 
     // 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 (and has a static spend key? TODO visit bounds)
+    // depending on how these transactions are coordinated
+
     // 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
-    // For the above TODO, also consider FROST's TODO of a global transcript instance
-    let mut transcript = Transcript::new(b"Input Mixins");
-    // Does dom-sep despite not being a proof because it's a unique section (and we have no dom-sep yet)
-    transcript.append_message("dom-sep", "inputs_outputs");
+    let mut transcript = Transcript::new(label);
     for input in &self.inputs {
+      // These outputs can only be spent once. Therefore, it forces all RNGs derived from this
+      // transcript (such as the one used to create one time keys) to be unique
+      transcript.append_message(b"input_hash", &input.tx.0);
+      transcript.append_message(b"input_output_index", &u64::try_from(input.o).unwrap().to_le_bytes());
+      // Not including this, with a doxxed list of payments, would allow brute forcing the inputs
+      // to determine RNG seeds and therefore the true spends
       transcript.append_message(b"input_shared_key", &input.key_offset.to_bytes());
     }
     for payment in &self.payments {
       transcript.append_message(b"payment_address", &payment.0.as_bytes());
       transcript.append_message(b"payment_amount", &payment.1.to_le_bytes());
     }
+    // Not only is this an output, but this locks to the base keys to be complete with the above key offsets
+    transcript.append_message(b"change", &self.change.as_bytes());
 
     // Select mixins
     let mixins = mixins::select(
-      &mut transcript.seeded_rng(b"mixins", None),
+      &mut ChaCha12Rng::from_seed(transcript.rng_seed(b"mixins", None)),
       rpc,
       height,
       &self.inputs
@@ -86,6 +95,7 @@ impl SignableTransaction {
       clsags.push(
         AlgorithmMachine::new(
           clsag::Multisig::new(
+            transcript.clone(),
             clsag::Input::new(
               mixins[i].2.clone(),
               mixins[i].1,
@@ -112,6 +122,7 @@ impl SignableTransaction {
     Ok(TransactionMachine {
       leader: keys.params().i() == included[0],
       signable: self,
+      transcript,
       our_images,
       mask_sum,
       msg,
@@ -122,19 +133,6 @@ impl SignableTransaction {
   }
 }
 
-// Seeded RNG so multisig participants agree on one time keys to use, preventing burning attacks
-fn outputs_rng(tx: &SignableTransaction, entropy: [u8; 32]) -> <Transcript as TranscriptTrait>::SeededRng {
-  let mut transcript = Transcript::new(b"Stealth Addresses");
-  // This output can only be spent once. Therefore, it forces all one time keys used here to be
-  // unique, even if the entropy is reused. While another transaction could use a different input
-  // ordering to swap which 0 is, that input set can't contain this input without being a double
-  // spend
-  transcript.append_message(b"dom-sep", b"input_0");
-  transcript.append_message(b"hash", &tx.inputs[0].tx.0);
-  transcript.append_message(b"index", &u64::try_from(tx.inputs[0].o).unwrap().to_le_bytes());
-  transcript.seeded_rng(b"tx_keys", Some(entropy))
-}
-
 impl StateMachine for TransactionMachine {
   type Signature = Transaction;
 
@@ -157,7 +155,7 @@ impl StateMachine for TransactionMachine {
       rng.fill_bytes(&mut entropy);
       serialized.extend(&entropy);
 
-      let mut rng = outputs_rng(&self.signable, entropy);
+      let mut rng = ChaCha12Rng::from_seed(self.transcript.rng_seed(b"tx_keys", Some(entropy)));
       // Safe to unwrap thanks to the dummy prepare
       let (commitments, mask_sum) = self.signable.prepare_outputs(&mut rng).unwrap();
       self.mask_sum.replace(mask_sum);
@@ -196,9 +194,11 @@ impl StateMachine for TransactionMachine {
         }
         let prep = prep.as_ref().unwrap();
 
-        let mut rng = outputs_rng(
-          &self.signable,
-          prep[clsag_lens .. (clsag_lens + 32)].try_into().map_err(|_| FrostError::InvalidShare(l))?
+        let mut rng = ChaCha12Rng::from_seed(
+          self.transcript.rng_seed(
+            b"tx_keys",
+            Some(prep[clsag_lens .. (clsag_lens + 32)].try_into().map_err(|_| FrostError::InvalidShare(l))?)
+          )
         );
         // Not invalid outputs due to doing a dummy prep as leader
         let (commitments, mask_sum) = self.signable.prepare_outputs(&mut rng).map_err(|_| FrostError::InvalidShare(l))?;

coins/monero/tests/clsag.rs

@@ -7,7 +7,7 @@ use curve25519_dalek::{constants::ED25519_BASEPOINT_TABLE, scalar::Scalar};
 
 use monero_serai::{random_scalar, Commitment, key_image, clsag};
 #[cfg(feature = "multisig")]
-use monero_serai::frost::MultisigError;
+use monero_serai::frost::{MultisigError, Transcript};
 
 #[cfg(feature = "multisig")]
 mod frost;
@@ -84,6 +84,7 @@ fn test_multisig() -> Result<(), MultisigError> {
     machines.push(
       sign::AlgorithmMachine::new(
         clsag::Multisig::new(
+          Transcript::new(b"Monero Serai CLSAG Test".to_vec()),
           clsag::Input::new(ring.clone(), RING_INDEX, Commitment::new(randomness, AMOUNT)).unwrap(),
           Rc::new(RefCell::new([1; 32])),
           Rc::new(RefCell::new(Scalar::from(42u64)))

coins/monero/tests/send_multisig.rs

@@ -32,7 +32,7 @@ pub async fn send_multisig() {
   let t = keys[0].params().t();
 
   // Generate an address
-  let view = Scalar::from_hash(Blake2b512::new().chain("Serai DEX")).0;
+  let view = Scalar::from_hash(Blake2b512::new().chain("Monero Serai Transaction Test")).0;
   let spend = keys[0].group_key().0;
   let addr = Address::standard(
     Network::Mainnet,
@@ -57,6 +57,7 @@ pub async fn send_multisig() {
       SignableTransaction::new(
         vec![output.clone()], vec![(addr, amount)], addr, fee_per_byte
       ).unwrap().multisig(
+        b"Monero Serai Test Transaction".to_vec(),
         &mut OsRng,
         &rpc,
         keys[i - 1].clone(),