Use dom-sep tags in the transcripts

Also simplifies form in some places

coins/monero/src/clsag/multisig.rs

@@ -26,7 +26,9 @@ use crate::{
 };
 
 impl Input {
-  pub fn transcript<T: TranscriptTrait>(&self, transcript: &mut T) {
+  fn transcript<T: TranscriptTrait>(&self, transcript: &mut T) {
+    // Doesn't dom-sep as this is considered part of the larger input signing proof
+
     // Ring index
     transcript.append_message(b"ring_index", &[self.i]);
 
@@ -170,8 +172,9 @@ impl Algorithm<Ed25519> for Multisig {
   }
 
   fn transcript(&self) -> Option<Self::Transcript> {
-    let mut transcript = Self::Transcript::new(b"CLSAG");
+    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

coins/monero/src/transaction/multisig.rs

@@ -52,25 +52,20 @@ 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
+    // can withstand the burning attack (and has a static spend key? TODO visit bounds)
     // 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 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");
     for input in &self.inputs {
-      shared_keys.extend(&input.key_offset.to_bytes());
+      transcript.append_message(b"input_shared_key", &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"payment_address", &payment.0.as_bytes());
+      transcript.append_message(b"payment_amount", &payment.1.to_le_bytes());
     }
-    transcript.append_message(b"payments", &payments);
 
     // Select mixins
     let mixins = mixins::select(
@@ -129,11 +124,12 @@ 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"StealthAddress");
+  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))

crypto/frost/src/sign.rs

@@ -218,15 +218,26 @@ fn sign_with_share<C: Curve, A: Algorithm<C>>(
   // While Merlin, which may or may not be the transcript used here, wants application level
   // transcripts passed around to proof systems, this maintains a desired level of abstraction and
   // works without issue
+  // Not to mention, mandating a global transcript would conflict with the IETF draft UNLESS an
+  // IetfTranscript was declared which ignores field names and solely does their values, with a
+  // fresh instantiation per sign round. That could likely be made to align without issue
+  // TODO: Consider Option<Transcript>?
   let mut transcript = params.algorithm.transcript();
+  if params.keys.offset.is_some() && transcript.is_none() {
+    transcript = Some(A::Transcript::new(b"FROST Offset"));
+  }
+  if transcript.is_some() {
+    // https://github.com/rust-lang/rust/issues/91345
+    transcript = transcript.map(|mut t| { t.append_message(b"dom-sep", b"FROST"); t });
+  }
 
   // If the offset functionality provided by this library is in use, include it in the transcript.
   // Not compliant with the IETF spec which doesn't have a concept of offsets, nor does it use
   // transcripts
   if params.keys.offset.is_some() {
-    let mut offset_transcript = transcript.unwrap_or(A::Transcript::new(b"FROST_offset"));
-    offset_transcript.append_message(b"offset", &C::F_to_le_bytes(&params.keys.offset.unwrap()));
-    transcript = Some(offset_transcript);
+    transcript = transcript.map(
+      |mut t| { t.append_message(b"offset", &C::F_to_le_bytes(&params.keys.offset.unwrap())); t }
+    );
   }
 
   // If a transcript was defined, move the commitments used for the binding factor into it

crypto/transcript/src/lib.rs

@@ -21,6 +21,8 @@ pub trait Transcript {
     label: &'static [u8],
     additional_entropy: Option<[u8; 32]>
   ) -> Self::SeededRng;
+
+  // TODO: Consider a domain_separate function
 }
 
 #[derive(Clone, Debug)]