Derive Delta discussion

chain-signatures/node/src/indexer.rs

@@ -103,8 +103,8 @@ impl NearIndexer {
         let payload = pending_request.payload;
         let epsilon = pending_request.epsilon;
 
-        // no longer taking entropy from logs, but this is merely for integration tests, so
-        // it doesn't matter as much as long as the IT nodes agree on the entropy.
+        // No longer taking entropy from logs, but this is merely for integration tests, so
+        // it doesn't matter as much as long as the integration tests nodes agree on the entropy.
         let entropy = self.derive_entropy_from_sign_id(&sign_id);
         // NOTE: path is not used at all currently in signature.rs during signing, so hardcoding
         // it here won't matter.

chain-signatures/node/src/indexer_eth/mod.rs

@@ -363,8 +363,11 @@ fn sign_request_from_filtered_log(log: Log) -> Option<IndexedSignRequest> {
         &event.path,
     );
 
-    // Use transaction hash as entropy
-    let entropy = log.transaction_hash.unwrap_or_default();
+    // Use Ethereum transaction hash as entropy
+    let Some(entropy) = log.transaction_hash else {
+        tracing::error!("log missing transaction hash, skipping sign request");
+        return None;
+    };
 
     let sign_id = SignId::new(event.generate_request_id());
     tracing::info!(?sign_id, "eth signature requested");

chain-signatures/node/src/indexer_hydration.rs

@@ -480,7 +480,7 @@ pub async fn run(
                     event
                 );
 
-                let entropy = sp_core::hashing::blake2_256(ev.bytes());
+                let entropy = entropy_hydration(&ev);
 
                 if let Err(e) = crate::stream::ops::process_sign_event(
                     Box::new(event),
@@ -533,8 +533,7 @@ pub async fn run(
                     "Hydration::Signet::SignBidirectionalRequested in block #{number} ({hash:?}): {:?}",
                 event
                 );
-
-                let entropy = sp_core::hashing::blake2_256(ev.bytes());
+                let entropy = entropy_hydration(&ev);
 
                 if let Err(e) = crate::stream::ops::process_sign_event(
                     Box::new(event),
@@ -595,6 +594,11 @@ pub fn spawn_runtime_updater(api: OnlineClient<SubstrateConfig>) {
     });
 }
 
+/// Blake2-256 hash of the raw Substrate event bytes.
+fn entropy_hydration(ev: &EventDetails<SubstrateConfig>) -> [u8; 32] {
+    sp_core::hashing::blake2_256(ev.bytes())
+}
+
 fn decode_signature_requested(
     ev: &EventDetails<SubstrateConfig>,
 ) -> anyhow::Result<HydrationSignatureRequestedEvent> {

chain-signatures/node/src/indexer_sol.rs

@@ -487,6 +487,8 @@ fn build_sign_request(
     sign_event: SignatureEventBox,
     tx_sig: Vec<u8>,
 ) -> anyhow::Result<IndexedSignRequest> {
+    // First 32 bytes of the Solana transaction signature,
+    // which serves as Solana's unique transaction identifier.
     let mut entropy = [0u8; 32];
     entropy.copy_from_slice(&tx_sig[..32]);
     sign_event.generate_sign_request(entropy)

chain-signatures/node/src/kdf.rs

@@ -6,9 +6,11 @@ use mpc_primitives::Signature;
 use near_primitives::hash::CryptoHash;
 use sha3::Sha3_256;
 
-// In case there are multiple requests in the same block (hence same entropy), we need to ensure
-// that we generate different random scalars as delta tweaks.
-// Receipt ID should be unique inside of a block, so it serves us as the request identifier.
+/// # Parameters
+/// - `request_id`: unique identifier of the sign request (hash of the event fields).
+/// - `entropy`: chain-specific, unique per-request data
+/// - `presignature_big_r`: the R point from the presignature, mixed into the
+///   derivation so delta is also bound to the specific presignature used.
 pub fn derive_delta(
     request_id: [u8; 32],
     entropy: [u8; 32],