From 48d528917ce9539da6a63ec9c90d32de763da4f9 Mon Sep 17 00:00:00 2001
From: Jan Nanista <jan@turnkey.io>
Date: Mon, 11 May 2026 10:13:30 -0700
Subject: [PATCH] DRY the interpolating value derivation code

---
 frost-core/src/lib.rs    | 50 +++++++++++++++++++---------------------
 frost-core/src/round2.rs | 18 +--------------
 2 files changed, 25 insertions(+), 43 deletions(-)

diff --git a/frost-core/src/lib.rs b/frost-core/src/lib.rs
index 7be4bc8..cbd6c6f 100644
--- a/frost-core/src/lib.rs
+++ b/frost-core/src/lib.rs
@@ -341,15 +341,29 @@ fn derive_interpolating_value<C: Ciphersuite>(
     signer_id: &Identifier<C>,
     signing_package: &SigningPackage<C>,
 ) -> Result<Scalar<C>, Error<C>> {
-    compute_lagrange_coefficient(
-        &signing_package
-            .signing_commitments()
-            .keys()
-            .cloned()
-            .collect(),
-        None,
-        *signer_id,
-    )
+    match &signing_package.signing_participants_groups {
+        Some(signing_participants_groups) => {
+            let signing_participants_group = signing_participants_groups.iter().find(|g| g.contains(signer_id));
+
+            match signing_participants_group {
+                Some(group) => compute_lagrange_coefficient(
+                    group,
+                    None,
+                    *signer_id,
+                ),
+                None => Err(Error::UnknownIdentifier),
+            }
+        }
+        None => compute_lagrange_coefficient(
+            &signing_package
+                .signing_commitments()
+                .keys()
+                .cloned()
+                .collect(),
+            None,
+            *signer_id,
+        ),
+    }
 }
 
 /// Generated by the coordinator of the signing operation and distributed to
@@ -874,23 +888,7 @@ fn verify_signature_share_precomputed<C: Ciphersuite>(
     verifying_share: &keys::VerifyingShare<C>,
     challenge: Challenge<C>,
 ) -> Result<(), Error<C>> {
-    let lambda_i = match signing_package.signing_participants_groups.clone() {
-        Some(signing_participants_groups) => {
-            let mut result: Result<Scalar<C>, Error<C>> = Err(Error::UnknownIdentifier);
-            for signing_participants_group in signing_participants_groups {
-                if signing_participants_group.contains(&signature_share_identifier) {
-                    result = compute_lagrange_coefficient(
-                        &signing_participants_group,
-                        None,
-                        signature_share_identifier,
-                    );
-                    break;
-                }
-            }
-            result?
-        }
-        None => derive_interpolating_value(&signature_share_identifier, signing_package)?,
-    };
+    let lambda_i = derive_interpolating_value(&signature_share_identifier, signing_package)?;
 
     let binding_factor = binding_factor_list
         .get(&signature_share_identifier)
diff --git a/frost-core/src/round2.rs b/frost-core/src/round2.rs
index 3fd3060..b7a1e90 100644
--- a/frost-core/src/round2.rs
+++ b/frost-core/src/round2.rs
@@ -160,23 +160,7 @@ pub fn sign<C: Ciphersuite>(
     let group_commitment = compute_group_commitment(&signing_package, &binding_factor_list)?;
 
     // Compute Lagrange coefficient.
-    let lambda_i = match signing_package.signing_participants_groups.clone() {
-        Some(signing_participants_groups) => {
-            let mut result: Result<Scalar<C>, Error<C>> = Err(Error::UnknownIdentifier);
-            for signing_participants_group in signing_participants_groups {
-                if signing_participants_group.contains(key_package.identifier()) {
-                    result = frost::compute_lagrange_coefficient(
-                        &signing_participants_group,
-                        None,
-                        *key_package.identifier(),
-                    );
-                    break;
-                }
-            }
-            result?
-        }
-        None => frost::derive_interpolating_value(key_package.identifier(), &signing_package)?,
-    };
+    let lambda_i = frost::derive_interpolating_value(key_package.identifier(), &signing_package)?;
 
     // Compute the per-message challenge.
     let challenge = <C>::challenge(