From 97e7ffda9fd275b5426c45b91dcfcaecc5ab09f6 Mon Sep 17 00:00:00 2001
From: Peter Nose <peter.nose@gmail.com>
Date: Thu, 5 Sep 2024 11:31:48 +0200
Subject: [PATCH] secret-sharing/src/churp: Verify the number of shareholders

---
 secret-sharing/src/churp/handoff.rs | 18 ++++++++++++++++++
 secret-sharing/src/churp/switch.rs  | 17 ++++-------------
 2 files changed, 22 insertions(+), 13 deletions(-)

diff --git a/secret-sharing/src/churp/handoff.rs b/secret-sharing/src/churp/handoff.rs
index 2ba19e469f7..1ea78bc8b3d 100644
--- a/secret-sharing/src/churp/handoff.rs
+++ b/secret-sharing/src/churp/handoff.rs
@@ -169,12 +169,21 @@ where
     /// Creates a new handoff where the given shareholders will generate
     /// a random secret and receive corresponding secret shares.
     pub fn new(threshold: u8, me: G::Scalar, shareholders: Vec<G::Scalar>) -> Result<Self> {
+        // The number of shareholders must be at least threshold t + 2,
+        // ensuring that even if t Byzantine dealers reveal their secret,
+        // an honest shareholder cannot compute the combined bivariate
+        // polynomial.
+        if shareholders.len() < threshold as usize + 2 {
+            return Err(Error::NotEnoughShareholders.into());
+        }
+
         let share_distribution = DimensionSwitch::new_full_share_distribution(
             threshold,
             me,
             shareholders,
             HandoffKind::DealingPhase,
         )?;
+
         share_distribution.skip_accumulating()?;
         share_distribution.start_merging(None)?;
 
@@ -219,12 +228,17 @@ where
     /// Creates a new handoff where the secret shares of the given shareholders
     /// will be randomized.
     pub fn new(threshold: u8, me: G::Scalar, shareholders: Vec<G::Scalar>) -> Result<Self> {
+        if shareholders.len() < threshold as usize + 1 {
+            return Err(Error::NotEnoughShareholders.into());
+        }
+
         let share_distribution = DimensionSwitch::new_full_share_distribution(
             threshold,
             me,
             shareholders,
             HandoffKind::CommitteeUnchanged,
         )?;
+
         share_distribution.skip_accumulating()?;
 
         Ok(Self { share_distribution })
@@ -278,6 +292,10 @@ where
     /// Creates a new handoff where the shared secret will be transferred
     /// to a new committee composed of the given shareholders.
     pub fn new(threshold: u8, me: G::Scalar, shareholders: Vec<G::Scalar>) -> Result<Self> {
+        if shareholders.len() < threshold as usize + 1 {
+            return Err(Error::NotEnoughShareholders.into());
+        }
+
         let share_reduction = DimensionSwitch::new_share_reduction(
             threshold,
             me,
diff --git a/secret-sharing/src/churp/switch.rs b/secret-sharing/src/churp/switch.rs
index af6a470648d..ca24399a0db 100644
--- a/secret-sharing/src/churp/switch.rs
+++ b/secret-sharing/src/churp/switch.rs
@@ -494,16 +494,7 @@ where
         handoff: HandoffKind,
         shareholder: Option<Arc<Shareholder<G>>>,
     ) -> Result<Self> {
-        // During the dealing phase, the number of shares must be at least
-        // threshold + 2, ensuring that even if t Byzantine dealers reveal
-        // their secret, an honest shareholder cannot compute the combined
-        // bivariate polynomial.
-        let min = match handoff {
-            HandoffKind::DealingPhase => threshold as usize + 2,
-            HandoffKind::CommitteeUnchanged => 1,
-            HandoffKind::CommitteeChanged => 1,
-        };
-        if shareholders.len() < min {
+        if shareholders.is_empty() {
             return Err(Error::NotEnoughShareholders.into());
         }
 
@@ -775,13 +766,13 @@ mod tests {
         let me = prepare_shareholder(1);
         let shareholders = prepare_shareholders(&[1, 2, 3]);
 
-        // Dealing phase requires at least threshold + 2 dealers.
+        // There should be at least 1 shareholder.
         let res = BivariateShares::<Group>::new(
             threshold,
             me,
-            shareholders.clone(),
+            vec![],
             DimensionSwitchKind::ShareReduction,
-            HandoffKind::DealingPhase,
+            hkind,
             None,
         );
         assert!(res.is_err());