From 0230ccb29dcb7a7965bcd09b8ee86475c2ea499e Mon Sep 17 00:00:00 2001
From: Conrado Gouvea <conradoplg@gmail.com>
Date: Thu, 1 Sep 2022 17:40:20 -0300
Subject: [PATCH] fix batch support after rebase

---
 frost-core/src/batch.rs         |  2 +-
 frost-core/src/lib.rs           | 36 +++++++++++++++++++++------------
 frost-core/src/verifying_key.rs |  2 +-
 3 files changed, 25 insertions(+), 15 deletions(-)

diff --git a/frost-core/src/batch.rs b/frost-core/src/batch.rs
index 8547120e..b8d22133 100644
--- a/frost-core/src/batch.rs
+++ b/frost-core/src/batch.rs
@@ -50,7 +50,7 @@ where
     /// requires borrowing the message data, the `Item` type is unlinked
     /// from the lifetime of the message.
     pub fn verify_single(self) -> Result<(), Error> {
-        self.vk.verify_prehashed(&self.sig, self.c)
+        verify_prehashed::<C>(self.c, &self.sig, &self.vk)
     }
 }
 
diff --git a/frost-core/src/lib.rs b/frost-core/src/lib.rs
index 272712b5..524d6ec0 100644
--- a/frost-core/src/lib.rs
+++ b/frost-core/src/lib.rs
@@ -24,6 +24,28 @@ pub use signature::Signature;
 pub use signing_key::SigningKey;
 pub use verifying_key::VerifyingKey;
 
+/// Verify a purported `signature` with a pre-hashed [`Challenge`] made by this verification
+/// key.
+pub(crate) fn verify_prehashed<C: Ciphersuite>(
+    challenge: Challenge<C>,
+    signature: &Signature<C>,
+    public_key: &VerifyingKey<C>,
+) -> Result<(), Error> {
+    // Verify check is h * ( - z * B + R  + c * A) == 0
+    //                 h * ( z * B - c * A - R) == 0
+    //
+    // where h is the cofactor
+    let zB = C::Group::generator() * signature.z;
+    let cA = public_key.element * challenge.0;
+    let check = (zB - cA - signature.R) * C::Group::cofactor();
+
+    if check == C::Group::identity() {
+        Ok(())
+    } else {
+        Err(Error::InvalidSignature)
+    }
+}
+
 /// A prime order finite field GF(q) over which all scalar values for our prime order group can be
 /// multiplied are defined.
 ///
@@ -208,19 +230,7 @@ pub trait Ciphersuite: Copy + Clone {
     ) -> Result<(), Error> {
         let c = crate::challenge::<Self>(&signature.R, &public_key.element, msg);
 
-        // Verify check is h * ( - z * B + R  + c * A) == 0
-        //                 h * ( z * B - c * A - R) == 0
-        //
-        // where h is the cofactor
-        let zB = Self::Group::generator() * signature.z;
-        let cA = public_key.element * c.0;
-        let check = (zB - cA - signature.R) * Self::Group::cofactor();
-
-        if check == Self::Group::identity() {
-            Ok(())
-        } else {
-            Err(Error::InvalidSignature)
-        }
+        verify_prehashed(c, signature, public_key)
     }
 }
 
diff --git a/frost-core/src/verifying_key.rs b/frost-core/src/verifying_key.rs
index 2d1429ec..fd68cee4 100644
--- a/frost-core/src/verifying_key.rs
+++ b/frost-core/src/verifying_key.rs
@@ -2,7 +2,7 @@ use std::fmt::{self, Debug};
 
 use hex::FromHex;
 
-use crate::{Challenge, Ciphersuite, Error, Group, Signature};
+use crate::{Ciphersuite, Error, Group, Signature};
 
 /// A valid verifying key for Schnorr signatures over a FROST [`Ciphersuite::Group`].
 #[derive(Copy, Clone, PartialEq)]