Add common_scalar method to Transcript.

src/transcript.rs

@@ -19,6 +19,10 @@ pub trait Transcript<C: CurveAffine> {
     /// Writing the point to the transcript without writing it to the proof,
     /// treating it as a common input.
     fn common_point(&mut self, point: C) -> io::Result<()>;
+
+    /// Writing the scalar to the transcript without writing it to the proof,
+    /// treating it as a common input.
+    fn common_scalar(&mut self, scalar: C::Scalar) -> io::Result<()>;
 }
 
 /// Transcript view from the perspective of a verifier that has access to an
@@ -84,7 +88,7 @@ impl<R: Read, C: CurveAffine> TranscriptRead<C> for Blake2bRead<R, C> {
                 "invalid field element encoding in proof",
             )
         })?;
-        self.state.update(&scalar.to_bytes());
+        self.common_scalar(scalar)?;
 
         Ok(scalar)
     }
@@ -104,6 +108,12 @@ impl<R: Read, C: CurveAffine> Transcript<C> for Blake2bRead<R, C> {
         Ok(())
     }
 
+    fn common_scalar(&mut self, scalar: C::Scalar) -> io::Result<()> {
+        self.state.update(&scalar.to_bytes());
+
+        Ok(())
+    }
+
     fn squeeze_challenge(&mut self) -> C::Base {
         let hasher = self.state.clone();
         let result: [u8; 64] = hasher.finalize().as_bytes().try_into().unwrap();
@@ -147,7 +157,7 @@ impl<W: Write, C: CurveAffine> TranscriptWrite<C> for Blake2bWrite<W, C> {
         self.writer.write_all(&compressed[..])
     }
     fn write_scalar(&mut self, scalar: C::Scalar) -> io::Result<()> {
-        self.state.update(&scalar.to_bytes());
+        self.common_scalar(scalar)?;
         let data = scalar.to_bytes();
         self.writer.write_all(&data[..])
     }
@@ -167,6 +177,12 @@ impl<W: Write, C: CurveAffine> Transcript<C> for Blake2bWrite<W, C> {
         Ok(())
     }
 
+    fn common_scalar(&mut self, scalar: C::Scalar) -> io::Result<()> {
+        self.state.update(&scalar.to_bytes());
+
+        Ok(())
+    }
+
     fn squeeze_challenge(&mut self) -> C::Base {
         let hasher = self.state.clone();
         let result: [u8; 64] = hasher.finalize().as_bytes().try_into().unwrap();