Fix the known instance of #295

crypto/frost/src/lib.rs

@@ -26,6 +26,9 @@ pub mod tests;
 /// Various errors possible during signing.
 #[derive(Clone, Copy, PartialEq, Eq, Debug, Error)]
 pub enum FrostError {
+  #[error("internal error: {0}")]
+  InternalError(&'static str),
+
   #[error("invalid participant (0 < participant <= {0}, yet participant is {1})")]
   InvalidParticipant(u16, Participant),
   #[error("invalid signing set ({0})")]

crypto/frost/src/sign.rs

@@ -516,6 +516,8 @@ impl<C: Curve, A: Algorithm<C>> SignatureMachine<A::Signature> for AlgorithmSign
     }
 
     // If everyone has a valid share, and there were enough participants, this should've worked
-    panic!("everyone had a valid share yet the signature was still invalid");
+    // The only known way to cause this, for valid parameters/algorithms, is to deserialize a
+    // semantically invalid FrostKeys
+    Err(FrostError::InternalError("everyone had a valid share yet the signature was still invalid"))
   }
 }