Fix some FROST nits (#63)

* Impl DefaultIsZeros for every type that uses jubjub::Fr/Scalar

This requires Copy and Clone along with Default. If we do not want to include those, we can impl Zeroize and Drop directly.

* Hash signature message with HStar before deriving the binding factor

To avoid a collision, we should hash our input message, our 'standard' hash is HStar, which uses a domain separator already, and is the same one that generates the binding factor.

* Add a comment about why we hash the signature message before generating the binding factor

* Add comments on how we Zeroize

* Consume nonces with sign()

We want to make sure that the nonces we use when signing are Drop'd
(and thus Zeroize'd) when they go out of scope, so we must move participant_nonces into sign()

src/frost.rs

@@ -22,31 +22,22 @@
 //! Internally, keygen_with_dealer generates keys using Verifiable Secret
 //! Sharing,  where shares are generated using Shamir Secret Sharing.
 
+use std::{collections::HashMap, convert::TryFrom, marker::PhantomData};
+
 use rand_core::{CryptoRng, RngCore};
-use std::convert::TryFrom;
-
-use std::{collections::HashMap, marker::PhantomData};
-
-use zeroize::Zeroize;
+use zeroize::DefaultIsZeroes;
 
 use crate::private::Sealed;
 use crate::{HStar, Scalar, Signature, SpendAuth, VerificationKey};
 
 /// A secret scalar value representing a single signer's secret key.
-#[derive(Clone, Default)]
+#[derive(Clone, Copy, Default)]
 pub struct Secret(Scalar);
 
-impl Zeroize for Secret {
-    fn zeroize(&mut self) {
-        self.0 = Scalar::zero();
-    }
-}
-
-impl Drop for Secret {
-    fn drop(&mut self) {
-        self.zeroize();
-    }
-}
+// Zeroizes `Secret` to be the `Default` value on drop (when it goes out of
+// scope).  Luckily the derived `Default` includes the `Default` impl of
+// jubjub::Fr/Scalar, which is four 0u64's under the hood.
+impl DefaultIsZeroes for Secret {}
 
 impl From<Scalar> for Secret {
     fn from(source: Scalar) -> Secret {
@@ -318,16 +309,16 @@ fn generate_shares<R: RngCore + CryptoRng>(
 /// Note that [`SigningNonces`] must be used *only once* for a signing
 /// operation; re-using nonces will result in leakage of a signer's long-lived
 /// signing key.
-#[derive(Clone)]
+#[derive(Clone, Copy, Default)]
 pub struct SigningNonces {
     hiding: Scalar,
     binding: Scalar,
 }
 
-// TODO finish drop
-impl Drop for SigningNonces {
-    fn drop(&mut self) {}
-}
+// Zeroizes `SigningNonces` to be the `Default` value on drop (when it goes out
+// of scope).  Luckily the derived `Default` includes the `Default` impl of the
+// `jubjub::Fr/Scalar`'s, which is four 0u64's under the hood.
+impl DefaultIsZeroes for SigningNonces {}
 
 impl SigningNonces {
     /// Generates a new signing nonce.
@@ -384,12 +375,20 @@ pub struct SigningPackage {
 
 /// A participant's signature share, which the coordinator will use to aggregate
 /// with all other signer's shares into the joint signature.
+#[derive(Clone, Copy, Default)]
 pub struct SignatureShare {
     /// Represents the participant index.
     pub(crate) index: u32,
     /// This participant's signature over the message.
     pub(crate) signature: Scalar,
 }
+
+// Zeroizes `SignatureShare` to be the `Default` value on drop (when it goes out
+// of scope).  Luckily the derived `Default` includes the `Default` impl of
+// jubjub::Fr/Scalar, which is four 0u64's under the hood, and u32, which is
+// 0u32.
+impl DefaultIsZeroes for SignatureShare {}
+
 impl SignatureShare {
     /// Tests if a signature share issued by a participant is valid before
     /// aggregating it into a final joint signature to publish.
@@ -439,11 +438,19 @@ where
 /// Generates the binding factor that ensures each signature share is strongly
 /// bound to a signing set, specific set of commitments, and a specific message.
 fn gen_rho_i(index: u32, signing_package: &SigningPackage) -> Scalar {
+    // Hash signature message with HStar before deriving the binding factor.
+    //
+    // To avoid a collision with other inputs to the hash that generates the
+    // binding factor, we should hash our input message first. Our 'standard'
+    // hash is HStar, which uses a domain separator already, and is the same one
+    // that generates the binding factor.
+    let message_hash = HStar::default().update(signing_package.message).finalize();
+
     let mut hasher = HStar::default();
     hasher
         .update("FROST_rho".as_bytes())
         .update(index.to_be_bytes())
-        .update(signing_package.message);
+        .update(message_hash.to_bytes());
 
     for item in signing_package.signing_commitments.iter() {
         hasher.update(item.index.to_be_bytes());
@@ -524,7 +531,7 @@ fn gen_lagrange_coeff(
 /// the commitment that was assigned by the coordinator in the SigningPackage.
 pub fn sign(
     signing_package: &SigningPackage,
-    participant_nonces: &SigningNonces, // TODO this should probably consume the nonce
+    participant_nonces: SigningNonces,
     share_package: &SharePackage,
 ) -> Result<SignatureShare, &'static str> {
     let mut bindings: HashMap<u32, Scalar> =

tests/frost.rs

@@ -39,9 +39,9 @@ fn check_sign_with_dealer() {
             .iter()
             .find(|share| participant_index == share.index)
             .unwrap();
-        let nonce_to_use = &nonce[0];
+        let nonce_to_use = nonce[0];
         // Each participant generates their signature share.
-        let signature_share = frost::sign(&signing_package, &nonce_to_use, share_package).unwrap();
+        let signature_share = frost::sign(&signing_package, nonce_to_use, share_package).unwrap();
         signature_shares.push(signature_share);
     }