Make Schnorr modular to its transcript

crypto/ciphersuite/src/lib.rs

@@ -53,7 +53,7 @@ pub trait Ciphersuite:
   type G: Group<Scalar = Self::F> + GroupOps + PrimeGroup + Zeroize + ConstantTimeEq;
   /// Hash algorithm used with this curve.
   // Requires BlockSizeUser so it can be used within Hkdf which requies that.
-  type H: Clone + BlockSizeUser + Digest + HashMarker + SecureDigest;
+  type H: Send + Clone + BlockSizeUser + Digest + HashMarker + SecureDigest;
 
   /// ID for this curve.
   const ID: &'static [u8];

crypto/frost/src/algorithm.rs

@@ -127,28 +127,42 @@ pub trait Hram<C: Curve>: Send + Clone {
   fn hram(R: &C::G, A: &C::G, m: &[u8]) -> C::F;
 }
 
-/// IETF-compliant Schnorr signature algorithm ((R, s) where s = r + cx).
+/// Schnorr signature algorithm ((R, s) where s = r + cx).
 #[derive(Clone)]
-pub struct Schnorr<C: Curve, H: Hram<C>> {
-  transcript: IetfTranscript,
+pub struct Schnorr<C: Curve, T: Clone + Debug + Transcript, H: Hram<C>> {
+  transcript: T,
   c: Option<C::F>,
   _hram: PhantomData<H>,
 }
 
-impl<C: Curve, H: Hram<C>> Default for Schnorr<C, H> {
-  fn default() -> Self {
-    Self::new()
+/// IETF-compliant Schnorr signature algorithm.
+///
+/// This algorithm specifically uses the transcript format defined in the FROST IETF draft.
+/// It's a naive transcript format not viable for usage in larger protocols, yet is presented here
+/// in order to provide compatibility.
+///
+/// Usage of this with key offsets will break the intended compatibility as the IETF draft does not
+/// specify a protocol for offsets.
+pub type IetfSchnorr<C, H> = Schnorr<C, IetfTranscript, H>;
+
+impl<C: Curve, T: Clone + Debug + Transcript, H: Hram<C>> Schnorr<C, T, H> {
+  /// Construct a Schnorr algorithm continuing the specified transcript.
+  pub fn new(transcript: T) -> Schnorr<C, T, H> {
+    Schnorr { transcript, c: None, _hram: PhantomData }
   }
 }
 
-impl<C: Curve, H: Hram<C>> Schnorr<C, H> {
-  pub fn new() -> Schnorr<C, H> {
-    Schnorr { transcript: IetfTranscript(vec![]), c: None, _hram: PhantomData }
+impl<C: Curve, H: Hram<C>> IetfSchnorr<C, H> {
+  /// Construct a IETF-compatible Schnorr algorithm.
+  ///
+  /// Please see the IetfSchnorr documentation for the full details of this.
+  pub fn ietf() -> IetfSchnorr<C, H> {
+    Schnorr::new(IetfTranscript(vec![]))
   }
 }
 
-impl<C: Curve, H: Hram<C>> Algorithm<C> for Schnorr<C, H> {
-  type Transcript = IetfTranscript;
+impl<C: Curve, T: Clone + Debug + Transcript, H: Hram<C>> Algorithm<C> for Schnorr<C, T, H> {
+  type Transcript = T;
   type Addendum = ();
   type Signature = SchnorrSignature<C>;
 

crypto/frost/src/tests/mod.rs

@@ -6,7 +6,7 @@ pub use dkg::tests::{key_gen, recover_key};
 
 use crate::{
   Curve, Participant, ThresholdKeys, FrostError,
-  algorithm::{Algorithm, Hram, Schnorr},
+  algorithm::{Algorithm, Hram, IetfSchnorr},
   sign::{Writable, PreprocessMachine, SignMachine, SignatureMachine, AlgorithmMachine},
 };
 
@@ -197,8 +197,8 @@ pub fn test_schnorr<R: RngCore + CryptoRng, C: Curve, H: Hram<C>>(rng: &mut R) {
   const MSG: &[u8] = b"Hello, World!";
 
   let keys = key_gen(&mut *rng);
-  let machines = algorithm_machines(&mut *rng, Schnorr::<C, H>::new(), &keys);
-  let sig = sign(&mut *rng, Schnorr::<C, H>::new(), keys.clone(), machines, MSG);
+  let machines = algorithm_machines(&mut *rng, IetfSchnorr::<C, H>::ietf(), &keys);
+  let sig = sign(&mut *rng, IetfSchnorr::<C, H>::ietf(), keys.clone(), machines, MSG);
   let group_key = keys[&Participant::new(1).unwrap()].group_key();
   assert!(sig.verify(group_key, H::hram(&sig.R, &group_key, MSG)));
 }
@@ -217,8 +217,8 @@ pub fn test_offset_schnorr<R: RngCore + CryptoRng, C: Curve, H: Hram<C>>(rng: &m
     assert_eq!(keys.group_key(), offset_key);
   }
 
-  let machines = algorithm_machines(&mut *rng, Schnorr::<C, H>::new(), &keys);
-  let sig = sign(&mut *rng, Schnorr::<C, H>::new(), keys.clone(), machines, MSG);
+  let machines = algorithm_machines(&mut *rng, IetfSchnorr::<C, H>::ietf(), &keys);
+  let sig = sign(&mut *rng, IetfSchnorr::<C, H>::ietf(), keys.clone(), machines, MSG);
   let group_key = keys[&Participant::new(1).unwrap()].group_key();
   assert!(sig.verify(offset_key, H::hram(&sig.R, &group_key, MSG)));
 }
@@ -228,7 +228,7 @@ pub fn test_schnorr_blame<R: RngCore + CryptoRng, C: Curve, H: Hram<C>>(rng: &mu
   const MSG: &[u8] = b"Hello, World!";
 
   let keys = key_gen(&mut *rng);
-  let machines = algorithm_machines(&mut *rng, Schnorr::<C, H>::new(), &keys);
+  let machines = algorithm_machines(&mut *rng, IetfSchnorr::<C, H>::ietf(), &keys);
 
   let (mut machines, shares) = preprocess_and_shares(&mut *rng, machines, |_, _| {}, MSG);
 

crypto/frost/src/tests/vectors.rs

@@ -14,7 +14,7 @@ use ciphersuite::group::{ff::PrimeField, GroupEncoding};
 use crate::{
   curve::Curve,
   Participant, ThresholdCore, ThresholdKeys,
-  algorithm::{IetfTranscript, Hram, Schnorr},
+  algorithm::{IetfTranscript, Hram, IetfSchnorr},
   sign::{
     Writable, Nonce, GeneratorCommitments, NonceCommitments, Commitments, Preprocess,
     PreprocessMachine, SignMachine, SignatureMachine, AlgorithmMachine,
@@ -160,7 +160,8 @@ pub fn test_with_vectors<R: RngCore + CryptoRng, C: Curve, H: Hram<C>>(
 
     let mut machines = vec![];
     for i in &vectors.included {
-      machines.push((i, AlgorithmMachine::new(Schnorr::<C, H>::new(), keys[i].clone()).unwrap()));
+      machines
+        .push((i, AlgorithmMachine::new(IetfSchnorr::<C, H>::ietf(), keys[i].clone()).unwrap()));
     }
 
     let mut commitments = HashMap::new();
@@ -342,7 +343,8 @@ pub fn test_with_vectors<R: RngCore + CryptoRng, C: Curve, H: Hram<C>>(
     // Create the machines
     let mut machines = vec![];
     for i in &vectors.included {
-      machines.push((i, AlgorithmMachine::new(Schnorr::<C, H>::new(), keys[i].clone()).unwrap()));
+      machines
+        .push((i, AlgorithmMachine::new(IetfSchnorr::<C, H>::ietf(), keys[i].clone()).unwrap()));
     }
 
     for (i, machine) in machines.drain(..) {

crypto/schnorr/src/aggregate.rs

@@ -16,7 +16,7 @@ use multiexp::multiexp_vartime;
 use crate::SchnorrSignature;
 
 // Returns a unbiased scalar weight to use on a signature in order to prevent malleability
-fn weight<D: Clone + SecureDigest, F: PrimeField>(digest: &mut DigestTranscript<D>) -> F {
+fn weight<D: Send + Clone + SecureDigest, F: PrimeField>(digest: &mut DigestTranscript<D>) -> F {
   let mut bytes = digest.challenge(b"aggregation_weight");
   debug_assert_eq!(bytes.len() % 8, 0);
   // This should be guaranteed thanks to SecureDigest

crypto/transcript/src/lib.rs

@@ -16,8 +16,8 @@ use digest::{
   Digest, Output, HashMarker,
 };
 
-pub trait Transcript {
-  type Challenge: Clone + Send + Sync + AsRef<[u8]>;
+pub trait Transcript: Send + Clone {
+  type Challenge: Send + Sync + Clone + AsRef<[u8]>;
 
   /// Create a new transcript with the specified name.
   fn new(name: &'static [u8]) -> Self;
@@ -83,9 +83,9 @@ where
 
 /// A simple transcript format constructed around the specified hash algorithm.
 #[derive(Clone, Debug)]
-pub struct DigestTranscript<D: Clone + SecureDigest>(D);
+pub struct DigestTranscript<D: Send + Clone + SecureDigest>(D);
 
-impl<D: Clone + SecureDigest> DigestTranscript<D> {
+impl<D: Send + Clone + SecureDigest> DigestTranscript<D> {
   fn append(&mut self, kind: DigestTranscriptMember, value: &[u8]) {
     self.0.update([kind.as_u8()]);
     // Assumes messages don't exceed 16 exabytes
@@ -94,7 +94,7 @@ impl<D: Clone + SecureDigest> DigestTranscript<D> {
   }
 }
 
-impl<D: Clone + SecureDigest> Transcript for DigestTranscript<D> {
+impl<D: Send + Clone + SecureDigest> Transcript for DigestTranscript<D> {
   type Challenge = Output<D>;
 
   fn new(name: &'static [u8]) -> Self {