Fix incorrect sig_hash generation

sig_hash was used as a challenge. challenges should be of the form H(R, A, m).
These sig hashes were solely H(A, m), allowing trivial forgeries.

coordinator/src/tributary/mod.rs

@@ -458,11 +458,9 @@ impl Transaction {
     signed_ref.signer = Ristretto::generator() * key.deref();
     signed_ref.nonce = nonce;
 
+    let sig_nonce = Zeroizing::new(<Ristretto as Ciphersuite>::F::random(rng));
+    signed(self).signature.R = <Ristretto as Ciphersuite>::generator() * sig_nonce.deref();
     let sig_hash = self.sig_hash(genesis);
-    signed(self).signature = SchnorrSignature::<Ristretto>::sign(
+    signed(self).signature = SchnorrSignature::<Ristretto>::sign(key, sig_nonce, sig_hash);
-      key,
-      Zeroizing::new(<Ristretto as Ciphersuite>::F::random(rng)),
-      sig_hash,
-    );
   }
 }

coordinator/tributary/src/tests/transaction/mod.rs

@@ -1,3 +1,4 @@
+use core::ops::Deref;
 use std::{io, collections::HashMap};
 
 use zeroize::Zeroizing;
@@ -114,11 +115,9 @@ pub fn signed_transaction<R: RngCore + CryptoRng>(
   let mut tx =
     SignedTransaction(data, Signed { signer, nonce, signature: random_signed(rng).signature });
 
-  tx.1.signature = SchnorrSignature::sign(
+  let sig_nonce = Zeroizing::new(<Ristretto as Ciphersuite>::F::random(rng));
-    key,
+  tx.1.signature.R = Ristretto::generator() * sig_nonce.deref();
-    Zeroizing::new(<Ristretto as Ciphersuite>::F::random(rng)),
+  tx.1.signature = SchnorrSignature::sign(key, sig_nonce, tx.sig_hash(genesis));
-    tx.sig_hash(genesis),
-  );
 
   let mut nonces = HashMap::from([(signer, nonce)]);
   verify_transaction(&tx, genesis, &mut nonces).unwrap();

coordinator/tributary/src/transaction.rs

@@ -98,10 +98,20 @@ pub trait Transaction: 'static + Send + Sync + Clone + Eq + Debug + ReadWrite {
   /// Obtain the challenge for this transaction's signature.
   ///
   /// Do not override this unless you know what you're doing.
+  ///
+  /// Panics if called on non-signed transactions.
   fn sig_hash(&self, genesis: [u8; 32]) -> <Ristretto as Ciphersuite>::F {
+    match self.kind() {
+      TransactionKind::Signed(Signed { signature, .. }) => {
         <Ristretto as Ciphersuite>::F::from_bytes_mod_order_wide(
-      &Blake2b512::digest([genesis, self.hash()].concat()).into(),
+          &Blake2b512::digest(
+            [genesis.as_ref(), &self.hash(), signature.R.to_bytes().as_ref()].concat(),
           )
+          .into(),
+        )
+      }
+      _ => panic!("sig_hash called on non-signed transaction"),
+    }
   }
 }