Make the group-key identifier a content fingerprint

GroupKey.Bytes() returned only {len(A), len(BTilde)} — the dimensions,
which every key of one parameter set shares — so two independent group
keys carried the identical identifier 0808. Anything keyed on it (a
registry, a route, a cache) treated every Corona group as the same
group. Return sha256 of the canonical serialization instead, so distinct
keys have distinct identifiers.

Co-authored-by: Hanzo Dev <[email protected]>

threshold/threshold.go

@@ -13,9 +13,10 @@
 package threshold
 
 import (
-	"fmt"
 	"crypto/rand"
+	"crypto/sha256"
 	"errors"
+	"fmt"
 	"io"
 	"math/big"
 
@@ -78,8 +79,16 @@ func (gk *GroupKey) Bytes() []byte {
 	if gk == nil || gk.BTilde == nil {
 		return nil
 	}
-	// Return size info as a simple representation
-	return []byte{byte(len(gk.A)), byte(len(gk.BTilde))}
+	// A content fingerprint over the canonical serialization, so distinct group
+	// keys have distinct identifiers. The previous value was only the (A, BTilde)
+	// dimensions, which every key of one parameter set shares — anything keyed on
+	// it collapsed every Corona group into one.
+	canonical, err := gk.MarshalBinary()
+	if err != nil {
+		return nil
+	}
+	sum := sha256.Sum256(canonical)
+	return sum[:]
 }
 
 // KeyShare holds a party's secret share data.

threshold/threshold_test.go

@@ -4,6 +4,8 @@
 package threshold
 
 import (
+	"bytes"
+	"crypto/rand"
 	"testing"
 )
 
@@ -248,3 +250,27 @@ func TestInvalidThreshold(t *testing.T) {
 		t.Errorf("expected ErrInvalidPartyCount, got %v", err)
 	}
 }
+
+// TestGroupKeyBytes_IsAContentFingerprint pins that the group-key identifier
+// distinguishes distinct keys. It was only the (A, BTilde) dimensions, which
+// every key of one parameter set shares, so anything keyed on it collapsed
+// every Corona group into one.
+func TestGroupKeyBytes_IsAContentFingerprint(t *testing.T) {
+	_, a, err := GenerateKeysTrustedDealer(1, 2, rand.Reader)
+	if err != nil {
+		t.Fatal(err)
+	}
+	_, b, err := GenerateKeysTrustedDealer(1, 2, rand.Reader)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if bytes.Equal(a.Bytes(), b.Bytes()) {
+		t.Fatalf("two distinct group keys share the identifier %x", a.Bytes())
+	}
+	if !bytes.Equal(a.Bytes(), a.Bytes()) {
+		t.Fatal("group-key identifier is not stable across calls")
+	}
+	if len(a.Bytes()) != 32 {
+		t.Fatalf("identifier length = %d, want a 32-byte content hash", len(a.Bytes()))
+	}
+}