From 40289cab24a5dc010f7e77b6f62f83966b4372ba Mon Sep 17 00:00:00 2001
From: maclane <maclane@nucypher.com>
Date: Tue, 9 Jun 2026 21:58:09 -0400
Subject: [PATCH] Pin cross-language coordinator selection vectors

The Rust engine ports Go's math/rand shuffle semantics
(pkg/tbtc/signer/src/go_math_rand.rs on the FROST signer mirror
branch) and pins concrete SelectCoordinator outcomes in
select_coordinator_matches_known_keep_core_vectors. The Go side only
asserted determinism properties, so a Go-side change to the selection
semantics (math/rand/v2, a different shuffle, a different seed
composition) would pass the Go suite while silently fracturing
coordinator agreement with the Rust engine.

Pin the same concrete vectors on the Go side so either side drifting
fails its own test suite.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
---
 pkg/frost/roast/coordinator_test.go | 61 +++++++++++++++++++++++++++++
 1 file changed, 61 insertions(+)

diff --git a/pkg/frost/roast/coordinator_test.go b/pkg/frost/roast/coordinator_test.go
index 0847685de6..f93b95f424 100644
--- a/pkg/frost/roast/coordinator_test.go
+++ b/pkg/frost/roast/coordinator_test.go
@@ -109,3 +109,64 @@ func TestSelectCoordinator_AffectedBySeed(t *testing.T) {
 		t.Fatal("coordinator did not change for any seed")
 	}
 }
+
+// TestSelectCoordinator_CrossLanguagePinnedVectors pins concrete
+// SelectCoordinator outputs so cross-language agreement with the Rust
+// engine is enforced by tests on both sides. The Rust port of Go's
+// math/rand (pkg/tbtc/signer/src/go_math_rand.rs,
+// select_coordinator_matches_known_keep_core_vectors) asserts these
+// exact values; the FROST/ROAST liveness path depends on every honest
+// node electing the same coordinator for the same attempt.
+//
+// If this test breaks, the Go selection semantics changed (for
+// example a move to math/rand/v2, a different shuffle, or a different
+// seed composition). That is a network-fracturing change: it must be
+// coordinated with the Rust engine and rolled out as a new versioned
+// selection rule, never shipped silently.
+func TestSelectCoordinator_CrossLanguagePinnedVectors(t *testing.T) {
+	const seed = int64(6879463052285329321)
+
+	vectors := []struct {
+		members     []group.MemberIndex
+		seed        int64
+		attempt     uint
+		coordinator group.MemberIndex
+	}{
+		{[]group.MemberIndex{1, 2}, seed, 1, 2},
+		{[]group.MemberIndex{1, 2}, seed, 2, 1},
+		{[]group.MemberIndex{1, 2}, seed, 3, 2},
+		{[]group.MemberIndex{1, 2, 3}, seed, 1, 3},
+		{[]group.MemberIndex{1, 2, 3}, seed, 2, 2},
+		{[]group.MemberIndex{1, 2, 3}, seed, 4, 1},
+		{[]group.MemberIndex{1, 2, 3, 4, 5, 6}, 333, 4, 4},
+	}
+
+	for _, vector := range vectors {
+		actual, err := SelectCoordinator(
+			vector.members,
+			vector.seed,
+			vector.attempt,
+		)
+		if err != nil {
+			t.Fatalf(
+				"selection failed for members [%v] seed [%d] attempt [%d]: [%v]",
+				vector.members,
+				vector.seed,
+				vector.attempt,
+				err,
+			)
+		}
+
+		if actual != vector.coordinator {
+			t.Fatalf(
+				"pinned vector drift for members [%v] seed [%d] attempt [%d]\n"+
+					"expected: [%v]\nactual:   [%v]",
+				vector.members,
+				vector.seed,
+				vector.attempt,
+				vector.coordinator,
+				actual,
+			)
+		}
+	}
+}