1 files changed, 50 insertions, 0 deletions
diff --git a/internal/server/auth_test.go b/internal/server/auth_test.go
new file mode 100644
index 0000000..ecd3c08
--- /dev/null
+++ b/internal/server/auth_test.go
@@ -0,0 +1,50 @@
+package server
+
+import (
+	"crypto/subtle"
+	"testing"
+)
+
+// TestConstantTimePasswordCompare verifies that password comparisons use
+// constant-time comparison to prevent timing side-channel attacks.
+// This is a regression test for the bug where `!=` was used directly,
+// leaking timing information to an attacker who can measure response latency.
+func TestConstantTimePasswordCompare(t *testing.T) {
+	t.Parallel()
+
+	tests := []struct {
+		name     string
+		a        string
+		b        string
+		wantSame bool
+	}{
+		{"equal passwords", "secret123", "secret123", true},
+		{"different passwords", "secret123", "wrongpass", false},
+		{"empty vs non-empty", "", "secret", false},
+		{"both empty", "", "", true},
+		{"prefix match only", "secret", "secretXYZ", false},
+		{"suffix match only", "XYZsecret", "secret", false},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			t.Parallel()
+
+			// secretsEqual is the function under test — it must use
+			// crypto/subtle.ConstantTimeCompare, not plain ==.
+			got := secretsEqual(tt.a, tt.b)
+			if got != tt.wantSame {
+				t.Errorf("secretsEqual(%q, %q) = %v, want %v",
+					tt.a, tt.b, got, tt.wantSame)
+			}
+
+			// Cross-check with the reference implementation to ensure our
+			// helper is semantically correct, not just timing-safe.
+			reference := subtle.ConstantTimeCompare([]byte(tt.a), []byte(tt.b)) == 1
+			if got != reference {
+				t.Errorf("secretsEqual(%q, %q) = %v but crypto/subtle gives %v",
+					tt.a, tt.b, got, reference)
+			}
+		})
+	}
+}