secretbox: add AES-256-GCM sealing of small secrets

Moved out of Hammerhead's internal package so Sandbar can use it too.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

secretbox/secretbox_test.go

@@ -0,0 +1,51 @@
+package secretbox
+
+import (
+	"strings"
+	"testing"
+)
+
+func TestSealOpenRoundTripAndKeys(t *testing.T) {
+	key := GenerateKey()
+	b, err := New(key)
+	if err != nil {
+		t.Fatal(err)
+	}
+	sealed := b.Seal("hunter2")
+	if !strings.HasPrefix(sealed, "enc1:") || strings.Contains(sealed, "hunter2") {
+		t.Fatalf("not sealed: %q", sealed)
+	}
+	if again := b.Seal("hunter2"); again == sealed {
+		t.Error("two seals of the same value must differ (random nonce)")
+	}
+	if b.Seal(sealed) != sealed {
+		t.Error("sealing an already sealed value must not double-encrypt")
+	}
+	if got, err := b.Open(sealed); err != nil || got != "hunter2" {
+		t.Errorf("Open = %q, %v", got, err)
+	}
+	if got, _ := b.Open("legacy plaintext"); got != "legacy plaintext" {
+		t.Errorf("unsealed legacy data should pass through, got %q", got)
+	}
+	if b.Seal("") != "" {
+		t.Error("empty stays empty")
+	}
+
+	other, _ := New(GenerateKey())
+	if _, err := other.Open(sealed); err == nil {
+		t.Error("the wrong key must fail loudly, not return garbage")
+	}
+	if _, err := b.Open(sealed[:len(sealed)-4] + "AAAA"); err == nil {
+		t.Error("tampered data must fail")
+	}
+
+	// a passphrase works as a key, and is stable
+	p1, _ := New("correct horse battery staple")
+	p2, _ := New("correct horse battery staple")
+	if got, err := p2.Open(p1.Seal("x")); err != nil || got != "x" {
+		t.Errorf("passphrase keys must be reproducible: %q %v", got, err)
+	}
+	if _, err := New("  "); err == nil {
+		t.Error("an empty key must be rejected")
+	}
+}