feat: establish secure update foundation

internal/secure/secure.go

@@ -37,6 +37,16 @@ type Cipher struct {
 }
 
 func LoadCipher(path string) (*Cipher, error) {
+	info, err := os.Lstat(path)
+	if err != nil {
+		return nil, fmt.Errorf("inspect master key: %w", err)
+	}
+	if !info.Mode().IsRegular() || info.Mode()&os.ModeSymlink != 0 {
+		return nil, errors.New("master key must be a regular file")
+	}
+	if err := os.Chmod(path, 0o600); err != nil {
+		return nil, fmt.Errorf("secure master key: %w", err)
+	}
 	raw, err := os.ReadFile(path)
 	if err != nil {
 		return nil, fmt.Errorf("read master key: %w", err)
@@ -47,6 +57,17 @@ func LoadCipher(path string) (*Cipher, error) {
 // LoadOrCreateCipher loads an existing master key or creates one on first start.
 // The key must live on a persistent volume separate from the application data.
 func LoadOrCreateCipher(path string) (*Cipher, error) {
+	directory := filepath.Dir(path)
+	if err := os.MkdirAll(directory, 0o700); err != nil {
+		return nil, fmt.Errorf("create master key directory: %w", err)
+	}
+	info, err := os.Lstat(directory)
+	if err != nil || !info.IsDir() || info.Mode()&os.ModeSymlink != 0 {
+		return nil, errors.New("master key directory must be a regular directory")
+	}
+	if err := os.Chmod(directory, 0o700); err != nil {
+		return nil, fmt.Errorf("secure master key directory: %w", err)
+	}
 	cipher, err := LoadCipher(path)
 	if err == nil {
 		return cipher, nil
@@ -55,9 +76,6 @@ func LoadOrCreateCipher(path string) (*Cipher, error) {
 		return nil, err
 	}
 
-	if err := os.MkdirAll(filepath.Dir(path), 0o700); err != nil {
-		return nil, fmt.Errorf("create master key directory: %w", err)
-	}
 	key := make([]byte, chacha20poly1305.KeySize)
 	if _, err := rand.Read(key); err != nil {
 		return nil, fmt.Errorf("create master key: %w", err)