feat: encrypt private keys saved on disk and in database (#682)

Co-authored-by: Kyle Mendell <kmendell@ofkm.us>

backend/internal/utils/jwk/key_provider.go

@@ -0,0 +1,50 @@
+package jwk
+
+import (
+	"fmt"
+
+	"github.com/lestrrat-go/jwx/v3/jwk"
+	"gorm.io/gorm"
+
+	"github.com/pocket-id/pocket-id/backend/internal/common"
+)
+
+type KeyProviderOpts struct {
+	EnvConfig *common.EnvConfigSchema
+	DB        *gorm.DB
+	Kek       []byte
+}
+
+type KeyProvider interface {
+	Init(opts KeyProviderOpts) error
+	LoadKey() (jwk.Key, error)
+	SaveKey(key jwk.Key) error
+}
+
+func GetKeyProvider(db *gorm.DB, envConfig *common.EnvConfigSchema, instanceID string) (keyProvider KeyProvider, err error) {
+	// Load the encryption key (KEK) if present
+	kek, err := LoadKeyEncryptionKey(envConfig, instanceID)
+	if err != nil {
+		return nil, fmt.Errorf("failed to load encryption key: %w", err)
+	}
+
+	// Get the key provider
+	switch envConfig.KeysStorage {
+	case "file", "":
+		keyProvider = &KeyProviderFile{}
+	case "database":
+		keyProvider = &KeyProviderDatabase{}
+	default:
+		return nil, fmt.Errorf("invalid key storage '%s'", envConfig.KeysStorage)
+	}
+	err = keyProvider.Init(KeyProviderOpts{
+		DB:        db,
+		EnvConfig: envConfig,
+		Kek:       kek,
+	})
+	if err != nil {
+		return nil, fmt.Errorf("failed to init key provider of type '%s': %w", envConfig.KeysStorage, err)
+	}
+
+	return keyProvider, nil
+}