add KMS support

Fixes #226

kms/gcp.go

@@ -0,0 +1,42 @@
+package kms
+
+const (
+	gcpProviderName = "GCP"
+)
+
+type gcpSecret struct {
+	baseGCloudSecret
+}
+
+func newGCPSecret(base baseSecret, url, masterKey string) SecretProvider {
+	return &gcpSecret{
+		baseGCloudSecret{
+			baseSecret: base,
+			url:        url,
+			masterKey:  masterKey,
+		},
+	}
+}
+
+func (s *gcpSecret) Name() string {
+	return gcpProviderName
+}
+
+func (s *gcpSecret) IsEncrypted() bool {
+	return s.Status == SecretStatusGCP
+}
+
+func (s *gcpSecret) Encrypt() error {
+	if err := s.baseGCloudSecret.Encrypt(); err != nil {
+		return err
+	}
+	s.Status = SecretStatusGCP
+	return nil
+}
+
+func (s *gcpSecret) Decrypt() error {
+	if !s.IsEncrypted() {
+		return errWrongSecretStatus
+	}
+	return s.baseGCloudSecret.Decrypt()
+}