Extract common server encryption logic (#65)

* refactor: extract common message encryption logic
* refactor: move letsencrypt logic to common
* refactor: rename common package to encryption
* test: add encryption tests

encryption/message.go

@@ -0,0 +1,40 @@
+package encryption
+
+import (
+	pb "github.com/golang/protobuf/proto" //nolint
+	log "github.com/sirupsen/logrus"
+	"golang.zx2c4.com/wireguard/wgctrl/wgtypes"
+)
+
+// EncryptMessage encrypts a body of the given protobuf Message
+func EncryptMessage(remotePubKey wgtypes.Key, ourPrivateKey wgtypes.Key, message pb.Message) ([]byte, error) {
+	byteResp, err := pb.Marshal(message)
+	if err != nil {
+		log.Errorf("failed marshalling message %v", err)
+		return nil, err
+	}
+
+	encryptedBytes, err := Encrypt(byteResp, remotePubKey, ourPrivateKey)
+	if err != nil {
+		log.Errorf("failed encrypting SyncResponse %v", err)
+		return nil, err
+	}
+
+	return encryptedBytes, nil
+}
+
+// DecryptMessage decrypts an encrypted message into given protobuf Message
+func DecryptMessage(remotePubKey wgtypes.Key, ourPrivateKey wgtypes.Key, encryptedMessage []byte, message pb.Message) error {
+	decrypted, err := Decrypt(encryptedMessage, remotePubKey, ourPrivateKey)
+	if err != nil {
+		log.Warnf("error while decrypting Sync request message from peer %s", remotePubKey.String())
+		return err
+	}
+
+	err = pb.Unmarshal(decrypted, message)
+	if err != nil {
+		log.Warnf("error while umarshalling Sync request message from peer %s", remotePubKey.String())
+		return err
+	}
+	return nil
+}