Validate the incoming nodekey with regex before attempting to parse

Signed-off-by: Kristoffer Dalby <kristoffer@tailscale.com>

utils.go

@@ -17,6 +17,7 @@ import (
 	"os"
 	"path/filepath"
 	"reflect"
+	"regexp"
 	"strconv"
 	"strings"
 
@@ -64,6 +65,8 @@ const (
 	ZstdCompression = "zstd"
 )
 
+var NodePublicKeyRegex = regexp.MustCompile("nodekey:[a-fA-F0-9]+")
+
 func MachinePublicKeyStripPrefix(machineKey key.MachinePublic) string {
 	return strings.TrimPrefix(machineKey.String(), machinePublicHexPrefix)
 }