<?xml version="1.0" encoding="utf-8"?>
<request 
        xmlns="http://www.olb.ru/commands" 
        xmlns:ss="http://www.olb.ru/commands/SecurityServer" 
        xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
  ... 
  <command xsi:type="ss:Crypt">
    <ss:provider>MessagePro_3. 0 </ss:provider>
    <ss:data>=34r3434r34=</ss:data>
    <ss:to name="12" password="1111111"/>
  </command>
  ...
</request>

XNamespace tns = "http://www.olb.ru/commands";
XNamespace ss = "http://www.olb.ru/commands/SecurityServer";
XNamespace xsi = "http://www.w3.org/2001/XMLSchema-instance";

var command_list = from command in doc.Root.Elements(tns + "command")
                           where command.Attribute(xsi + "type").Value =="ss:Сrypt"
                           select command;

XNamespace tns = "http://www.olb.ru/commands";
XNamespace xsi = "http://www.w3.org/2001/XMLSchema-instance";
XNamespace ss = "http://www.olb.ru/commands/SecurityServer";
            
XDocument doc = XDocument.Load("XMLFile2.xml");

string local_ss = doc.Root.Attributes().Where((a, b) => a.Value == "http://www.olb.ru/commands/SecurityServer").Single().Name.LocalName;
var command_list = from command in doc.Root.Elements(tns + "command")
                           where command.Attribute(xsi + "type").Value == local_ss + ":Crypt"
                           select command;

foreach (var command in command_list)
      Console.WriteLine(command);