C# static class not in Plugins folder

I found this MD5 code somewhere around here. Why won't this work if I don't put it in the Plugins folder? With Javascript I can make classes with static functions & variables and put it anywhere I want in the project and still reference it. I generally stick to Javascript in Unity so I'm not familiar with the quirks of C# in Unity.

using UnityEngine; using System.Collections;

public class MD5 : MonoBehaviour { public static string encode(string strToEncrypt) { System.Text.UTF8Encoding ue = new System.Text.UTF8Encoding(); byte[] bytes = ue.GetBytes(strToEncrypt);

     // encrypt bytes
     System.Security.Cryptography.MD5CryptoServiceProvider md5 = new System.Security.Cryptography.MD5CryptoServiceProvider();
     byte[] hashBytes = md5.ComputeHash(bytes);

     // Convert the encrypted bytes back to a string (base 16)
     string hashString = "";

     for (int i = 0; i </pre>