Class, struct, or interface method must have a return type

I'm trying to convert the commented JS to C#. It's stalling on the "public static Md5Sum(string strToEncrypt)" line with this error:

error CS1520: Class, struct, or interface method must have a return type

 //    function doLogin() {
 //    var sHash = Md5Sum(strUsername+strPassword+sYourID);
 //    var tURL = sURL;
 //    tURL=tURL+"?mdwCMD=loginRequest";
 //    tURL=tURL+"&user="+strUsername;
 //    tURL=tURL+"&pass="+strPassword;
 //    tURL=tURL+"&md5="+sHash;
 //    var rLogin = WWW(tURL);
 //    yield rLogin;
 //    switch(rLogin.text) {
 //        case "loginTrue":
 //            // GUI OR REDIRECT
 //            Application.ExternalEval("document.location.replace('"+sMemArea+"')");
 //            break;
 //        case "loginFalse":
 //            sLoginStatus = "Login Status: Failed, Please Try Again";
 //            yield WaitForSeconds(3);
 //            sLoginStatus = "";
 //            break;
 //    };
 //}
     
     IEnumerator doLogin()
     {
         string sHash = Md5Sum(strUsername+strPassword+yourID);
         string tURL = phpURL;
         
         tURL = tURL+"?mdwCMD=loginRequest";
         tURL = tURL+"&user="+strUsername;
         tURL = tURL+"&pass="+strPassword;
         tURL = tURL+"&md5="+sHash;
         
         WWW rLogin = WWW(tURL);
         yield return rLogin;
         
         switch(rLogin.text)
         {
         case "loginTrue":
             //GUI or redirect
             Application.ExternalEval("document.location.replace('"+sMemArea+"')");
             break;
         case "loginFalse":
             sLoginStatus = "Login Status: FAILED, please try again";
             yield return new WaitForSeconds(3);
             sLoginStatus = "";
             break;
         }
     }
     
     //============================================
     
 //    static function Md5Sum(strToEncrypt: String) {
 //        var encoding = System.Text.UTF8Encoding();
 //        var bytes = encoding.GetBytes(strToEncrypt);
 //        var md5 = System.Security.Cryptography.MD5CryptoServiceProvider();    //-Encrypt bytes
 //        var hashBytes:byte[] = md5.ComputeHash(bytes);                      //-Convert the encrypted bytes back to a string (base 16)
 //        var hashString = "";
 //        for (var i=0; i<hashBytes.Length; i++) {
 //            hashString += System.Convert.ToString(hashBytes[i],16).PadLeft(2,"0"[0]);
 //        } return hashString.PadLeft(32,"0"[0]);
 //    }
     
     public static Md5Sum(string strToEncrypt)
     {
         var encoding = System.Text.UTF8Encoding();
         var bytes = encoding.GetBytes(strToEncrypt);
         var md5 = System.Security.Cryptography.MD5CryptoServiceProvider(); // Encrypt bytes
         byte[] hashbytes = md5.ComputeHash(bytes);                           // Convert encrypted bytes back to string (base 16)
         string hashString = "";
         
         for (int i = 0; i < hashbytes.Length; i++)
         {
             hashString += System.Convert.ToString(hashbytes[i],16).PadLeft(2,"0"[0]);
         }
         
         return hashString.PadLeft(32,"0"[0]);
     }