private string GetAuthentificationCertSerialNum()
        {
            string sn = null;
            ServiceSecurityContext ssc = ServiceSecurityContext.Current;
            if (ssc != null && ssc.PrimaryIdentity.IsAuthenticated)
            {
                X509CertificateClaimSet certClaims = null;
                foreach (var cs in ssc.AuthorizationContext.ClaimSets)
                {
                    certClaims = cs as X509CertificateClaimSet;
                    if (certClaims != null)
                    {
                        sn = certClaims.X509Certificate.SerialNumber;
                        break;
                    }
                }   
            }
            return sn;
        }