public static class Certificate
    {
        public static X509Certificate2 CurrentCertificate { get; set; }
        public static bool SelectCert()
        {
            X509Store store = new X509Store("MY", StoreLocation.CurrentUser);
            store.Open(OpenFlags.ReadOnly | OpenFlags.OpenExistingOnly);
            X509Certificate2Collection collection = store.Certificates;
            X509Certificate2Collection scollection = X509Certificate2UI.SelectFromCollection(collection, "Выбор сертификата", "", X509SelectionFlag.SingleSelection);

            if (scollection.Count > 0)
            {
                CurrentCertificate = scollection[0];
                return true;
            }
            else return false;
        }
    }

Gost3410CryptoServiceProvider Gost3410 = new Gost3410CryptoServiceProvider();
Gost3410.ContainerCertificate = Certificate.CurrentCertificate;