The store is current store. The thing is its run perfectly from a windows
form but not from a web page?
The code look like.

X509CertificateStore store =
X509CertificateStore.CurrentUserStore(X509CertificateStore.MyStore);
            store.OpenRead();
            X509CertificateCollection certs =
store.FindCertificateByKeyIdentifier(Convert.FromBase64String(keyId));
            store.Close();
            return new X509SecurityToken(((X509Certificate)certs[0]));