Using Holder-of-Key Web Browser SSO Profile on a SimpleSAMLphp SP

This document describes how to enable the SAML V2.0 Holder-of-Key (HoK) Web Browser SSO Profile on a SimpleSAMLphp Service Provider (SP).

The SAML V2.0 HoK Web Browser SSO Profile is an alternate version of the standard SAML Web Browser SSO Profile. Its primary benefit is the enhanced security of the SSO process while preserving maximum compatibility with existing deployments on client and server side.

When using this profile the communication between the user and the SP is required to be protected by the TLS protocol. Additionally, the user needs a TLS client certificate. This certificate is usually selfsigned and stored in the certificate store of the browser or the underlying operating system.

Configuring Apache

The SP requests a client certificate from the user agent during the TLS handshake. This behaviour is enabled with the following Apache webserver configuration:

  SSLEngine on
  SSLCertificateFile /etc/openssl/certs/server.crt
  SSLCertificateKeyFile /etc/openssl/private/server.key
  SSLVerifyClient optional_no_ca
  SSLOptions +ExportCertData

If the user agent can successfully prove possession of the private key associated to the public key from the certificate, the received certificate is stored in the environment variable SSL_CLIENT_CERT of the webserver.

Enable HoK on SP

To enable support for the HoK SSO Profile in the SP, the saml20.hok.assertion option must be set to TRUE in the SP configuration. This option can also be enabled in the saml20-idp-remote metadata file, but in that case the endpoint will not be added to the SP metadata. You must also send authentication requests specifying the Holder-of-Key profile to the IdP. This is controlled by the ProtocolBinding option in the SP configuration.

  'hok-sp' => [
      'saml20.hok.assertion' => TRUE,
      'ProtocolBinding' => 'urn:oasis:names:tc:SAML:2.0:profiles:holder-of-key:SSO:browser',

When this is done, you can add the metadata of your SP to the IdP and test the authentication.