Second Factor Only (SFO) authentication allows a service provider (SP) to authenticate only the second factor of a user. SFO is suitable for situations where the SP must perform the fist factor authentication of the user. We see this requirement when the service is an application gateway (e.g. Citrix Netscaler or F5 BIG-IP) or an authentication or authorization gateway (e.g. Microsoft ADFS or Novell/NetIQ) of an institution. In these cases the first factor authentication is performed by the service and the service uses SFO to authenticate the second factor of the user. SFO authentication has its own authentication endpoint at the SURFsecureID gateway.

Once a user has activated their second factor (aka token) in SURFsecureID both SFO authentication and the standard SURFsecureID authentication can be used.

Differences between 'standard' SURFsecureID and SFO authentication

The table below lists the differences between standard authentication and second factor (SFO) authentication that is offered by SURFsecureID gateway to a service provider.

DescriptionStandard authenticationSFO authenticaton
Authentication of the user's first factor by the SURFsecureID gatewayAlwaysNever
Authentication of second factor by the SURFsecureID gatewayBased on policy between IdP and SPAlways
User registrationUsing SURFsecureID selfservice registration and/or vetting by an RA
Standard SURFconext features that can be usedAttributes, Authorization, persistent identifiersNone
The service provider must specify the identity of the user in the authentication requestNeverAlways

In an SFO authentication SURFconext is bypassed (see image below). This means that SURFconext functionality (e.g. attributes from the user's home IdP, the definition of authorization rules and persistent user identifiers) is not available. During a standard authentication the identity of the user is determined by SURFconext during the 1st factor authentication. Because this step is omitted during SFO, the service provider must provide the identity of the user to SURFsecureID during authentication instead.

Note that SFO can only be used for authentication of the user's second factor. The registration, vetting and revocation of users / and tokens is always performed though SURFsecuereID.

SAML AuthRequest

To start an SFO authentiction, the SP must send a SAML 2.0 AuthnRequest to the SFO endpoint of the SURFsecureID Gateway. Production and Test use different endpoints. This authentication request MUST:

  • use the urn:oasis:names:tc:SAML:2.0:bindings:HTTP-Redirect binding. The HTTP-Redirect binding is the recommended binding for making a SFO request to SURFsecureID.
  • be signed using the algorithm. Unsigned authentications requests or request that are signed using a different algorithm are not accepted. Note that the HTTP-Redirect binding does not use XML signatures. The HTTP-Redirect binding specifies its own signature scheme. See Bindings for the OASIS Security Assertion Markup Language (SAML) V2.0, section 3.4.
  • include a RequestedAuthnContext with an AuthnContextClassRef with the URI for one of the defined authentication levels for the SURFsecureID environment that your are authenticating to.
  • include the SURFconext identifier of the user in the Subject element as a NameID (with Format="urn:oasis:names:tc:SAML:1.1:nameid-format:unspecified", see description of AuthnRequest in, line 2001).

SURFsecureID Implementation notes regarding RequestedAuthnContext

  • The SAML standard allows multiple AuthnContextClassRef elements to be specified in the RequestedAuthnContext. Currently SURFsecureID will only look at the first AuthnContextClassRef element.
  • Specifying an AuthnContextClassRef other than one the of the defined authentication levels for SFO will result in an error.
  • The SAML standard allows a Comparision attribute to be added to the the RequestedAuthnContext element. Currently SURFsecureID does not interpret the value of this attribute and behaves as if "minimum" was specified as value for the Comparison attribute, which is a deviation of the SAML standard which specifies "exact" as the default. "minimum" means that the authentication context in the authentication statement that is returned after a successful authentication will either be the requested authentication context, or the the authentication context of a stronger (i.e. higher level) authentication. SURFsecureID currently always returns the authentication context corresponding to the highest level at which the user could be authenticated.

Future SURFsecureID versions may support more complex processing of RequestedAuthnContext options and add new AuthnContextClassRef "families" to support different registration policies.

Example AuthnRequest

Below is an example SAML 2.0 SFO AuthnRequest request for the SURFsecureID production environment:

Example SFO AuthRequest to the SURFsecureID production environment
<samlp:AuthnRequest xmlns:samlp="urn:oasis:names:tc:SAML:2.0:protocol"
        <saml:NameID Format="urn:oasis:names:tc:SAML:1.1:nameid-format:unspecified"></saml:NameID>

Note that the signature is not visible in the XML of the above AuthnRequest: it will be encoded in the HTTP GET parameters according to the specification of the HTTP-Redirect binding.

Note that SFO uses a different SingleSignOn Location and a different AuthnConextClassRef identifiers than a standard authentication to SURFsecureID. See SURFsecureID Metadata for Service Providers for the diffenrent AuthnConextClassRef identifiers that are being used by SURFsecureID.

Determining the SURFconext identifier of a user

The SURFconext identifier of a user is built from the values if two different attributes that the identity provider (IdP) of the user's institution sends to SURFconext during authentication. The two attributes that are used to create the SURFconext user identitfier are:

  1. the value of this attribute identifies the user's institution.
  2. urn:mace:dir:attribute-def:uid: the value of this attribute identifies the user withing the institution


  • urn:collab:person:
    = fixed prefix.
  • {{}}
    = value of schacHomeOrganization attribute of the user; typically the same for all users of one institution and will be something like "".
  • {{urn:mace:dir:attribute-def:uid}}
    = value of uid attribute of the user. Replace each "@" (at) character in the uid with an "_" (underscore) character.

For the value of last two items: ask the administrator of the IdP .


SAML Response

The result of a successful authentication is a SAML Response. Note that it does not contain an AttributeStatement and that the Assertion element is signed and that the Response element is not signed. Response signing is not currently supported by SURFsecureID, it may be added in future versions.

Example Response
 <samlp:Response xmlns:samlp="urn:oasis:names:tc:SAML:2.0:protocol"
        <samlp:StatusCode Value="urn:oasis:names:tc:SAML:2.0:status:Success" />
    <saml:Assertion xmlns:xsi=""
        <ds:Signature xmlns:ds="">
                <ds:CanonicalizationMethod Algorithm="" />
                <ds:SignatureMethod Algorithm="" />
                <ds:Reference URI="#_1ROuxGVzJi5bbre6W4woNza82aK41HKjp6aKtw9r">
                        <ds:Transform Algorithm="" />
                        <ds:Transform Algorithm="" />
                    <ds:DigestMethod Algorithm="" />
            <saml:NameID Format="urn:oasis:names:tc:SAML:1.1:nameid-format:unspecified"></saml:NameID>
            <saml:SubjectConfirmation Method="urn:oasis:names:tc:SAML:2.0:cm:bearer">
                <saml:SubjectConfirmationData NotOnOrAfter="2016-03-10T15:14:25Z"
        <saml:Conditions NotBefore="2016-03-10T15:09:25Z"
        <saml:AuthnStatement AuthnInstant="2016-03-10T15:09:25Z">

Error handling

For specific scenarios, when the authentication fails, the SURFsecureID gateway sends a SAMLResponse to the SP with a non success status:

  • urn:oasis:names:tc:SAML:2.0:status:Responder with subcode urn:oasis:names:tc:SAML:2.0:status:AuthnFailed = 
    Authentication was not successful. This specifically happens when the user cancels the authentication.
  • urn:oasis:names:tc:SAML:2.0:status:Responder with subcode urn:oasis:names:tc:SAML:2.0:status:NoAuthnContext = 
    The user could not be authenticated at the requested level. The user does not have an activated (vetted) token at the requested level.
  • Other situations can also lead to an error response. Specifically:
    • No RequestedAuthnContext or AuthnContextClassRef in the AuthnRequest
    • An unsupported URI in the AuthnContextClassRef
    • No Subject NameID in the AuthnRequest
    • The service provider is not authorised to authenticate the user specified in the Subject NameID

A service provider SHOULD be able to handle the first two errors scenarios above (AuthnFailed and NoAuthnContext) in a user friendly manner. These error responses will occur during normal use: users can and do cancel the authentication process and users that do no not yet, or no longer have, a registered second factor will try to authenticate to your service, and fail.

SURFsecureID does not currently add a StatusMessage to an "error" response. We plan to add a StatusMessage in a future version of SURFsecureID that provides more context about the error to the operator of the service than can be conveyed using the standard statusCodes.

Example Error Response

Below is an example SAML "error" Response:

Example Error Response
<samlp:Response xmlns:samlp="urn:oasis:names:tc:SAML:2.0:protocol"
        <samlp:StatusCode Value="urn:oasis:names:tc:SAML:2.0:status:Requester">
            <samlp:StatusCode Value="urn:oasis:names:tc:SAML:2.0:status:NoAuthnContext" />

Level: authentication strength

See explanation at "Levels of Assurance".


SFO must be implemented at the SP. The authentication protocol is similar to the one used by the SURFsecureID gateway and by SURFconext. The main difference is that the SP must send the identifier of the user in the Subject element of the SAML AuthnRequest (see description of AuthnRequest, line 2017).

  • The SP will be registered at the SURFsecureID gateway as an SFO SP
  • There is a white-list of SURFconext identities allowing SFO authentication. The SP must indicate in advance the institutions from which he wants to authenticate users with SFO.

You can find the metadata of the SFO endpoints on SURFsecureID Metadata for Service Providers.

Always do a first factor authentication before starting a SFO authentication

Starting an SFO authentication will immediately start an authentication at the SURFsecureID gateway: a push notification (tqr) or an SMS will be sent to the user being authenticated. If authentication is started for the wrong user, this will derange the targeted user and in case of SMS, incur a cost to the institution and possibly for the user.

By observing the behavior of the SFO authentication it is possible to determine whether a username exists.

For this reasons we advise to perform a first factor authentication before starting a SFO authentication.

Use an existing SAML library

Creating a secure implementation of SAML is hard, see e.g The OWASP SAML cheat sheet. We strongly recommend that implementers do not create their own implementation of the SAML 2.0 protocol for implementing SFO, but instead base their implementation on an existing well tested SAML 2.0 implementation.


An example code for using SFO with SimpleSAMLphp can be found at:

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