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Second Factor Only authentication allows a SP to authenticate only the second factor of a user. With SFO you can add two factor authentication to your institutions SFO is suitable for situations where the application must perform the fist factor authentication of the user like that application gateway (e.g. Citrix Netscaler or F5 BIG-IP) or to the authentication or authorization gateway (e.g. Microsoft ADFS or Novell/NetIQ) of an institution.  SFO SFO has its own authentication endpoint at the SURFsecureID gateway.

Once a user is registered with a second factor in SURFsecureID both SFO authentication and SURFsecureID and the standard SURFsecureID authentication can be used.

Differences between 'standard' SURFsecureID and SFO authentication

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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 vetting by an RA
Standard SURFconext features that can be usedAttributes, Authorization, persistent identifiersNone
The service provider specifies the identity of the user during authenticationNeverAlways

With SFO the authentication via 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 detemind by SURFconext during the 1st factor authentication. Because this step is omitted during SFO, the service provider must provide the identity of the user's to SURFsecureID during authentication instead.

Note that also with SFO the registration of users (i.e. linking second factors to user identities) will be done by the institutions (IdP's): there is no work to be done for the SP.

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  • use the urn:oasis:names:tc:SAML:2.0:bindings:HTTP-Redirect binding. The HTTP-Redirect binding is currently the only suppored binding for making standard or SFO request to SURFsecureID.
  • be signed using the http://www.w3.org/2001/04/xmldsig-more#rsa-sha256 algorithm (XML signatures cannot be used).. 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 URN URI for one of the defined authentication levels for the SURFsecureID environment that your are usingauthenticating 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 https://docs.oasis-open.org/security/saml/v2.0/saml-core-2.0-os.pdf, line 2001).

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SURFsecureID Implementation notes regarding RequestedAuthnContext

  • The SAML standard allows multiple AuthnContextClassRef elements to be specified in the RequestedAuthnContext. Currenly SURFsecureID will only look at the first AuthnContextClassRef element.
  • Specifying an AuthnContextClassRef other than on the of the defined authentication level 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 successfull 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 corrsponding to the highst level at which the user could be authentictated.

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

Example AuthnRequest

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

Code Block
languagexml
titleExample SFO AuthRequest to the SURFsecureID production environment
collapsetrue
<samlp:AuthnRequest xmlns:samlp="urn:oasis:names:tc:SAML:2.0:protocol"
    				xmlns:saml="urn:oasis:names:tc:SAML:2.0:assertion"
					ID="_zQIibz9FKixdlgX8E7bHqE29wfatcgbsPdVn0NN"
    				Version="2.0" 
					IssueInstant="2016-03-10T15:09:21Z"
    				Destination="https://sa-gw.stepupsurfconext.example.org/gssp/2ndnl/second-factor-only/single-sign-on"
    				AssertionConsumerServiceURL="https://application-gateway.some-organisation.example.org/consume-assertion"
    				ProtocolBinding="urn:oasis:names:tc:SAML:2.0:bindings:HTTP-POST">
    <saml:Issuer>https://application-gateway.some-organisation.example.org/metadata</saml:Issuer>
    <saml:Subject>
        <saml:NameID Format="urn:oasis:names:tc:SAML:1.1:nameid-format:unspecified">urn:collab:person:some-organisation.example.org:m1234567890</saml:NameID>
    </saml:Subject>
    <samlp:RequestedAuthnContext>
        <saml:AuthnContextClassRef>http://surfconext.nl/assurance/sfo-level2</saml:AuthnContextClassRef>
    </samlp:RequestedAuthnContext>
</samlp:AuthnRequest>

Note that the signature is not visible in the XML of the above request 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 identifiers the values if two different attributes that the identity provider (IdP) of the user's institution sends to SURFconext during authentication
urn:. The two attributes that are used to create the SURFconext user identitfier are:

  1. urn:mace:terena.org:attribute-def:schacHomeOrganization: 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:{{urn:mace:terena.org:attribute-def:schacHomeOrganization}}:{{urn:mace:dir:attribute-def:uid}} 
where:

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

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

Example: urn:collab:person:some-organisation.example.org:m1234567890

SAML Response

The result of a successful authentication is a SAML Response. It 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.

Code Block
languagexml
titleExample Response
collapsetrue
 <samlp:Response xmlns:samlp="urn:oasis:names:tc:SAML:2.0:protocol"
                 xmlns:saml="urn:oasis:names:tc:SAML:2.0:assertion"
                 ID="_ECAokbn0lm7lfVT7THQUl+dSbMrpeyAgiTv0+q16"
                 Version="2.0"
                 IssueInstant="2016-03-10T15:09:25Z"
                 Destination="https://application-gateway.some-organisation.example.org/consume-assertion"
                 InResponseTo="_zQIibz9FKixdlgX8E7bHqE29wfatcgbsPdVn0NN"
                >
    <saml:Issuer>https://sa-gw.stepupsurfconext.example.orgnl/second-factoryfactor-only/metadata</saml:Issuer>
    <samlp:Status>
        <samlp:StatusCode Value="urn:oasis:names:tc:SAML:2.0:status:Success" />
    </samlp:Status>
    <saml:Assertion xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
                    xmlns:xs="http://www.w3.org/2001/XMLSchema"
                    ID="_zQIibz9FKixdlgX8E7bHqE29wfatcgbsPdVn0NN"
                    Version="2.0"
                    IssueInstant="2016-03-10T15:09:25Z"
                    >
        <saml:Issuer>https://gw.stepup.example.org/second-factory-only/metadata</saml:Issuer>
        <ds:Signature xmlns:ds="http://www.w3.org/2000/09/xmldsig#">
            <ds:SignedInfo>
                <ds:CanonicalizationMethod Algorithm="http://www.w3.org/2001/10/xml-exc-c14n#" />
                <ds:SignatureMethod Algorithm="http://www.w3.org/2001/04/xmldsig-more#rsa-sha256" />
                <ds:Reference URI="#_1ROuxGVzJi5bbre6W4woNza82aK41HKjp6aKtw9r">
                    <ds:Transforms>
                        <ds:Transform Algorithm="http://www.w3.org/2000/09/xmldsig#enveloped-signature" />
                        <ds:Transform Algorithm="http://www.w3.org/2001/10/xml-exc-c14n#" />
                    </ds:Transforms>
                    <ds:DigestMethod Algorithm="http://www.w3.org/2001/04/xmlenc#sha256" />
                    <ds:DigestValue>YR5JFfJc1JZIKm7Ao3kXtDupEfeMrhKpD9T1lF1z0Lg=</ds:DigestValue>
                </ds:Reference>
            </ds:SignedInfo>
            <ds:SignatureValue>...</ds:SignatureValue>
            <ds:KeyInfo>
                <ds:X509Data>
                    <ds:X509Certificate>...</ds:X509Certificate>
                </ds:X509Data>
            </ds:KeyInfo>
        </ds:Signature>
        <saml:Subject>
            <saml:NameID Format="urn:oasis:names:tc:SAML:1.1:nameid-format:unspecified">urn:collab:person:some-organisation.example.org:m1234567890</saml:NameID>
            <saml:SubjectConfirmation Method="urn:oasis:names:tc:SAML:2.0:cm:bearer">
                <saml:SubjectConfirmationData NotOnOrAfter="2016-03-10T15:14:25Z"
                                              Recipient="https://application-gateway.some-organisation.example.org/consume-assertion"
                                              InResponseTo="_zQIibz9FKixdlgX8E7bHqE29wfatcgbsPdVn0NN"
                                              />
            </saml:SubjectConfirmation>
        </saml:Subject>
        <saml:Conditions NotBefore="2016-03-10T15:09:25Z"
                         NotOnOrAfter="2016-03-10T15:14:25Z"
                         >
            <saml:AudienceRestriction>
                <saml:Audience>https://application-gateway.some-organisation.example.org/metadata</saml:Audience>
            </saml:AudienceRestriction>
        </saml:Conditions>
        <saml:AuthnStatement AuthnInstant="2016-03-10T15:09:25Z">
            <saml:AuthnContext>
                <saml:AuthnContextClassRef>http://stepup.example.org/verified-second-factor/level2</saml:AuthnContextClassRef>
            </saml:AuthnContext>
        </saml:AuthnStatement>
    </saml:Assertion>
</samlp:Response> 

Error handling

When a user cannot be authenticatedFor specifiec scenario's, when the authenticated fails, the SURFsecureID gateway sends a SAML Response 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 requests 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 seond factor will try to authneticate to your service, and fail.

SURFsecureID does not currenly 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:

Code Block
languagexml
titleExample Error Response
collapsetrue
<samlp:Response xmlns:samlp="urn:oasis:names:tc:SAML:2.0:protocol"
                xmlns:saml="urn:oasis:names:tc:SAML:2.0:assertion"
                ID="YaszKubip05bTwe7hIWOc5AsNxwmEliPJ88nUQ"
                Version="2.0"
                IssueInstant="2015-05-12T12:17:38Z"
                Destination="https://your-sp.example.com/acs-location"
                InResponseTo="_6d93f735ccfb8d98454999b4016d515834211b0dde"
                >
    <saml:Issuer>https://sa-gw.surfconext.nl/authentication/metadata</saml:Issuer>
    <samlp:Status>
        <samlp:StatusCode Value="urn:oasis:names:tc:SAML:2.0:status:Requester">
            <samlp:StatusCode Value="urn:oasis:names:tc:SAML:2.0:status:NoAuthnContext" />
        </samlp:StatusCode>      
    </samlp:Status>
</samlp:Response>


Level: authentication strength

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An example code for using SFO with SimpleSAMLphp can be found at: https://github.com/SURFnet/Stepup-SFO-demo