Introduction
Second Factor Only authentication allows a SP to authenticate only the second factor of a user. 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 has its own authentication endpoint at the SURFsecureID gateway.
Once a user is registered with a second factor 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.
Description | Standard authentication | SFO authenticaton |
---|---|---|
Authentication of the user's first factor by the SURFsecureID gateway | Always | Never |
Authentication of second factor by the SURFsecureID gateway | Based on policy between IdP and SP | Always |
User registration | Using SURFsecureID selfservice registration and vetting by an RA | |
Standard SURFconext features that can be used | Attributes, Authorization, persistent identifiers | None |
The service provider specifies the identity of the user during authentication | Never | Always |
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.
SAML AuthRequest
To start a SFO the SP must send a SAML 2.0 AuthnRequest
to the SFO endpoint of the SURFsecureID Gateway. This request MUST:
- use the
urn:oasis:names:tc:SAML:2.0:bindings:HTTP-Redirect
binding. TheHTTP-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. Unsigned authentications requests or request that are signed using a different algorithm are not accepted. Note that theHTTP-Redirect
binding does not use XML signatures. TheHTTP-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 anAuthnContextClassRef
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 aNameID
(withFormat="urn:oasis:names:tc:SAML:1.1:nameid-format:unspecified",
see description ofAuthnRequest
in https://docs.oasis-open.org/security/saml/v2.0/saml-core-2.0-os.pdf, line 2001).
SURFsecureID Implementation notes regarding RequestedAuthnContext
- The SAML standard allows multiple
AuthnContextClassRef
elements to be specified in theRequestedAuthnContext
. Currenly SURFsecureID will only look at the firstAuthnContextClassRef
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 theRequestedAuthnContext
element. Currently SURFsecureID does not interpret the value of this attribute and behaves as if"minimum"
was specified as value for theComparison
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 corrsponding to the highest level at which the user could be authentictated.
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:
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:
urn:mace:terena.org:attribute-def:schacHomeOrganization
: the value of this attribute identifies the user's institution.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 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 ofuid
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 .
Examples: urn:collab:person:some-organisation.example.org:m1234567890
urn:collab:person:uniharderwijk.nl:jan.jansen_uniharderwijk.nl
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.
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 subcodeurn: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 subcodeurn: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
orAuthnContextClassRef
in theAuthnRequest
- An unsupported URI in the
AuthnContextClassRef
- No
Subject
NameID
in theAuthnRequest
- The service provider is not authorised to authenticate the user specified in the
Subject
NameID
- No
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 authenticate 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 statusCode
s.
Example Error Response
Below is an example SAML "error" Response:
Level: authentication strength
See explanation at "Levels of Assurance".
Implementation
SFO must be implemented at the SP. The authentication protocol is similar to the one used by the SURFsecureID gateway. 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 either SFO SP or a 'standard' SURFsecureID SP: this determines which endpoint he can access. If a SP implementation needs both, he can register an additional EnityID and use it to access the other endpoint.
- 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.
Example
An example code for using SFO with SimpleSAMLphp can be found at: https://github.com/SURFnet/Stepup-SFO-demo