Deep-dive on SAML 2.0 vs. WS-Federation

After my previous blog entries on WS-Federation I received some requests for a more in-depth analysis of WS-Federation and in particular how it compares to SAML 2.0. It took me a while to get to it but I finally manage to spend some time  to do just that.

Below is a table where I compare both specs on various features and technical details. Note that each blue highlight identifies what I think is a plus for the specification (when compared to the other).


Topic

WS-Federation

SAML 2.0

Target

- Browser Redirect (messages in URL)

- Browser POST (messages in HTML form)

- SOAP (over HTTP)

- Artifact

- Browser Redirect (messages in URL)

- Browser POST (messages in HTML form)

- Artifact (reference to assertion + SOAP call)

- SOAP (over HTTP)

- Reverse SOAP (over HTTP)

Security tokens supported

- Those supported by WS-Trust (SAML assertions, X509 certificates, kerberos...)

- SAML assertions

- Any other token types (embedded in a SAML assertions via the SubjectConfirmation element)

Dependencies

- WS-Trust [1], WS-Policy, WS-SecurityPolicy.

- WS-Eventing to subscribe to Single Sign Out messages.

- WS-Transfer & WS-ResourceTransfer.

- None!

Identity federation

- Performed by the Pseudonym service (optional...) which provides identity mapping and its management.

- A Pseudonym service may be independent of an IP/STS and could store tokens associated to a pseudonym. 

- Identity mapping is part of the IdP. Although less (?) flexible it avoids the need for yet another protocol between the pseudonym service and the assertion generator (IP/STS in WS-\*).

- Mapping can be created either by the requestor (principal...) or the owner of the resource (SP).

- Mapping is created by the IdP but can be changed by either the IdP or an SP.


- All operations on pseudonyms (get, set, create or delete) are done via WS-Transfer (and its extension WS-ResourceTransfer to filter the scope of these operations).

- Client-based pseudonyms: a requestor can specify (in an RST) ad-hoc data for a pseudonym it wants to be used by the STS (e.g. PPID, DisplayName, email...)

- SAML does not provide a similar concept to the ClientPseudonym in its AuthNRequest. Is this one of the active requestor “benefit”?
The Name ID management protocol (and SPProviderID) is not meant for transient ID mapping.

Metadata

- Description of the federation metadata format.

- Description of a secure transfer of this metadata.

- Can hold info about several federations.

- Description of metadata for SSO and more.

- Organized by roles (IdP, SP, Attribute requester, PDP...)

Single Logout

- Can be initiated by either an SP or the (primary) STS which will send sign-out messages to all RP.

- Similar

Artifact

- Based on the use of a reference token (i.e. an EPR to which a WS-Transfer GET can be made to retrieve the actual token).

- Artifact profile (complete SAML response)

- URI binding (to only obtain SAML assertion)

- SAML also defines mechanisms to request or query existing assertions (by subject or statement type).

Authorization service

- Again a specialized STS.

- Concept of authorization context (name-scope-value) to condition the issuance of a token.

- The context seems to be a kind of pendant to the SAML2 XACML profile...

Authentication freshness

- A requestor can specify its freshness requirements (allow caching of security tokens etc.)

- Similar with Conditions and ForceAuthN

Authentication level

- WS-Trust defines the parameter (AuthenticationType). WS-Fed specifies predefined values (e.g. Ssl, SslSndKey, smartcard).

- SAML 2.0 offers a much broader & extensible set of authentication contexts.

Privacy

- A requestor can express its protection requirements for security tokens it requests (protectData w/h claims & confirmation from STS).


- Privacy statements can be retrieved via WS-Transfer.

- SAML offers a range of options to constraint the use & scope of an assertion (audience, advice, proxyRestriction, oneTimeUse, condition) [2]


- Those constraints can originate from both the SP or the IdP.

 
[1] WS-Federation basically extends WS-Trust to allow the issuance of tokens that can carry attributes or pseudonyms.

[2] One would argue that this could be achieved with WS-Policy but SAML has the advantage of offering built-in ones.

 
So what to think of all that? Well as already mentioned in previous blog I find WS-Federation to be very similar to SAML. I also think SAML is a more self contained specification; because of its composability approach, WS-Federation allows you to tune your deployment in many ways but it is done at the expense of simplicity. One has to go tweak WS-Trust or WS-Policy etc.

Some examples of what I can easily do with SAML 2.0:

  • I want to silently check whether the user is authenticated (I don't want the IdP to perform authN): set <IsPassive> to TRUE

  • I need an assertion with constrained validity or use: apply SAML's <Conditions>

  • I only accept authentication assertion from a list of IdPs: use <Scoping>

  • etc.

I have no doubt you can achieve this with the "right" combination of WS-Fed / WS-Trust / WS-Policy (and probably other specs) but with SAML it's all there...

 



Comments:

To be fair, one more plus for WS-Federation is that the format for an authentication request (in the browser profile) is much simpler - just an HTTP GET with a bunch of parameters, rather than a deflated, base64'd, URL encoded XML document. Of course, this is a two edged sword, since I would say that the request mechanism is thus less flexible.

Posted by Pat on March 02, 2007 at 03:21 AM PST #

How about including economic comparisons as well? For example, is WS-Federation running on Windows cheaper than running SAML using Open Source from Sun? Please don't forget to include support and consulting costs...

Posted by James on March 04, 2007 at 07:09 PM PST #

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