I. Introduction

This document is the specification of levels of assurance (LOA) in CAS clients and server.

The LOA is the confidence the CAS server has in the given credentials to say who the principal is.

There are a number of different factors that affect LOA:

• Registration and Identity Proofing
• Assurance level of the authentication token
• Security of the authentication mechanism (channel encryption, etc.)
• Other details of the authentication (e.g. how stale is it?)
• Other policies in place (such as revocation policy)

Related documentation:

• Discussion on the cas-dev mailing list:  https://lists.wisc.edu/read/messages?id=18431743
• NIST Electronic Authentication Guideline
  • Key areas of interest:
    • Section 5: Registration and Issuance Process
    • Section 6.3 - Token Assurance Levels (pp 48-54)
    • Really, the whole document is very interesting and directly applicable
• E-Authenticaiton Guidelines for Federal Agencies
  • Key areas of interest
    • Section 2.2: This is an interesting discussion of risk assessment - addresses the question: "Why do we want LOA?"
    • Section 2.4: Examples of various levels
• Incommon Assurance Policies Bronze and Silver
  • Key areas of interest
    • Section 4.1:  Summary of Identity Assurance Criteria
• Authentication Contexts for SAML 2.0
  • Key areas of interest
    • Section 3.4: Authentication Context Classes list

II. Authentication API in CAS server

A) CAS 3.5.0

In CAS server 3.5.0, an authentication manager is in charge of validating credentials. It takes credentials as input.

An authentication manager has :

• many authentication handlers, each supporting a kind of credentials and returning a principal
• many credentials to principal resolvers, each supporting a kind of credentials
• potentially many authentication meta data populators which add attributes to the authentication.

A credentials to principal resolver can retrieve user attributes from a specific repository and populates them in the authentication.

An authentication has :

• an authentication date
• attributes
• a principal with identifier and attributes.

B) CAS 4.0.0

TODO Marvin : describe briefly the new design of the authentication API in CAS server 4.0.0

III. New concepts or updated ones for LOA

A) Assurance policy (new)

The CAS server has one and only assurance policy. It consists of many levels of assurance (at least two).

Nathan's Recommendation:

The assurance policy is basically a map from a set of authentication attribute constraints, defined in an expression, to descriptive names and numbers.

Example Assurance Policy

Authentication handlers defined for this example:

The policy looks like this:

* I just noticed that with this design (and if reusing the "renew" parameter to specify the names of accepted levels), then adding a level named "true" might actually provide backwards compatibility automatically without needing any special code.  Haha!    I'm not sure if I'd actually recommend this practice, but it is kind of interesting. -Nathan

Authentication Constraint Expressions

An authentication constraint expression describes a combination of authentication handlers and authentication attributes derived from those handlers that are required to achieve the given level of authentication.  Acceptable authentication handlers can be combined using logical and and or and grouped using parentheses.  Authentication attribute constraints can be defined for each authentication handler. (In this example, they are defined within braces following the authentication handler.)

Note that authentication constraint expressions need to be run both forwards and backwards.  The server needs to be able to determine the levels and names that are applicable to the user's current set of authentication objects (i.e. "determine the current LOA").  The server also needs to be able to determine which authentication handlers should be satisfied, and therefore which interactions (see below) should be invoked, in order to raise the user's LOA to the desired level if it is not there yet.  Note that the server needs to be smart enough to know that it cannot upgrade someone from "ldap{password_strength==5}" to "ldap{password_strength==8}" just by sending them back through the "ldap" authentication handler and corresponding interaction.

NOTE: As I'm working on my use cases, I'm discovering that the attributes aspect of authentication handlers (other than the pseudo-attributes of "initial" and "timestamp") might be better handled proactively.  In other words, a service that would require a stronger password will probably already be using role-based authorization to allow/deny access.  If there is an interrupt screen in CAS that enforces the stronger password policy based on this role membership (e.g. by looking at Grouper or LDAP group membership), then can be assured that the user has the stronger password if they get past the interrupt screen code.  In that case, we probably don't need to handle attributes at the constraint expression level.  This would reduce the complexity of the assurance evaluator.  On the other hand, it still might be useful to compute a stronger LOA number (or name) based on attributes such as password strength.  Especially when federating with someone who wants NIST-like levels, it might be nice to be able to pass that extra info along.  -Nathan

Authentication Attributes (update)

Authentication attributes come in two flavors: credential attributes and event attributes.  For the most part, these are stored in the Authentication object and placed there by the authentication handler.  However, some of these attributes used in constraint expressions (such as the "initial" attribute in the above examples) will likely be computed on the fly or pulled from other objects.

Credential attributes describe the credentials for a particular principal, such as (but not limited to) the key strength used for a certificate, the password strength for an individual user, any identity verification that was used for issuance, and revocation policies.  Credential attributes for a user can, in theory, be determined about a user even before that user attempts to authenticate with a given authentication handler.

Event attributes describe a particular authentication event, indicating details such as the authentication handler used, the timestamp of the authentication, the encryption level of the communication channel, and whether this was an initial authentication or part of a session (i.e. related to CAS's "renew" functionality).

B) Level of assurance (new)

A level of assurance has :

• many names (at least one)
• a level (positive numeric value). 0 means not authenticated. The higher the value is, the more secure the level is.
• a constraint expression.  see above. -Nathan

So far, an authentication handler authenticates credentials and is defined to support a kind of credentials.

An authentication handler will be added :

• a name
• a supported level of assurance (strictly positive numeric value).

D) Interaction (new)

An interaction is a way to get credentials. Each interaction is defined to support a kind of credentials it will return when called.

There are two types of interactions :

• user interaction means the user has to give inputs as credentials. A user interaction is also defined with a view to display to get user inputs. At least, one user interaction is defined in CAS server : the login page interaction which supports username password credentials.
• system interaction means the CAS server will get by itself credentials from the coming HTTP request.

E) Interaction manager (new)

IV. Extension to CAS protocol

So far, the CAS server supports the renew parameter set to true to force re-authentication whatever the current authentication is.

For backward compatibility, these parameter and value will remain but new values will be possible for the renew parameter to handle LOA :

• renew=value, the numeric value is strictly positive, it's meant to be a requested numeric LOA
• renew=name, name is the name of an authentication handler or the name of a level of assurance.  (I recommend allowing the client to specify either a minimum number or a list of names of the level of assurance and not an individual authentication handler.  If that level of flexibility is desired, then the client should be able to specify a complete authentication constraint expression and not just an authentication handler name. Noe that with a list of names, only exact matches count. Even if the user has fulfilled a higher number but not fulfilled the requested type of authentication specified by the named level, then it doesn't count. -Nathan)

V. Main algorithm

A) Diagram

B) Requested LOA

1) Client requested LOA

A client requested LOA is computed from the renew parameter if this one is defined :

• If renew is a numeric value, all authentication handlers whose LOA is superior or equals are selected
• If renew is a LOA name, the associated numeric value is used to find the authentication handlers as previously
• If renew is an authentication handler name, the authentication handler is selected.

2) Server requested LOA

A server requested LOA is computed from the requested registered service (service parameter).

The requested registered service gives the requested authentication handlers.

If no service is defined, all authentication handlers are requested.

3) Computed requested LOA

The requested authentication handlers are the intersection between the ones computed for client and the ones computed for server.

If no authentication handler is selected, a CAS error page is displayed : "The CAS server cannot grant the level of assurance requested by service and/or by renew parameter".

The requested LOA is the lowest numeric value of all selected authentication handlers.

C) Current LOA

The current LOA is retrieved from the authentication using the assurance evaluator.

D) Compare requested LOA and current LOA

If the current LOA is equals or superior to requested LOA, a service ticket is generated and the user is redirected to the service with this service ticket or to the generic login successfull view is displayed (if no service is requested).

If the current LOA is strictly inferior to the requested LOA, an authentication process should be started.

E) Define the needed interactions

At this step, we have the requested authentication handlers and LOA.

From these requested authentication handlers, the needed interactions can be computed using the interaction manager.

If no interaction is returned, a CAS error page is displayed : "The CAS server cannot grant the level of assurance requested by service and/or by renew parameter".

F) Process interactions

At this step, we have a list of ordered interactions to use for authentication process.

Let's start with the first interaction if it exists.

If the interaction is a system one, credentials are returned by this interaction.

• If credentials are null, we move to the next interaction. If credentials are not null, the authentication manager is called with these credentials and the requested LOA. A principal is returned.
  • If the principal is null, we move to the next interaction. If the principal is not null, the user is authenticated and we can generate a service ticket and redirect him to the service or display the generic login successfull view (if no requested service).

G) Communication with Client

Ideas for a new CAS Protocol for Ticket Validation Response   -Nathan

<cas:serviceResponse xmlns:cas='http://www.yale.edu/tp/cas'>
  <cas:authenticationSuccess>
    <cas:user>${principal.id}</cas:user>
    <cas:proxyGrantingTicket>${pgtIou}</cas:proxyGrantingTicket>

    <cas:proxies>
       <cas:proxy>${proxy.principal.id}</cas:proxy>
       ...
    </cas:proxies>

    <cas:user-attributes>
      <cas:attrib name="${attrib.key}">${attrib.value}</cas:attrib>
      ...
    </cas:user-attributes>

    <cas:loa-number>${max_loa_number}</cas:loa-number>
 
    <cas:levels-satisfied>
       <cas:level>${level.name}<cas:level>
       ...
    </cas:levels-satisfied>

    <cas:auth-handlers>
      <cas:auth-handler name="${authhandler.name}">
        <cas:attrib name="${attrib.key}">${attrib.value}</cas:attrib>
        ...
      </cas:auth-handler>
      ...
    </cas:auth-handlers>
  </cas:authentication>

  </cas:authenticationSuccess>
</cas:serviceResponse>

VI. Use cases

A) 2 webapps supporting remember-me and authentication by credentials / client requested LOA

- I access app1 and app2 as remember-me (round-trips on /login)
- I access a more secured area (requiring login/password authentication) in app2 -> I'm redirected to CAS server with renew=credentials, I need to re-authenticate, I fill my password and I'm redirected to app2 -> I get access as I'm now authenticated (not remembered)
- I access a more secured area (requiring login/password authentication) in app1 : what happens ? I'm still remembered in this one, so I'm redirected back to CAS server with renew=credentials and as I'm now authenticated in SSO session with the right LOA, I'm redirected back to application with a valid service ticket.

B) Other use cases

Note that some of these are various versions of the same story but using different scenarios.

NOTE: The following were written by Nathan.

1. Guest -> Student -> Employee
   1. I first interact with CAS when registering for an event as a guest.
      1. For convenience I choose to use Facebook authentication.
   2. A few months later, I join the organization as a student.
      1. Through a process out of the scope of this document, I sign up for a username and password.  These credentials are connected to my existing identity, which up to this point only contained Facebook authentication information.
      2. If I attempt to access a resource intended for guests (app1), I see a login page that does include the Facebook button.
      3. If I attempt to access a resource intended for students (app2), I see a login page that does not include the Facebook authentication option.
      4. Let's say I login to app1 using Facebook, then I navigate to app2.
         
         1. I am presented with the login page again, this time not showing the Facebook button and including a message stating, "You must login with your username and password.  Facebook Connect is not supported for the site you are trying to reach."
         2. After I login, I am taken to the page.
   3. A few years later, I join the organization as an employee.
      1. Through a process out of the scope of this document, I am forced to increase the strength of my password to meet employee standards.  This is likely handled through a business process but also enforced using an interrupt screen in CAS that analyzes my password strength (probably by looking at authentication attributes, the assurance policy, and my organizational roles) and, if necessary, sends me to a self-service change password page after logging in.  The change password page will check my role (in my case by looking in Grouper) and will enforce the employee password policy.
      2. In this case, because password strength has been enforced prior to my accessing a service (through the use of interrupt screens and validation), the service can assume that my password is strong enough.
2. Remember me
   1. I'm a student.
   2. I read a blog on the organization's blog site and I want to leave a comment.  I log in and check the "remember me" box.
   3. When reading another blog a few days later, I am able to leave a comment without logging in again because the "remember me" feature worked for the blog.
   4. I access the event registration web app, and it presents me with the login page because it requires an LOA number higher than that of the "remember me" feature.
3. Trusted external IdP  - Note that this is very similar to using a public IdP like Facebook, and (like Facebook Connect) is basically a form of federation
   1. I'm an employee at a partner organization.
   2. I attempt to access the event registration web app.  On the login page, I click the link "login with username and password from partner organization."  (yes, the real message would be much less generic... more along the lines of "login with facebook" but for the partner org)
   3. I attempt to access an employee portal website, which does not accept employees from partner org.  I am shown the login screen again, with a message saying something like, "You must login with your username and password.  Partner Org login is not supported for the site you are trying to reach."
4. Add a second factor (selected by user) when accessing enterprise app
   1. I access the employee portal and login with my username and password.
   2. I navigate to the organization PeopleSoft HR system.
   3. The system looks up some info in my account and determines that I have my cell phone registered to receive SMS messages and I have Google Authenticator configured.  It presents me with a screen showing the two options and asking me to click one.  I click on the "SMS message" option.  (Note that if I had only previously set up one of these options, the system would have selected it automatically.  If I had not previously set up either option, the system would have given me instructions telling me to set up at least one of these.)
   4. My phone receives a code as an SMS message.  I type this code into a text box on the screen and click "continue".
   5. I am granted access to the PeopleSoft HR system.
5. Extra-strong password policy enforced for user with power-admin role - Note that this is an important LOA scenario, though it is somewhat outside of the scope of LOA in CAS as we're talking about now.  It really highlights how interrupt screens can play an important role in LOA.
   1. I'm an employee with access to the PeopleSoft HR system.
   2. I'm granted a special role in PeopleSoft that gives me lots of power.  This is registered in the organization's RBAC system (in this case Grouper).
   3. The next time I login with my username and password, some code in CAS intercepts my login and realizes that my password is not strong enough for this role.  I get sent to a screen where I must choose a stronger password.
   4. After choosing the stronger password, I login again and then am granted access to PeopleSoft.
   5. Note that PeopleSoft itself might not specifically immediately request any special password strength (beyond what would be true for all users), since it doesn't know who is accessing it when it redirects the first time.  However, after it has received and validated the ticket, it might take the opportunity to look up the person's assigned roles to ensure that their current password strength or LOA level is sufficient.  In that case it could redirect the user back to CAS with a more specific LOA request.  Alternatively, it would probably block access and notify an administrator, because the interrupt screens should have already done the work to ensure that this aspect of the LOA policy was fulfilled.

VII. Roadmap

A) Version 1.0 : only client requested LOA

For a first version, the idea is to target only the LOA requested by the client (renew parameter).

It means creating the assurance policy, level of assurance and assurance evaluator, updating the authentication handler and the authentication manager and implementing the main algorithm (impacts on webflow).

B) Version 2.0 : add server requested LOA

For a second version, we can add the requested LOA defined in CAS server.

It means updating the registered service, the services management webapp and the main algorithm.

C) Version 3.0 : add interactions

For a third version, we can finally add the interactions and interactions manager and update the main algorithm (impacts on webflow).