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How to: Create a Custom Authorization Policy

The Identity Model infrastructure in Windows Communication Foundation (WCF) supports a claim-based authorization model. Claims are extracted from tokens, optionally processed by custom authorization policy, and then placed into an AuthorizationContext that can then be examined to make authorization decisions. A custom policy can be used to transform claims from incoming tokens into claims expected by the application. In this way, the application layer can be insulated from the details on the differing claims served up by the different token types that WCF supports. This topic shows how to implement a custom authorization policy and how to add that policy to the collection of policies used by a service.

To implement a custom authorization policy

  1. Define a new class that derives from IAuthorizationPolicy.

  2. Implement the read-only Id property by generating a unique string in the constructor for the class and returning that string whenever the property is accessed.

  3. Implement the read-only Issuer property by returning a ClaimSet that represents the policy issuer. This could be a ClaimSet that represents the application or a built-in ClaimSet (for example, the ClaimSet returned by the static System property.

  4. Implement the Evaluate method as described in the following procedure.

To implement the Evaluate method

  1. Two parameters are passed to this method: an instance of the EvaluationContext class and an object reference.

  2. If the custom authorization policy adds ClaimSet instances without regard to the current content of the EvaluationContext, then add each ClaimSet by calling the AddClaimSet method and return true from the Evaluate method. Returning true indicates to the authorization infrastructure that the authorization policy has performed its work and does not need to be called again.

  3. If the custom authorization policy adds claim sets only if certain claims are already present in the EvaluationContext, then look for those claims by examining the ClaimSet instances returned by the ClaimSets property. If the claims are present, then add the new claim sets by calling the AddClaimSet method and, if no more claim sets are to be added, return true, indicating to the authorization infrastructure that the authorization policy has completed its work. If the claims are not present, return false, indicating that the authorization policy should be called again if other authorization policies add more claim sets to the EvaluationContext.

  4. In more complex processing scenarios, the second parameter of the Evaluate method is used to store a state variable that the authorization infrastructure will pass back during each subsequent call to the Evaluate method for a particular evaluation.

To specify a custom authorization policy through configuration

  1. Specify the type of the custom authorization policy in the policyType attribute in the add element in the authorizationPolicies element in the serviceAuthorization element.

    <configuration>
     <system.serviceModel>
      <behaviors>
        <serviceAuthorization serviceAuthorizationManagerType=
                  "Samples.MyServiceAuthorizationManager" >
          <authorizationPolicies>       
            <add policyType="Samples.MyAuthorizationPolicy"
          </authorizationPolicies>
        </serviceAuthorization>
      </behaviors>
     </system.serviceModel>
    </configuration>
    

To specify a custom authorization policy through code

  1. Create a List of IAuthorizationPolicy.

  2. Create an instance of the custom authorization policy.

  3. Add the authorization policy instance to the list.

  4. Repeat steps 2 and 3 for each custom authorization policy.

  5. Assign a read-only version of the list to the ExternalAuthorizationPolicies property.

    // Add a custom authorization policy to the service authorization behavior.
    List<IAuthorizationPolicy> policies = new List<IAuthorizationPolicy>();
    policies.Add(new MyAuthorizationPolicy());
    serviceHost.Authorization.ExternalAuthorizationPolicies = policies.AsReadOnly();
    
    

Example

The following example shows a complete IAuthorizationPolicy implementation.

public class MyAuthorizationPolicy : IAuthorizationPolicy
{
  string id;
  
  public MyAuthorizationPolicy()
  {
    id =  Guid.NewGuid().ToString();
  }

  public bool Evaluate(EvaluationContext evaluationContext, ref object state)
  {
    bool bRet = false;
    CustomAuthState customstate = null;

    // If the state is null, then this has not been called before so 
    // set up a custom state.
    if (state == null)
    {
      customstate = new CustomAuthState();
      state = customstate;
    }
    else
      customstate = (CustomAuthState)state;

    // If claims have not been added yet...
    if (!customstate.ClaimsAdded)
    {
      // Create an empty list of claims.
      IList<Claim> claims = new List<Claim>();

      // Iterate through each of the claim sets in the evaluation context.
      foreach (ClaimSet cs in evaluationContext.ClaimSets)
        // Look for Name claims in the current claimset.
        foreach (Claim c in cs.FindClaims(ClaimTypes.Name, Rights.PossessProperty))
          // Get the list of operations the given username is allowed to call.
          foreach (string s in GetAllowedOpList(c.Resource.ToString()))
          {
            // Add claims to the list.
            claims.Add(new Claim("http://example.org/claims/allowedoperation", s, Rights.PossessProperty));
            Console.WriteLine("Claim added {0}", s);
          }
      
      // Add claims to the evaluation context.
      evaluationContext.AddClaimSet(this, new DefaultClaimSet(this.Issuer,claims));
      
      // Record that claims were added.
      customstate.ClaimsAdded = true;
      
      // Return true, indicating that this method does not need to be called again.
      bRet = true;
    }
    else
    {
      // Should never get here, but just in case, return true.
      bRet = true;
    }
    
    
    return bRet;
  }

  public ClaimSet Issuer
  {
    get { return ClaimSet.System; }
  }
  
  public string Id
  {
    get { return id; }
  }

  // This method returns a collection of action strings that indicate the 
  // operations the specified username is allowed to call.
  private IEnumerable<string> GetAllowedOpList(string username)
  {
    IList<string> ret = new List<string>();
    
    if (username == "test1")
    {
      ret.Add ( "http://Microsoft.ServiceModel.Samples/ICalculator/Add");
      ret.Add ("http://Microsoft.ServiceModel.Samples/ICalculator/Multiply");
      ret.Add("http://Microsoft.ServiceModel.Samples/ICalculator/Subtract");
    }
    else if (username == "test2")
    {
      ret.Add ( "http://Microsoft.ServiceModel.Samples/ICalculator/Add");
      ret.Add ("http://Microsoft.ServiceModel.Samples/ICalculator/Subtract");
    }
    return ret;
  }

  // Internal class for keeping track of state.
  class CustomAuthState
  {
    bool bClaimsAdded;
    
    public CustomAuthState()
    {
      bClaimsAdded = false;
    }
    
    public bool ClaimsAdded { get { return bClaimsAdded; } 
    set {  bClaimsAdded = value; } }
  }
}

See Also


© 2007 Microsoft Corporation. All rights reserved.
Build Date: 2009-08-07

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