It sounds like you’re looking for the Windows Communication Foundation Authentication Service.
EDIT:
After re-reading the question more carefully (and after Ariel’s comment) I’d like to retract the above suggestion. The WCF Authentication Service won’t add much to this scenario.
I haven’t done this between WCF and ASP.NET, however I have configured ASP.NET applications to share forms authenticated users, perhaps I can help in some way.
To ensure that both applications can encrypt/decrypt the forms authentication cookie in the same way you should configure the <machineKey> element for both applications (in web.config or machine.config depending on whether you want to do this at the machine or application level). You should look at the validation, validationKey, decryption and decryptionKey attributes.
Ensure that your <forms> elements in both web.config files are configured similarly. Specifically the name, path and domain attributes.
It’s likely that this only applies to cookies passed to/from a web browser (but may be useful in this case): To allow cookies to be passed between the websites www.foo.com and bar.foo.com you would configure the forms element as follows to allow cookies to be set on one site and successfully passed to the other:
<forms ... domain=".foo.com" ... />
Passing the cookie to the WCF service is likely to be the tricky bit. I’m not very experienced with WCF, so I’ve adapted code from kennyw.com:
HttpRequestMessageProperty httpRequestProperty = new HttpRequestMessageProperty();
httpRequestProperty.Headers.Add(HttpRequestHeader.Cookie, "<Forms Authentication Cookie>");
using (OperationContextScope scope = new OperationContextScope(serviceClient.InnerChannel))
{
OperationContext.Current.OutgoingMessageProperties[HttpRequestMessageProperty.Name] = httpRequestProperty;
serviceClient.MethodName();
}
If you’re hosting WCF within IIS (and not self-hosting) you can pass the WCF request through the ASP.NET processing pipeline by setting
<system.serviceModel>
<serviceHostingEnvironment aspNetCompatibilityEnabled="true" ... />
</system.serviceModel>
If you’re self hosting you could examine the request headers using the incoming message’s properties in OperationContext.Current.IncomingMessageProperties and get the forms authentication cookie value and decrypt it using FormsAuthentication.Decrypt(string).
I have no idea whether any of this would work, but would love to hear if it does!