As per Microsoft How to install custom security extensions
] the following -
A new interface was introduced that can be implemented via IRSRequestContext, which provides the more common properties used by extensions to make decisions related to authentication.
The most generic example is accessing HttpContext.Current to read request information such as headers and cookies. In order to allow extensions to make the same decisions, we introduced a new method in the extension that provides request information and is called when authenticating from the portal.
Custom extensions have to implement the IAuthenticationExtension2 interface in order to use this new interface. The extensions will need to implement both versions of the GetUserInfo method, as one is called by the reportserver context and other used in Microsoft.ReportingServices.Portal.WebHost.exe process. The following sample shows one of the simple implementations for the portal where the identity resolved by the reportserver is the one used in C#.
public void GetUserInfo(IRSRequestContext requestContext, out IIdentity userIdentity, out IntPtr userId)
userIdentity = null;
if (requestContext.User != null)
userIdentity = requestContext.User;
userId = IntPtr.Zero;
Read more on the provided link on deployment and configurations as well as using machine keys.