TIdAuthentication = class(TIdPersistent);

TIdAuthentication is a TIdPersistent descendant, and the common ancestor class that specifies the properties and methods available in the Indy authentication framework. TIdAuthentication represents the common functionality needed to implement the authentication methods described in the Internet Standards documents: 

RFC 2616 - Hypertext Transfer Protocol [HTTP/1.1]  

RFC 2617 - HTTP Authentication: Basic and Digest Access Authentication  

Applications should create instances of descendant classes that implement the specifics described in the preceeding documents, such as TIdBasicAuthentication and TIdDigestAuthentication.