Scott has a great post in his blog about how to share authentication between ASP.NET 1.1 and ASP.NET 2.0.

It’s useful for those of you who actually needs to be able to share the same authentication between an older ASP.NET 1.1 application and a new ASP.NET 2.0 application.

This article exposes some information I didn’t know (or didn’t want to know 😉 ).

First, ASP.NET 1.1 encrypted cookie uses the 3DES (a.k.a Triple DES) encryption. This key is per ASP.NET application (unless configured otherwise in the configuration).

According to the Scott’s post, ASP.NET 2.0 uses a stronger encryption (I assume AES since 3DES is AES predecessor).
Therefore, if you do use this feature mind that it will weaken your ecnrypted cookine in your ASP.NET 2.0 application.