We have a cookie management library that writes a cookie containing some sensitive information, encrypted with Rijndael. The cookie encrypts and decrypts fine in unit tests (using Moq), works fine for MVC web applications, but when called from an ASP.net 2.0 website, the cookie cannot be decrypted. "Padding is invalid and cannot be removed."

We are sure that the cookie value is valid because we tested it 10,000 times with random data in a unit test. There is something about what ASP.NET 2.0 does when it reads and writes the cookie that causes trouble.

There has to be a gotcha. Any suggestions?