What is the algorithm to construct FormsAuthentication cookie data (after installing update mentioned in Microsoft Security Bulletin MS11-100)

This link below has described the algorithm to construct FormsAuthentication cookie data
http://www.codeproject.com/Articles/16822/The-Anatomy-of-Forms-Authentication .
However, after installing update mentioned in Microsoft Security Bulletin MS11-100, the algorithm has been changed.
By guessing and experimenting, I've worked out most part of the new algorithm.
However, the way how the first 24 bytes of the cookie data are generated is still unknown.
Is the new algorithm publicly available?
(The reason I'd like to know the algorithm is because I need to authenticate .NET generated cookie data in a non .NET language.)
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