Is it possible to decode EventValidation and ViewState in ASP.NET?

I'm not sure about EventValidation, but you can decode ViewState by using Fritz Onion's ViewState Decoder.

I answered a similar question recently, Getting values from viewstate using JQuery?.

Basically, by default ViewState is just Base64-encoded, so you can decode it as long as the administrator hasn't configured the site to encrypt it. Quoting from my previous answer:

If you are writing the control for your own consumption and you only need to read from ViewState, you could do so, but I wouldn't recommend it unless you find a well-debugged library to parse it for you. The format is a bit hairy (see ViewState: All You Wanted to Know for more details).

That link provides an extremely thorough and clear introduction to ViewState.

As for Event Validation, I'm unsure whether it's Base64-encoded or if it just looks like Base64 (I can't find a conclusive, authoritative reference). This Rexiology article might help though.