I accept compression as generalized, not as encrypted, and compress the string.
Alternatively, I would suggest Microsoft Ajax Minified 4.0 that I use it with sussex on very complex and large javascript files (including jQuery) to compress them in real time / minimize them ...
http://aspnet.codeplex.com/releases/view/40584
and next to zip, I suggest SharpZipLib, but also DotNetZip
http://dotnetzip.codeplex.com/