The Extensions for Microsoft AJAX Framework project over at CodePlex provides cryptographic services including secure encoding and decoding of data, as well as hashing and random number generation, and new methods for JavaScript native objects. This project is pretty cool, and could really grow into a huge library offering us what Microsoft isn't putting into the AJAX Extensions themselves. And, now we have an object oriented, JavaScript way of doing MD5, SHA1, HMAC and AES!
Go check it out: http://www.codeplex.com/jsxfx