Brill says: In the GWT forums someone asked is anyone had tried encryption from the client side in GWT... so I did.
Maven users see: Usage
Non-Maven users can download the jar from the download tab, with code examples at Usage
Update 2009-10-08:mooreds says: I updated the code to work with GWT 1.5.3 and tested it in 1.6.4 and 1.7.1, and fixed a bug or two. Please download it and give it a try. If you are using it and it needs updating, let me know or send in a patch.
Also, make sure you understand what client side encryption does and does not do for you: ClientSideEncryptionConsiderations