Do you need to encrypt sesitive data or to verify it has not been modified? If you need to encrypt the data, use SSL - simpler!

Otherwise you are dealing with an activeX control that the user need to install, etc. If you are looking for a java solution, you would need to design the encryption algorythm on a Jar file that resides server side, since wily users can modify the java routines on the client side (take a look at the yahoo.com login page's source - they use an encrytion routine on the client side, but they do not show the process on the 'other side').. Another method that I have seen is to xor each of the text strings with an arbitrary number (decoding requires the same number), but again, the code on the client is transparent and hackable. Here is a simple version: http://www.freevbcode.com/ShowCode.asp?ID=5676 (this will work on IE 5+ only, unless you adapt it to java - here is a nice example: http://tools.devshed.com/c/a/Web-Development/XOREncryption/). OnSuibmit you would encrypt the strings and execute the mailto.

Are your clients that sophisticated? Well, even if they are not, if you have a method that can be easily defeated is the same as not having it at all.

Cheers,