This is a modified version of "Encryption/Decryption with .NET" by Frank Fang. This version has been ported to VB and is also a CCW. A COM Callable Wrapper allows you to use the .NET class in VB 6.0.
I have included two tester applications, just forms that you can use to test the encryption/decryption. The key is hard coded, you can change it in the form variables.
Here is a link to the original author's submission: Encryption/Decryption with .NET.