Verify keys are not too similar

so this it the mitigation that @udidahan recommended. the problem is that not  being able to reproduce the issue (cant find too keys that cause the problem) i dont know if the "string being encrypted" is a variable that impacts the outcome.

Can anyone come up with anything better?

For reference here are the SO articles 

 * http://stackoverflow.com/questions/1556683/does-rijndael-always-throw-an-exception-if-the-password-isnt-correct
 * http://stackoverflow.com/questions/5812232/c-sharp-aes-rijndael-detecting-invalid-passwords
ok will make that change