Clever thief wants to lead any investigations away from him and onto the innocent 3rd party
![Wink](https://bitcointalk.org/Smileys/default/wink.gif)
There are better ways to launder stolen bitcoins. And if the thief will be caught, he will be prosecuted anyway in most cases.
The only way to be 100% secure is to not give your key to anyone else, ever and only decrypt on your own machine.
I assume that my data is never 100% secure, because any today's computer system is way too complex to be absolutely sure about anything. But of course, such practices, combined with strong passwords, can make you at least 99.99% sure.