Aside from the issues regarding decrypting code that was encrypted for whatever reason, as I all too well know, if the code was written by an employee, for employment purposes, the copyright then would belong to the employer as a "Work for hire" even if the employee was never compensated for the work ... painful memories coming back now ... anyway, ,that would make it your employers call on whether to allow it to be decrypted ... I'll not go into the whole story, but suffice to say I went through this battle with a previous employer in 2002 .... my lawyer finally advised me that regardless of the outcome of the lawsuit, that they would still have complete control over the code and it's use because there was no written licensing agreement.
So...
Did this employee write it on company time? If so there is no problem with decrypting it, provided it can be decrypted.
Did the employee provide a licensing agreement for the company? If no, then the company has all legal right to it's use, and reverse engineering has been held to be "fair use" by the courts and DMCA.
If the employer agreed to a licensing agreement that prohibits reverse engineering, then it is likely not ok to decrypt it.
If the employee did not create it while working at the companyor did not do it on company time, then the case would be very sticky and I would recommend staying clear of it.
All in all, I think that Se7en is right though ... it would make you a much better programmer if you simply recreated the program from scratch.
Now ask me if I have ever decrypted a program ... yep, plenty of times, would I do it and provide you with the source ... nope ... I have no assurances it would not be an unlawful enterprise ... But .... if I were in your shoes I would do it ... but only after direction from my employer.
