The Dec 14, 2008 paper "Fingerprint and Inkjet-trace Imaging using Disulfur Dinitride", in ChemComm (Paul F. Kelly, Roberto S. P. King and Roger J. Mortimer), outlines a new real-world forensics technique which is mentioned in this episode. A mirrored, Gaussian-smoothed version of the S₂N₂ treated envelope is shown below. It's still difficult to read, though, even more than a typical Captcha. The really adventuresome can try using an optical character recognition system. Previous puzzles can be seen at the Math behind NUMB3RS pages.