Eat your words

Purdue Pharma of Connecticut is patenting a system for ensuring that
documents are destroyed soon after they have been read (WO 98/58321). With
Microsoft Word documents, a special instruction, or macro, is added to the
file—just like a Word virus. Plain text is encrypted so it can be read
only by a special program. Either way, however, when a document is opened, the
software checks the date and erases it if it has been on a computer for more
than a set period. The same system can be used to send documents over the
Internet, which recipients can keep only if they pay for a key code.