  This paper is a proof-of-concept demonstration for a specific digital
signatures vulnerability that shows the ineffectiveness of the WYSIWYS (What
You See Is What You Sign) concept. The algorithm is fairly simple: the attacker
generates a polymorphic file that has two different types of content (text, as
a PDF document for example, and image: TIFF - two of the most widely used file
formats). When the victim signs the dual content file, he/ she only sees a PDF
document and is unaware of the hidden content inside the file. After obtaining
the legally signed document from the victim, the attacker simply has to change
the extension to the other file format. This will not invalidate the digital
signature, as no bits were altered. The destructive potential of the attack is
considerable, as the Portable Document Format (PDF) is widely used in
e-government and in e-business contexts.
