New technologies for tax stamps are evolving into more complex and complementary solutions. Tax stamp designers are being forced to make tradeoffs between, on one hand, maximimising security while allowing for ubiquitous non-proprietary reading systems, and, on the other hand, achieving a cheap and fast solution. Unique elements that are impossible or impractical to reproduce have a key role to play in this context, and one of the current challenges is to design and print a tax stamp both with all the traditional security elements and a unique, machine-readable and unclonable element.

In this paper we present the UniQode system for tax stamps. UniQode allows for the use of a cryptographic graphic code along with holograms and/or ink mixed with special visible metal particles (glitter). The hologram or glitter inks will produce a random pattern which, according to the disorder and chaos theory, is not possible to reproduce as is, thus constituting a physically unclonable function. To avoid the need for a database, a cryptographic code is printed with some minutia of the unique elements, thus allowing for an offline validation of the authenticity of the tax stamp.