Microprinting

That's the hinge Daniel's getting at.

Microtext on currency exists because a photocopier can't reproduce what it can't resolve.

The marking is the authentication.

The same scale that defeats the eye defeats the scanner.

A marking is information.

The smaller the mark, the less information it can carry before it becomes noise.

Microtext is the edge case.

You can put a paragraph on a speck of gold, but only with a tool that costs more than a house.