A formal definition and a new security mechanism of physical unclonable functions

TL;DR

This paper provides a formal definition of physical unclonable functions (PUFs), distinguishes them from related concepts, and introduces a new security mechanism that enables the creation of theoretically uncloneable quantum-PUFs without large secret storage.

Contribution

It offers a precise formal definition of PUFs, clarifies their relation to similar concepts, and proposes a novel security mechanism based on challenge-dependent erasure of information.

Findings

A formal, clear definition of PUFs for security evaluation.

Introduction of a new challenge-dependent erasure security mechanism.

Proposal of a quantum-PUF concept with absolute security.

Abstract

The characteristic novelty of what is generally meant by a "physical unclonable function" (PUF) is precisely defined, in order to supply a firm basis for security evaluations and the proposal of new security mechanisms. A PUF is defined as a hardware device which implements a physical function with an output value that changes with its argument. A PUF can be clonable, but a secure PUF must be unclonable. This proposed meaning of a PUF is cleanly delineated from the closely related concepts of "conventional unclonable function", "physically obfuscated key", "random-number generator", "controlled PUF" and "strong PUF". The structure of a systematic security evaluation of a PUF enabled by the proposed formal definition is outlined. Practically all current and novel physical (but not conventional) unclonable physical functions are PUFs by our definition. Thereby the proposed definition captures the existing intuition about what is a PUF and remains flexible enough to encompass further research. In a second part we quantitatively characterize two classes of PUF security mechanisms, the standard one, based on a minimum secret read-out time, and a novel one, based on challenge-dependent erasure of stored information. The new mechanism is shown to allow in principle the construction of a "quantum-PUF", that is absolutely secure while not requiring the storage of an exponentially large secret. The construction of a PUF that is mathematically and physically unclonable in principle does not contradict the laws of physics.