# Techniques: Hybrid Authentication Protocols for Advanced Quantum Networks

## What the paper contributes (one paragraph)

The paper introduces the first entanglement-based hybrid authentication (identification) protocols for quantum networks, combining a weak off-the-shelf classical Physical Unclonable Function (PUF) with the quantum property of local indistinguishability to obtain provable, exponential security. It gives two protocols. The offline protocol consumes pre-shared trusted Bell pairs and authenticates a prover using only LOCC and classical communication, with no quantum communication during authentication, suitable as a ping test on entangled network nodes. The online protocol removes the trusted source by introducing a new hardware module, the Hybrid Entangled PUF (HEPUF), which encodes a classical PUF output into a bipartite entangled state whose subsystem is sent over the quantum channel. Full completeness and soundness (unforgeability) proofs are given for both, including a fidelity-robust version for imperfect Bell pairs via entropic uncertainty. The arxiv id is 2504.11552.

## Techniques offered

- **Offline entanglement-based hybrid PUF authentication protocol** - An identification protocol over pre-shared trusted Bell pairs that authenticates a prover using only LOCC and classical communication, never transmitting the PUF response over the channel.
  - guarantee: completeness with negligible error and exponential soundness (universal unforgeability), with adversary forgery probability bounded by (1/2)^m against unbounded quantum eavesdroppers given trusted perfect Bell pairs, and by 2^{-m(1-mu(epsilon))} when the shared pairs are imperfect with fidelity at least 1-epsilon.
  - quote: "Protocol~\ref{prot_bell} satisfies completeness and exponential security against any unbounded quantum eavesdropper, given trusted perfect Bell-pairs."

- **Online HEPUF authentication protocol using local indistinguishability** - An identification protocol with no pre-shared entanglement in which the prover encodes a classical PUF output into a locally indistinguishable bipartite entangled state and sends a subsystem over the quantum channel, with security resting on the limits of local state discrimination.
  - guarantee: completeness with negligible error and exponential soundness (universal unforgeability) against QPT adversaries and unbounded channel eavesdroppers, with forgery probability bounded by (1/2 + delta*sqrt((1+4*delta^2)/2))^m for a classical PUF of bias delta.
  - quote: "Protocol \ref{online_prot_2Bell} satisfies security against QPT adversaries and unbounded quantum eavesdroppers (during authentication)."

- **Hybrid Entangled PUF (HEPUF) construction** - A new hybrid hardware module that takes a classical PUF challenge and outputs a bipartite entangled state encoding the response, with a local-decoding measurement, packaged as a reusable primitive for entanglement-based authentication.
  - guarantee: none (it is a hardware construction; its assurance is the unforgeability of the online protocol built on it, given an underlying classical PUF that is not fully broken).
  - quote: "We now introduce a new hybrid PUF, which takes a classical input and produces an entangled quantum output."

## Where it could apply

- Target primitive(s): none (this is a quantum-communication and cryptography result, not a gate-model algorithm primitive such as VQE, QAOA, QPE, sampling, or Trotterised simulation).
- Target application group(s): platforms_tooling, networking and quantum cryptography (entity authentication, identification, and link authentication on entanglement-enabled quantum networks).
- Code family / hardware assumptions: no quantum error correction; bipartite maximally entangled (Bell) states with measurements in the computational and Hadamard bases, a weak classical PUF as the hardware root of trust, and entanglement distribution, stated as well suited to photonic platforms.

## Caveats

These are quantum-communication and cryptography techniques rather than capabilities suppliable to a fault-tolerant or gate-model computational result. Security rests on hardware assumptions: the classical PUF is assumed not to be fully broken or corrupted by a colluding manufacturer, and no PUF can be secure against unbounded adversaries in the setup phase, so the online protocol assumes a polynomially bounded (QPT) adversary during setup. The offline protocol additionally assumes a trusted source of (near-)perfect Bell pairs. Key agreement and challenge re-usability are conjectured but left without formal proof, and the work is theoretical with no experimental implementation.