# Proof-of-forgery for hash-based signatures

## TL;DR

This paper explores a property of hash-based signatures that allows detection of forgery events through hash collisions, enhancing security by signaling when a hash function becomes insecure.

## Contribution

It demonstrates that Lamport and Winternitz one-time signatures can be adjusted to detect forgeries, providing a new security feature for hash-based signature schemes.

## Key findings

- Forgery detection is possible via hash collisions.
- Proper parameter adjustment enables forgery detection.
- This property supports crypto-agility by signaling hash function insecurity.

## Abstract

In the present work, a peculiar property of hash-based signatures allowing detection of their forgery event is explored. This property relies on the fact that a successful forgery of a hash-based signature most likely results in a collision with respect to the employed hash function, while the demonstration of this collision could serve as convincing evidence of the forgery. Here we prove that with properly adjusted parameters Lamport and Winternitz one-time signatures schemes could exhibit a forgery detection availability property. This property is of significant importance in the framework of crypto-agility paradigm since the considered forgery detection serves as an alarm that the employed cryptographic hash function becomes insecure to use and the corresponding scheme has to be replaced.