Quantum blind-unforgeability proves stronger than plus-one unforgeability
Blind Unforgeability implies Plus-One Unforgeability
Cryptography and Security
Summary
Some types of security protections for quantum cryptography are stronger than others, but it wasn't clear exactly how they relate. The authors showed that if a system is secure against a type of attack called quantum blind-unforgeability, it also protects against a related, weaker attack called plus-one unforgeability. They developed a new technique that reshapes how queries in the quantum algorithm interact, which helped prove this relationship. This means one security notion is strictly better, settling a question that was open before.
What this means in practice
- •For quantum security engineers: Improve the evaluation of quantum-resistant cryptographic protocols by prioritizing blind-unforgeability as a stronger security guarantee.
- •For cryptographic hardware designers: Design quantum-proof authentication hardware that exploits the stricter security implications of blind-unforgeability to resist advanced forgery.
A theory result. No direct application yet.
Authors
Mehil Agarwal, Calliope S. Reimann, Fang Song
Abstract
We prove that quantum blind-unforgeability implies quantum plus-one unforgeability, settling an open question in the literature. Since plus-one unforgeability is known not to imply blind-unforgeability, our result establishes that blind-unforgeability is \emph{strictly stronger} than plus-one unforgeability in the quantum setting. The implication is proven using a new \emph{smoothing} technique that coherently rescales the query histories of a quantum-query algorithm in a black-box manner. This enables us to transform any successful plus-one attacker into a blind-unforgeability attacker with only inverse-polynomial loss. Our proof represents a conceptual shift from merely extracting a useful query history as in earlier attempts, which we show inevitably fails in general, to first coherently reshaping how query histories interfere.