State Management for Hash-Based Signatures

The unavoidable transition to post-quantum cryptography requires dependable quantum-safe digital signature schemes. Hash-based signatures are well-understood and promising candidates, and the object of current standardization efforts. In the scope of this standardization process, the most commonly raised concern is statefulness, due to the use of one-time signature schemes. While the theory of hash-based signatures is mature, a discussion of the system security issues arising from the concrete management of their state has been lacking. In this paper, we analyze state management in N-time hash-based signature schemes, considering both security and performance, and categorize the security issues that can occur due to state synchronization failures. We describe a state reservation and nonvolatile storage, and show that it can be naturally realized in a hierarchical signature scheme. To protect against unintentional copying of the private key state, we consider a hybrid stateless/stateful scheme, which provides a graceful security degradation in the face of unintentional copying, at the cost of increased signature size. Compared to a completely stateless scheme, the hybrid approach realizes the essential benefits, with smaller signatures and faster signing.