XMSS

Stateful hash-based signature scheme standardized in RFC 8391 and NIST SP 800-208, built from Merkle trees.

Core Principles & Analysis

• Each private-key state may sign exactly once — reusing a state can forfeit the entire key's security.
• That statefulness restricts it to controlled environments like firmware signing, not general protocols.
• Far smaller and faster than stateless SPHINCS+ when the state can be managed safely.

Specifications & Metrics

• FAMILY: Hash-based (stateful)
• STATUS: Standardized
• STANDARD: RFC 8391 / SP 800-208
• YEAR: 2018