Core engineering and protocols for FIPS 205 Hypertree Architecture.
Early hash-based signature schemes (like LMS and XMSS, standardized in RFC 8554 and FIPS 206) are stateful: every private key maintains a strictly incrementing counter. If a virtual machine snapshot is rolled back or cloned, the state counter repeats, reusing a one-time signature key and allowing adversaries to completely forge master signatures. In contrast, FIPS 205 SLH-DSA is completely stateless.
SLH-DSA structures its signature space into a virtual hypertree composed of multiple layers of Merkle trees (d layers, each of height h/d). Message digests are signed using a Few-Time Signature scheme (FORS), which in turn is authenticated by a chain of Winternitz One-Time Signatures (WOTS+), culminating in the immutable public root key. Because leaves are selected pseudorandomly, the system can sign unlimited messages safely.