SPHINCS+

SPHINCS+ is a digital signature algorithm whose security depends only on the collision resistance of a cryptographic hash function, an assumption with a much longer track record than the lattice problems behind Dilithium and Kyber. NIST standardized it in 2024 as SLH-DSA (FIPS 205), explicitly as a backup: if a future mathematical advance ever weakened lattice-based cryptography, hash-based signatures like SPHINCS+ would still hold, since they rest on a structurally different and independently studied hardness assumption. The tradeoff is size and speed. SPHINCS+ signatures run tens of kilobytes, far larger than Dilithium's, and signing is noticeably slower. Most deployments are expected to use Dilithium as the default and reserve SPHINCS+ for high-assurance cases such as firmware signing or long-lived root certificates, where the performance cost is acceptable in exchange for a second, unrelated line of defense.