Pruning Hypertrees for the Lax and Lazy

Hypertree pruning is a novel algorithmic technique for SLH-DSA (SPHINCS+) that trades a keypair's signature use limit for significantly faster key generation and/or signing. By replacing unused WOTS leaf nodes in the XMSS hypertree with cheap pseudorandom surrogate hashes, signers can reduce keygen time by ~500x or achieve multi-fold signing speedups. Crucially, signatures produced remain valid under standard SLH-DSA verification. The article derives an optimal pruning strategy (prune layers equally, biased toward the root tree first), analyzes security implications including reduced safe signature limits, discusses privacy fingerprinting risks, warns about signer incompatibility footguns, and evaluates the technique across NIST SLH-DSA-SHA2-128s and SHRINCS parameter sets with concrete performance tables.