Module fors 

FORS: Forest of Random Subsets few-time signature scheme. FORS: Forest of Random Subsets (FIPS 205, Algorithms 14-17).

FORS is a few-time signature scheme that signs the message digest by selecting one leaf from each of k independent binary trees of height a. Each tree has 2^a leaves; the message digest is split into k indices of a bits each, and the signature reveals the secret leaf value plus an authentication path for each tree.

In the SLH-DSA hierarchy, FORS sits between the message hash and the hypertree: the FORS public key (a compression of the k tree roots) becomes the message that the hypertree signs. This two-level structure is what makes SLH-DSA stateless – FORS absorbs the message entropy, and the hypertree provides many-time security.

Functions

fors_node

Compute the root of a FORS subtree of height z at leaf index i.

fors_pk_from_sig

Compute a FORS public key from a FORS signature — constant-time.

fors_sign

Sign a message digest using FORS.

fors_sign_into

Streaming variant of fors_sign — writes the K * (1 + A) * N byte FORS signature directly into the start of out (which must be at least that size).

fors_sign_into_redundant

Recompute-and-compare FORS signing — T1-C redundancy against single-fault grafting-tree forgery.