Function slh_sign_internal 

pub fn slh_sign_internal<P: Params>(
    m: &[u8],
    sk: &[u8],
    addrnd: &[u8],
) -> Result<Vec<u8>, SlhDsaError>

Internal SLH-DSA signing function.

Implements Algorithm 19 of FIPS 205. Performs the full signing pipeline:

1. Generates the per-signature randomizer R via PRF_msg(SK.prf, addrnd, M).
2. Hashes the message with H_msg to produce a digest, then splits it into FORS indices (md), a tree index (idx_tree), and a leaf index (idx_leaf).
3. Signs md with FORS to obtain SIG_FORS, then computes the FORS public key.
4. Signs the FORS public key with the hypertree to obtain SIG_HT.

The addrnd parameter is n bytes of additional randomness for hedged signing. Passing PK.seed instead yields deterministic signing.

Returns the full signature: R || SIG_FORS || SIG_HT.

Feature: sca-fors-redundancy

When this cargo feature is enabled, the FORS sub-signature is produced by fors::fors_sign_into_redundant (T1-C — recompute-and-compare against single-fault grafting-tree forgery, per :cite:genet2023_protecting_sphincs_faults, :cite:castelnovi2018_grafting_trees, :cite:adiletta2025_slashdsa_rowhammer). On a detected divergence the function returns Err(SlhDsaError::FaultDetected) — the faulted signature never leaves the device. The validated FORS public key returned by the redundant routine is fed straight into the hypertree, saving a third FORS-root derivation.

Without the feature, the result is always Ok — the unified Result envelope is paid only to keep one signature across the two configurations. CAVP / KAT output bytes are unchanged.