slhdsa-c

A portable C90 implementation of SLH-DSA ("Stateless Hash-Based Digital Signature Standard") as described in FIPS 205.

• Supports all 12 parameter sets in FIPS 205, both "pure" and "internal" functions (without recompiling for various parameter sets), as well as prehash modes.
• Self-contained implementation without external dependencies. Can be easily included into applications.
• Passes NIST ACVP tests (all 1248 test cases currently in the default set.) Simple test wrapper included.
• ACVP tests for prehash modes with: SHA2-224, SHA2-256, SHA2-384, SHA2-512, SHA2-512/224, SHA2-512/256, SHA3-224, SHA3-256, SHA3-384, SHA3-512, SHAKE-128, SHAKE-256

This code was derived from SLotH driver code written by Markku-Juhani O. Saarinen between 2023 and 2025. This source code has been relicensed and donated to the slhdsa-c project.

FIPS 205 Name	Cat	PK	SK	Sig
SLH-DSA-SHA2-128s	1	32	64	7856
SLH-DSA-SHAKE-128s	1	32	64	7856
SLH-DSA-SHA2-128f	1	32	64	17088
SLH-DSA-SHAKE-128f	1	32	64	17088
SLH-DSA-SHA2-192s	3	48	96	16224
SLH-DSA-SHAKE-192s	3	48	96	16224
SLH-DSA-SHA2-192f	3	48	96	35664
SLH-DSA-SHAKE-192f	3	48	96	35664
SLH-DSA-SHA2-256s	5	64	128	29792
SLH-DSA-SHAKE-256s	5	64	128	29792
SLH-DSA-SHA2-256f	5	64	128	49856
SLH-DSA-SHAKE-256f	5	64	128	49856

Status

slhdsa-c is work in progress. WE DO NOT CURRENTLY RECOMMEND RELYING ON THIS LIBRARY IN A PRODUCTION ENVIRONMENT OR TO PROTECT ANY SENSITIVE DATA. Once we have the first stable version, this notice will be removed.

Building and Running Known Answer Tests

The implementation in this directory includes the necessary hash functions and, hence, has no external library dependencies. On a Linux system, you can typically use make to build the test wrapper executable xfips205.

$ make
gcc -Wall -Wextra -Werror=unused-result -Wpedantic -Werror -Wmissing-prototypes -Wshadow -Wpointer-arith -Wredundant-decls -Wno-long-long -Wno-unknown-pragmas -O3 -fomit-frame-pointer -std=c99 -pedantic -c sha2_256.c -o sha2_256.o
...
-O3 -fomit-frame-pointer -std=c99 -pedantic -o xfips205 sha2_256.o sha2_512.o sha3_api.o sha3_f1600.o slh_dsa.o slh_prehash.o slh_sha2.o slh_shake.o test/xfips205.o

Running the ACVP tests

test/acvp_client.py implement ACVP tests and can also be executed through make test. The ACVP version can be specified by passing the --version argument to the test/acvp_client.py. The static test vectors are automatically fetched from NIST's ACVP-Server repository on first execution.s

$ make test
python3 test/acvp_client.py
Using ACVP test vectors version v1.1.0.40
Running 1248 tests with 16 parallel jobs
[PASS] keyGen SLH-DSA-SHA2-128s [1] slh_keygen_internal()
...
[PASS] sigVer SLH-DSA-SHAKE-256s [497] slh_verify_internal()

=== test summary ===
PASS: 1248
FAIL: 0
ALL GOOD!

Structure of the implementation

External applications should include slh_dsa.h and optionally slh_prehash.h if prehash modes are required, and link the files in the slhdsa-c directory (not test).

slhdsa-c
├── LICENSE             # "MIT or Apache 2.0" licenses
├── Makefile            # generic makefile
├── plat_local.h        # macros for rotations, endianness
├── README.md           # this file
├── sha2_256.c          # SHA2-256 core implementation
├── sha2_512.c          # SHA2-512 core implementation
├── sha2_api.h          # SHA2 hash API
├── sha3_api.c          # SHA3/SHAKE core implementation
├── sha3_api.h          # SHA2 hash API
├── sha3_f1600.c        # Keccak-f1600 permutation for SHA3
├── slh_adrs.h          # SLH-DSA address manipulation
├── slh_dsa.c           # implementation file for internal and pure functions
├── slh_dsa.h           # SLH-DSA API (include this externally)
├── slh_param.h         # SLH-DSA parameter set / instantiation structure
├── slh_prehash.c       # implementation of the pre-hash wrapper
├── slh_prehash.h       # HashSLH API (include this externally if you need it)
├── slh_sha2.c          # SLH-DSA instantiation for SHA2 hash family
├── slh_shake.c         # SLH-DSA instantiation for SHA3/SHAKE hash family
├── slh_var.h           # internal SLH-DSA context structure
└── test                # testing stuff (not for application)
    ├── Makefile        # makefile for local test tasks
    ├── acvp_client.py  # ACVP client
    └── xfips205.c      # command-line test harness