- NAME
- DESCRIPTION
  - SLH-DSA Signature Parameters
- NOTES
- EXAMPLES
- SEE ALSO
- HISTORY
- COPYRIGHT

# NAME

EVP_SIGNATURE-SLH-DSA, EVP_SIGNATURE-SLH-DSA-SHA2-128s, EVP_SIGNATURE-SLH-DSA-SHA2-128f, EVP_SIGNATURE-SLH-DSA-SHA2-192s, EVP_SIGNATURE-SLH-DSA-SHA2-192f, EVP_SIGNATURE-SLH-DSA-SHA2-256s, EVP_SIGNATURE-SLH-DSA-SHA2-256f, EVP_SIGNATURE-SLH-DSA-SHAKE-128s, EVP_SIGNATURE-SLH-DSA-SHAKE-128f, EVP_SIGNATURE-SLH-DSA-SHAKE-192s, EVP_SIGNATURE-SLH-DSA-SHAKE-192f, EVP_SIGNATURE-SLH-DSA-SHAKE-256s, EVP_SIGNATURE-SLH-DSA-SHAKE-256f - The EVP_PKEY SLH-DSA signature implementations

# DESCRIPTION

The SLH-DSA-SHA2-128s, EVP_PKEY-SLH-DSA-SHA2-128f, SLH-DSA-SHA2-192s, EVP_PKEY-SLH-DSA-SHA2-192f, SLH-DSA-SHA2-256s, EVP_PKEY-SLH-DSA-SHA2-256f, SLH-DSA-SHAKE-128s, EVP_PKEY-SLH-DSA-SHAKE-128f, SLH-DSA-SHAKE-192s, EVP_PKEY-SLH-DSA-SHAKE-192f, SLH-DSA-SHAKE-256s and EVP_PKEY-SLH-DSA-SHAKE-256f EVP_PKEY implementations supports key generation, one-shot sign and verify using the SLH-DSA signature schemes described in FIPS 205.

The different algorithms names correspond to the parameter sets defined in FIPS 205 Section 11 Table 2. `s` types have smaller signature sizes, and the `f` variants are faster, (The signatures range from ~8K to ~50K depending on the type chosen). There are 3 different security categories also depending on the type.

EVP_SIGNATURE_fetch(3) can be used to explicitly fetch one of the 12 algorithms which can then be used with EVP_PKEY_sign_message_init(3), EVP_PKEY_sign(3), EVP_PKEY_verify_message_init(3), and EVP_PKEY_verify(3) to perform one-shot message signing or verification.

The normal signing process (called Pure SLH-DSA Signature Generation) encodes the message internally as 0x00 || len(ctx) || ctx || message. where ctx is some optional value of size 0x00..0xFF. OpenSSL also allows the message to not be encoded which is required for testing. OpenSSL does not support Pre Hash SLH-DSA Signature Generation, but this may be done by the user by doing Pre hash encoding externally and then choosing the option to not encode the message.

## SLH-DSA Signature Parameters

The `context-string` parameter, described below, can be used for both signing and verification. It may be set by passing an OSSL_PARAM array to EVP_PKEY_sign_init_ex2(3) or EVP_PKEY_verify_init_ex2(3)

"context-string" (OSSL_SIGNATURE_PARAM_CONTEXT_STRING) <octet string>

  A string of octets with length at most 255. By default it is the empty string.

The following parameters can be used when signing: They can be set by passing an OSSL_PARAM array to EVP_PKEY_sign_init_ex2(3).

"message-encoding" (OSSL_SIGNATURE_PARAM_MESSAGE_ENCODING) <integer>

  The default value of 1 uses 'Pure SLH-DSA Signature Generation' as described above. Setting it to 0 does not encode the message, which is used for testing, but can also be used for 'Pre Hash SLH-DSA Signature Generation'.

"test-entropy" (OSSL_SIGNATURE_PARAM_TEST_ENTROPY <octet string

  Used for testing to pass a optional random value.

"deterministic" (OSSL_SIGNATURE_PARAM_DETERMINISTIC) <integer>

  The default value of 0 generates a random value (using a DRBG) this is used when processing the message. Setting this to 1 causes the private key seed to be used instead. This value is ignored if "test-entropy" is set.

See EVP_PKEY-SLH-DSA(7) for information related to SLH-DSA keys.

# NOTES

For backwards compatibility reasons EVP_DigestSignInit_ex(), EVP_DigestSign(), EVP_DigestVerifyInit_ex() and EVP_DigestVerify() may also be used, but the digest passed in mdname must be NULL.

# EXAMPLES

To sign a message using an SLH-DSA EVP_PKEY structure:

    void do_sign(EVP_PKEY *key, unsigned char *msg, size_t msg_len)
    {
        size_t sig_len;
        unsigned char *sig = NULL;
        const OSSL_PARAM params[] = {
            OSSL_PARAM_octet_string("context-string", (unsigned char *)"A context string", 33),
            OSSL_PARAM_END
        };
        EVP_PKEY_CTX *sctx = EVP_PKEY_CTX_new_from_pkey(NULL, pkey, NULL);
        EVP_SIGNATURE *sig_alg = EVP_SIGNATURE_fetch(NULL, "SLH-DSA-SHA2-128s", NULL);

        EVP_PKEY_sign_message_init(sctx, sig_alg, params);
        /* Calculate the required size for the signature by passing a NULL buffer. */
        EVP_PKEY_sign(sctx, NULL, &sig_len, msg, msg_len);
        sig = OPENSSL_zalloc(sig_len);
        EVP_PKEY_sign(sctx, sig, &sig_len, msg, msg_len);
        ...
        OPENSSL_free(sig);
        EVP_SIGNATURE_free(sig_alg);
        EVP_PKEY_CTX_free(sctx);
    }

# SEE ALSO

EVP_PKEY-SLH-DSA(7) provider-signature(7), EVP_PKEY_sign(3), EVP_PKEY_verify(3),

# HISTORY

This functionality was added in OpenSSL 3.5.

# COPYRIGHT

Copyright 2024-2026 The OpenSSL Project Authors. All Rights Reserved.

Licensed under the Apache License 2.0 (the "License"). You may not use this file except in compliance with the License. You can obtain a copy in the file LICENSE in the source distribution or at https://www.openssl.org/source/license.html.