=pod

=head1 NAME

EVP_KDF-KB - The Key-Based EVP_KDF implementation

=head1 DESCRIPTION

The EVP_KDF-KB algorithm implements the Key-Based key derivation function
(KBKDF).  KBKDF derives a key from repeated application of a keyed MAC to an
input secret (and other optional values).

=head2 Identity

"KBKDF" is the name for this implementation; it can be used with the
EVP_KDF_fetch() function.

=head2 Supported parameters

The supported parameters are:

=over 4

=item B<OSSL_KDF_PARAM_PROPERTIES> ("properties") <UTF8 string>

=item B<OSSL_KDF_PARAM_DIGEST> ("digest") <UTF8 string>

=item B<OSSL_KDF_PARAM_DIGEST> ("mac") <UTF8 string>

=item B<OSSL_KDF_PARAM_KEY> ("key") <octet string>

=item B<OSSL_KDF_PARAM_SALT> ("salt") <octet string>

=item B<OSSL_KDF_PARAM_INFO> ("info") <octet string>

=back

The parameters key, salt, and info correspond to KI, Label, and Context
(respectively) in SP800-108.  As in that document, salt and info are optional
and may be omitted.  Currently, only HMAC is supported for mac.

=head1 NOTES

A context for KBKDF can be obtained by calling:

 EVP_KDF *kdf = EVP_KDF_fetch(NULL, "KBKDF", NULL);
 EVP_KDF_CTX *kctx = EVP_KDF_CTX_new(kdf);

The output length of an KBKDF is specified via the C<keylen>
parameter to the L<EVP_KDF_derive(3)> function.

Note that currently OpenSSL only implements Counter mode with HMAC.  Other
variants may be supported in the future.

=head1 EXAMPLES

This example derives 10 bytes using COUNTER-HMAC-SHA256, with KI "secret",
Label "label", and Context "context".

 EVP_KDF *kdf;
 EVP_KDF_CTX *kctx;
 unsigned char out[10];
 OSSL_PARAM params[6], *p = params;

 kdf = EVP_KDF_fetch(NULL, "KBKDF", NULL);
 kctx = EVP_KDF_CTX_new(kdf);
 EVP_KDF_free(kdf);

 *p++ = OSSL_PARAM_construct_utf8_string(OSSL_KDF_PARAM_DIGEST,
                                         "SHA256", 0);
 *p++ = OSSL_PARAM_construct_utf8_string(OSSL_KDF_PARAM_MAC,
                                         "HMAC", 0);
 *p++ = OSSL_PARAM_construct_octet_string(OSSL_KDF_PARAM_KEY,
                                          "secret", strlen("secret"))
 *p++ = OSSL_PARAM_construct_octet_string(OSSL_KDF_PARAM_SALT,
                                          "context", strlen("context"));
 *p++ = OSSL_PARAM_construct_octet_string(OSSL_KDF_PARAM_INFO,
                                          "label", strlen("label"));
 *p = OSSL_PARAM_construct_end();
 if (EVP_KDF_CTX_set_params(kctx, params) <= 0)
     error("EVP_KDF_CTX_set_params");
 else if (EVP_KDF_derive(kctx, out, sizeof(out)) <= 0)
     error("EVP_KDF_derive");

 EVP_KDF_CTX_free(kctx);

=head1 CONFORMING TO

NIST SP800-108, IETF RFC 8009.

=head1 SEE ALSO

L<EVP_KDF(3)>,
L<EVP_KDF_CTX_new_id(3)>,
L<EVP_KDF_CTX_free(3)>,
L<EVP_KDF_ctrl(3)>,
L<EVP_KDF_size(3)>,
L<EVP_KDF_derive(3)>,
L<EVP_KDF(3)/PARAMETERS>

=head1 HISTORY

This functionality was added to OpenSSL 3.0.

=head1 COPYRIGHT

Copyright 2019 The OpenSSL Project Authors. All Rights Reserved.
Copyright 2019 Red Hat, Inc.

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