=pod

=head1 NAME

EVP_KDF-KRB5KDF - The RFC3961 Krb5 KDF EVP_KDF implementation

=head1 DESCRIPTION

Support for computing the B<KRB5KDF> KDF through the B<EVP_KDF> API.

The EVP_KDF-KRB5KDF algorithm implements the key derivation function defined
in RFC 3961, section 5.1 and is used by Krb5 to derive session keys.
Three inputs are required to perform key derivation: a cipher, (for example
AES-128-CBC), the initial key, and a constant.

=head2 Identity

"KRB5KDF" 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 "properties" (B<OSSL_KDF_PARAM_PROPERTIES>) <UTF8 string>

=item "cipher" (B<OSSL_KDF_PARAM_CIPHER>) <UTF8 string>

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

These parameters work as described in L<EVP_KDF(3)/PARAMETERS>.

=item "constant" (B<OSSL_KDF_PARAM_CONSTANT>) <octet string>

This parameter sets the constant value for the KDF.
If a value is already set, the contents are replaced.

=back

=head1 NOTES

A context for KRB5KDF can be obtained by calling:

 EVP_KDF *kdf = EVP_KDF_fetch(NULL, "KRB5KDF", NULL);
 EVP_KDF_CTX *kctx = EVP_KDF_CTX_new(kdf);

The output length of the KRB5KDF derivation is specified via the I<keylen>
parameter to the L<EVP_KDF_derive(3)> function, and MUST match the key
length for the chosen cipher or an error is returned. Moreover, the
constant's length must not exceed the block size of the cipher.
Since the KRB5KDF output length depends on the chosen cipher, calling
L<EVP_KDF_CTX_get_kdf_size(3)> to obtain the requisite length returns the correct length
only after the cipher is set. Prior to that B<EVP_MAX_KEY_LENGTH> is returned.
The caller must allocate a buffer of the correct length for the chosen
cipher, and pass that buffer to the L<EVP_KDF_derive(3)> function along
with that length.

=head1 EXAMPLES

This example derives a key using the AES-128-CBC cipher:

 EVP_KDF *kdf;
 EVP_KDF_CTX *kctx;
 unsigned char key[16] = "01234...";
 unsigned char constant[] = "I'm a constant";
 unsigned char out[16];
 size_t outlen = sizeof(out);
 OSSL_PARAM params[4], *p = params;

 kdf = EVP_KDF_fetch(NULL, "KRB5KDF", NULL);
 kctx = EVP_KDF_CTX_new(kdf);
 EVP_KDF_free(kdf);

 *p++ = OSSL_PARAM_construct_utf8_string(OSSL_KDF_PARAM_CIPHER,
                                         SN_aes_128_cbc,
                                         strlen(SN_aes_128_cbc));
 *p++ = OSSL_PARAM_construct_octet_string(OSSL_KDF_PARAM_KEY,
                                          key, (size_t)16);
 *p++ = OSSL_PARAM_construct_octet_string(OSSL_KDF_PARAM_CONSTANT,
                                          constant, strlen(constant));
 *p = OSSL_PARAM_construct_end();
 if (EVP_KDF_derive(kctx, out, outlen, params) <= 0)
     /* Error */

 EVP_KDF_CTX_free(kctx);

=head1 CONFORMING TO

RFC 3961

=head1 SEE ALSO

L<EVP_KDF(3)>,
L<EVP_KDF_CTX_free(3)>,
L<EVP_KDF_CTX_get_kdf_size(3)>,
L<EVP_KDF_derive(3)>,
L<EVP_KDF(3)/PARAMETERS>

=head1 HISTORY

This functionality was added in OpenSSL 3.0.

=head1 COPYRIGHT

Copyright 2016-2021 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