=pod

=head1 NAME

EVP_PKEY_CTX_set_hkdf_md, EVP_PKEY_CTX_set1_hkdf_salt,

EVP_PKEY_CTX_set1_hkdf_key, EVP_PKEY_CTX_add1_hkdf_info,

EVP_PKEY_CTX_hkdf_mode -

HMAC-based Extract-and-Expand key derivation algorithm

=head1 SYNOPSIS

 #include <openssl/kdf.h>

 int EVP_PKEY_CTX_hkdf_mode(EVP_PKEY_CTX *pctx, int mode);

 int EVP_PKEY_CTX_set_hkdf_md(EVP_PKEY_CTX *pctx, const EVP_MD *md);

 int EVP_PKEY_CTX_set1_hkdf_salt(EVP_PKEY_CTX *pctx, unsigned char *salt,

                                 int saltlen);

 int EVP_PKEY_CTX_set1_hkdf_key(EVP_PKEY_CTX *pctx, unsigned char *key,

                                int keylen);

 int EVP_PKEY_CTX_add1_hkdf_info(EVP_PKEY_CTX *pctx, unsigned char *info,

                                 int infolen);

=head1 DESCRIPTION

The EVP_PKEY_HKDF algorithm implements the HKDF key derivation function.

HKDF follows the "extract-then-expand" paradigm, where the KDF logically

consists of two modules. The first stage takes the input keying material

and "extracts" from it a fixed-length pseudorandom key K. The second stage

"expands" the key K into several additional pseudorandom keys (the output

of the KDF).

EVP_PKEY_CTX_hkdf_mode() sets the mode for the HKDF operation. There are three

modes that are currently defined:

=over 4

=item EVP_PKEY_HKDEF_MODE_EXTRACT_AND_EXPAND

This is the default mode. Calling L<EVP_PKEY_derive(3)> on an EVP_PKEY_CTX set

up for HKDF will perform an extract followed by an expand operation in one go.

The derived key returned will be the result after the expand operation. The

intermediate fixed-length pseudorandom key K is not returned.

In this mode the digest, key, salt and info values must be set before a key is

derived or an error occurs.

=item EVP_PKEY_HKDEF_MODE_EXTRACT_ONLY

In this mode calling L<EVP_PKEY_derive(3)> will just perform the extract

operation. The value returned will be the intermediate fixed-length pseudorandom

key K.

The digest, key and salt values must be set before a key is derived or an

error occurs.

=item EVP_PKEY_HKDEF_MODE_EXPAND_ONLY

In this mode calling L<EVP_PKEY_derive(3)> will just perform the expand

operation. The input key should be set to the intermediate fixed-length

pseudorandom key K returned from a previous extract operation.

The digest, key and info values must be set before a key is derived or an

error occurs.

=back

EVP_PKEY_CTX_set_hkdf_md() sets the message digest associated with the HKDF.

EVP_PKEY_CTX_set1_hkdf_salt() sets the salt to B<saltlen> bytes of the

buffer B<salt>. Any existing value is replaced.

EVP_PKEY_CTX_set1_hkdf_key() sets the key to B<keylen> bytes of the buffer

B<key>. Any existing value is replaced.

EVP_PKEY_CTX_add1_hkdf_info() sets the info value to B<infolen> bytes of the

buffer B<info>. If a value is already set, it is appended to the existing

value.

=head1 STRING CTRLS

HKDF also supports string based control operations via

L<EVP_PKEY_CTX_ctrl_str(3)>.

The B<type> parameter "md" uses the supplied B<value> as the name of the digest

algorithm to use.

The B<type> parameter "mode" uses the values "EXTRACT_AND_EXPAND",

"EXTRACT_ONLY" and "EXPAND_ONLY" to determine the mode to use.

The B<type> parameters "salt", "key" and "info" use the supplied B<value>

parameter as a B<seed>, B<key> or B<info> value.

The names "hexsalt", "hexkey" and "hexinfo" are similar except they take a hex

string which is converted to binary.

=head1 NOTES

All these functions are implemented as macros.

A context for HKDF can be obtained by calling:

 EVP_PKEY_CTX *pctx = EVP_PKEY_CTX_new_id(EVP_PKEY_HKDF, NULL);

The total length of the info buffer cannot exceed 1024 bytes in length: this

should be more than enough for any normal use of HKDF.

The output length of an HKDF expand operation is specified via the length

parameter to the L<EVP_PKEY_derive(3)> function.

Since the HKDF output length is variable, passing a B<NULL> buffer as a means

to obtain the requisite length is not meaningful with HKDF in any mode that

performs an expand operation. Instead, the caller must allocate a buffer of the

desired length, and pass that buffer to L<EVP_PKEY_derive(3)> along with (a

pointer initialized to) the desired length. Passing a B<NULL> buffer to obtain

the length is allowed when using EVP_PKEY_HKDEF_MODE_EXTRACT_ONLY.

Optimised versions of HKDF can be implemented in an ENGINE.

=head1 RETURN VALUES

All these functions return 1 for success and 0 or a negative value for failure.

In particular a return value of -2 indicates the operation is not supported by

the public key algorithm.

=head1 EXAMPLES

This example derives 10 bytes using SHA-256 with the secret key "secret",

salt value "salt" and info value "label":

 EVP_PKEY_CTX *pctx;

 unsigned char out[10];

 size_t outlen = sizeof(out);

 pctx = EVP_PKEY_CTX_new_id(EVP_PKEY_HKDF, NULL);

 if (EVP_PKEY_derive_init(pctx) <= 0)

     /* Error */

 if (EVP_PKEY_CTX_set_hkdf_md(pctx, EVP_sha256()) <= 0)

     /* Error */

 if (EVP_PKEY_CTX_set1_hkdf_salt(pctx, "salt", 4) <= 0)

     /* Error */

 if (EVP_PKEY_CTX_set1_hkdf_key(pctx, "secret", 6) <= 0)

     /* Error */

 if (EVP_PKEY_CTX_add1_hkdf_info(pctx, "label", 5) <= 0)

     /* Error */

 if (EVP_PKEY_derive(pctx, out, &outlen) <= 0)

     /* Error */

=head1 CONFORMING TO

RFC 5869

=head1 SEE ALSO

L<EVP_PKEY_CTX_new(3)>,

L<EVP_PKEY_CTX_ctrl_str(3)>,

L<EVP_PKEY_derive(3)>

=head1 COPYRIGHT

Copyright 2016-2019 The OpenSSL Project Authors. All Rights Reserved.

Licensed under the OpenSSL license (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