=pod

=head1 NAME

EVP_PKEY_HKDF, EVP_PKEY_CTX_set_hkdf_md, EVP_PKEY_CTX_set1_hkdf_salt,
EVP_PKEY_CTX_set1_hkdf_key, EVP_PKEY_CTX_add1_hkdf_info -
HMAC-based Extract-and-Expand key derivation algorithm

=head1 SYNOPSIS

 #include <openssl/kdf.h>

 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_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_set_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> 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_new_id(EVP_PKEY_HKDF, NULL);

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

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 the KDF 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.
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.

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 EXAMPLE

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_salt(pctx, "salt", 4) <= 0)
    /* Error */
 if (EVP_PKEY_CTX_set1_key(pctx, "secret", 6) <= 0)
    /* Error */
 if (EVP_PKEY_CTX_add1_hkdf_info(pctx, "label", 6) <= 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)>

=cut