Add some documentation for the new HKDF modes

=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_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,

"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_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

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

 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.

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.

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.

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.