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_set_hkdf_mode - HMAC-based Extract-and-Expand key derivation
       algorithm

SYNOPSIS

        #include <openssl/kdf.h>

        int EVP_PKEY_CTX_set_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);

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_set_hkdf_mode() sets the mode for the HKDF operation. There are three modes that are
       currently defined:

       EVP_PKEY_HKDEF_MODE_EXTRACT_AND_EXPAND
           This is the default mode. Calling 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.

       EVP_PKEY_HKDEF_MODE_EXTRACT_ONLY
           In this mode calling 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.

       EVP_PKEY_HKDEF_MODE_EXPAND_ONLY
           In this mode calling 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.

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

       EVP_PKEY_CTX_set1_hkdf_salt() sets the salt to saltlen bytes of the buffer salt. Any existing value is
       replaced.

       EVP_PKEY_CTX_set1_hkdf_key() sets the key to keylen bytes of the buffer key. Any existing value is
       replaced.

       EVP_PKEY_CTX_add1_hkdf_info() sets the info value to infolen bytes of the buffer info. If a value is
       already set, it is appended to the existing value.

STRING CTRLS

       HKDF also supports string based control operations via EVP_PKEY_CTX_ctrl_str(3).  The type parameter "md"
       uses the supplied value as the name of the digest algorithm to use.  The type parameter "mode" uses the
       values "EXTRACT_AND_EXPAND", "EXTRACT_ONLY" and "EXPAND_ONLY" to determine the mode to use.  The type
       parameters "salt", "key" and "info" use the supplied value parameter as a seed, key or info value.  The
       names "hexsalt", "hexkey" and "hexinfo" are similar except they take a hex string which is converted to
       binary.

NOTES

       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 2048 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
       EVP_PKEY_derive(3) function.  Since the HKDF output length is variable, passing a 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
       EVP_PKEY_derive(3) along with (a pointer initialized to) the desired length. Passing a 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.

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.

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 */

CONFORMING TO

       RFC 5869

SEE ALSO

       EVP_PKEY_CTX_new(3), EVP_PKEY_CTX_ctrl_str(3), EVP_PKEY_derive(3)

HISTORY

       All of the functions described here were converted from macros to functions in OpenSSL 3.0.

COPYRIGHT

       Copyright 2016-2022 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
       <https://www.openssl.org/source/license.html>.