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EVP_PKEY_CTX_set_hkdf_md(3)         OpenSSL        EVP_PKEY_CTX_set_hkdf_md(3)



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

LIBRARY
       libcrypto, -lcrypto

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);

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:

       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_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_set_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
       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 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.

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_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)

COPYRIGHT
       Copyright 2016-2018 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
       <https://www.openssl.org/source/license.html>.



1.1.1                             2018-09-17       EVP_PKEY_CTX_set_hkdf_md(3)