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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

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_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

       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>.