man > EVP_KDF-HKDF(7)

EVP_KDF-HKDF(7SSL)                             OpenSSL                            EVP_KDF-HKDF(7SSL)

NAME
       EVP_KDF-HKDF - The HKDF EVP_KDF implementation

DESCRIPTION
       Support for computing the HKDF KDF through the EVP_KDF API.

       The EVP_KDF-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).

   Identity
       "HKDF" is the name for this implementation; it can be used with the EVP_KDF_fetch() function.

   Supported parameters
       The supported parameters are:

       "properties" (OSSL_KDF_PARAM_PROPERTIES) <UTF8 string>
       "digest" (OSSL_KDF_PARAM_DIGEST) <UTF8 string>
       "key" (OSSL_KDF_PARAM_KEY) <octet string>
       "salt" (OSSL_KDF_PARAM_SALT) <octet string>
           These parameters work as described in "PARAMETERS" in EVP_KDF(3).

       "info" (OSSL_KDF_PARAM_INFO) <octet string>
           This  parameter sets the info value.  The length of the context info buffer cannot exceed
           1024 bytes; this should be more than enough for any normal use of HKDF.

       "mode" (OSSL_KDF_PARAM_MODE) <UTF8 string> or <integer>
           This parameter sets the mode for the HKDF operation.  There  are  three  modes  that  are
           currently defined:

           "EXTRACT_AND_EXPAND" or EVP_KDF_HKDF_MODE_EXTRACT_AND_EXPAND
               This  is  the  default  mode.  Calling EVP_KDF_derive(3) on an EVP_KDF_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 otherwise an error will occur.

           "EXTRACT_ONLY" or EVP_KDF_HKDF_MODE_EXTRACT_ONLY
               In  this  mode calling EVP_KDF_derive(3) will just perform the extract operation. The
               value returned will be the intermediate fixed-length pseudorandom key K.  The  keylen
               parameter   must   match   the  size  of  K,  which  can  be  looked  up  by  calling
               EVP_KDF_CTX_get_kdf_size() after setting the mode and digest.

               The digest, key and salt values must be set before a  key  is  derived  otherwise  an
               error will occur.

           "EXPAND_ONLY" or EVP_KDF_HKDF_MODE_EXPAND_ONLY
               In  this  mode  calling EVP_KDF_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 otherwise an
               error will occur.

NOTES
       A context for HKDF can be obtained by calling:

        EVP_KDF *kdf = EVP_KDF_fetch(NULL, "HKDF", NULL);
        EVP_KDF_CTX *kctx = EVP_KDF_CTX_new(kdf);

       The output length of an HKDF expand operation is specified via the keylen  parameter  to  the
       EVP_KDF_derive(3)  function.   When using EVP_KDF_HKDF_MODE_EXTRACT_ONLY the keylen parameter
       must equal the size of the intermediate fixed-length pseudorandom key otherwise an error will
       occur.   For  that  mode,  the  fixed   output   size   can   be   looked   up   by   calling
       EVP_KDF_CTX_get_kdf_size() after setting the mode and digest on the EVP_KDF_CTX.

EXAMPLES
       This  example  derives 10 bytes using SHA-256 with the secret key "secret", salt value "salt"
       and info value "label":

        EVP_KDF *kdf;
        EVP_KDF_CTX *kctx;
        unsigned char out[10];
        OSSL_PARAM params[5], *p = params;

        kdf = EVP_KDF_fetch(NULL, "HKDF", NULL);
        kctx = EVP_KDF_CTX_new(kdf);
        EVP_KDF_free(kdf);

        *p++ = OSSL_PARAM_construct_utf8_string(OSSL_KDF_PARAM_DIGEST,
                                                SN_sha256, strlen(SN_sha256));
        *p++ = OSSL_PARAM_construct_octet_string(OSSL_KDF_PARAM_KEY,
                                                 "secret", (size_t)6);
        *p++ = OSSL_PARAM_construct_octet_string(OSSL_KDF_PARAM_INFO,
                                                 "label", (size_t)5);
        *p++ = OSSL_PARAM_construct_octet_string(OSSL_KDF_PARAM_SALT,
                                                 "salt", (size_t)4);
        *p = OSSL_PARAM_construct_end();
        if (EVP_KDF_derive(kctx, out, sizeof(out), params) <= 0) {
            error("EVP_KDF_derive");
        }

        EVP_KDF_CTX_free(kctx);

CONFORMING TO
       RFC 5869

SEE ALSO
       EVP_KDF(3),     EVP_KDF_CTX_new(3),     EVP_KDF_CTX_free(3),     EVP_KDF_CTX_get_kdf_size(3),
       EVP_KDF_CTX_set_params(3),      EVP_KDF_derive(3),      "PARAMETERS"      in      EVP_KDF(3),
       EVP_KDF-TLS13_KDF(7)

HISTORY
       This functionality was added in OpenSSL 3.0.

COPYRIGHT
       Copyright 2016-2021 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>.

3.0.13                                       2026-07-29                           EVP_KDF-HKDF(7SSL)