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



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>.



3.0.12                            2023-05-07                   EVP_KDF-HKDF(7)