Ubuntu Manpage: EVP_KDF-HKDF - The HKDF EVP

Provided by: openssl_3.5.0-2ubuntu1_amd64

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).

The output is considered to be keying material.

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.

The OpenSSL FIPS provider also supports the following parameters:

"fips-indicator" (OSSL_KDF_PARAM_FIPS_APPROVED_INDICATOR) <integer>
A getter that returns 1 if the operation is FIPS approved, or 0 otherwise. This may be used after
calling EVP_KDF_derive. It returns 0 if "key-check" is set to 0 and the check fails.

"key-check" (OSSL_KDF_PARAM_FIPS_KEY_CHECK) <integer>
The default value of 1 causes an error during EVP_KDF_CTX_set_params() if the length of used key-
derivation key (OSSL_KDF_PARAM_KEY) is shorter than 112 bits. Setting this to zero will ignore the
error and set the approved "fips-indicator" to 0. This option breaks FIPS compliance if it causes
the approved "fips-indicator" to return 0.

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

HISTORY

       This functionality was added in OpenSSL 3.0.

COPYRIGHT

       Copyright 2016-2024 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>.

3.5.0                                              2025-06-04                                 EVP_KDF-HKDF(7SSL)