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 2026-06-02 EVP_KDF-HKDF(7SSL)
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