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EVP_PKEY_CTX_set_hkdf_md(3) OpenSSL EVP_PKEY_CTX_set_hkdf_md(3)
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_set_hkdf_mode - HMAC-based Extract-and-Expand key
derivation algorithm
LIBRARY
libcrypto, -lcrypto
SYNOPSIS
#include <openssl/kdf.h>
int EVP_PKEY_CTX_set_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_set_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
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 2048 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.
EXAMPLES
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)
HISTORY
All of the functions described here were converted from macros to
functions in OpenSSL 3.0.
COPYRIGHT
Copyright 2016-2022 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_PKEY_CTX_set_hkdf_md(3)