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 - HMAC-based Extract-and-Expand key derivation algorithm

SYNOPSIS
 #include <openssl/kdf.h>

 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_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_set_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 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
All these functions are implemented as macros.

A context for HKDF can be obtained by calling:

 EVP_PKEY_CTX *pctx = EVP_PKEY_CTX_new_id(EVP_PKEY_HKDF, NULL);

The digest, key, salt and info values must be set before a key is derived or an error occurs.

The total length of the info buffer cannot exceed 1024 bytes in length: this should be more than enough for any normal use of HKDF.

The output length of the KDF 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. 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.

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.

EXAMPLE
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_salt(pctx, "salt", 4) <= 0)
    /* Error */
 if (EVP_PKEY_CTX_set1_key(pctx, "secret", 6) <= 0)
    /* Error */
 if (EVP_PKEY_CTX_add1_hkdf_info(pctx, "label", 6) <= 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)

COPYRIGHT
Copyright 2016-2018 The OpenSSL Project Authors. All Rights Reserved.

Licensed under the OpenSSL license (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>.