EVP_PKEY_CTX_SET_HKDF_MD(3SSL)

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_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 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>.