EVP_PKEY_CTX_set_hkdf_md (3)

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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 algorithm implements the key derivation function. follows the ``extract-then-expand'' paradigm, where the 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 ).

EVP_PKEY_set_hkdf_md() sets the message digest associated with the

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

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

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

The output length of the

is specified via the length parameter to the EVP_PKEY_derive(3) function. Since the output length is variable, passing a buffer as a means to obtain the requisite length is not meaningful with 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

can be implemented in an

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 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

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

in the source distribution or at <www.openssl.org/source/license.html>.