166 lines
5.5 KiB
Text
166 lines
5.5 KiB
Text
=pod
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=head1 NAME
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EVP_PKEY_CTX_set_hkdf_md, EVP_PKEY_CTX_set1_hkdf_salt,
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EVP_PKEY_CTX_set1_hkdf_key, EVP_PKEY_CTX_add1_hkdf_info,
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EVP_PKEY_CTX_hkdf_mode -
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HMAC-based Extract-and-Expand key derivation algorithm
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=head1 SYNOPSIS
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#include <openssl/kdf.h>
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int EVP_PKEY_CTX_hkdf_mode(EVP_PKEY_CTX *pctx, int mode);
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int EVP_PKEY_CTX_set_hkdf_md(EVP_PKEY_CTX *pctx, const EVP_MD *md);
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int EVP_PKEY_CTX_set1_hkdf_salt(EVP_PKEY_CTX *pctx, unsigned char *salt,
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int saltlen);
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int EVP_PKEY_CTX_set1_hkdf_key(EVP_PKEY_CTX *pctx, unsigned char *key,
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int keylen);
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int EVP_PKEY_CTX_add1_hkdf_info(EVP_PKEY_CTX *pctx, unsigned char *info,
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int infolen);
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=head1 DESCRIPTION
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The EVP_PKEY_HKDF algorithm implements the HKDF key derivation function.
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HKDF follows the "extract-then-expand" paradigm, where the KDF logically
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consists of two modules. The first stage takes the input keying material
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and "extracts" from it a fixed-length pseudorandom key K. The second stage
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"expands" the key K into several additional pseudorandom keys (the output
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of the KDF).
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EVP_PKEY_CTX_hkdf_mode() sets the mode for the HKDF operation. There are three
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modes that are currently defined:
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=over 4
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=item EVP_PKEY_HKDEF_MODE_EXTRACT_AND_EXPAND
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This is the default mode. Calling L<EVP_PKEY_derive(3)> on an EVP_PKEY_CTX set
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up for HKDF will perform an extract followed by an expand operation in one go.
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The derived key returned will be the result after the expand operation. The
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intermediate fixed-length pseudorandom key K is not returned.
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In this mode the digest, key, salt and info values must be set before a key is
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derived or an error occurs.
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=item EVP_PKEY_HKDEF_MODE_EXTRACT_ONLY
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In this mode calling L<EVP_PKEY_derive(3)> will just perform the extract
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operation. The value returned will be the intermediate fixed-length pseudorandom
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key K.
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The digest, key and salt values must be set before a key is derived or an
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error occurs.
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=item EVP_PKEY_HKDEF_MODE_EXPAND_ONLY
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In this mode calling L<EVP_PKEY_derive(3)> will just perform the expand
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operation. The input key should be set to the intermediate fixed-length
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pseudorandom key K returned from a previous extract operation.
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The digest, key and info values must be set before a key is derived or an
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error occurs.
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=back
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EVP_PKEY_set_hkdf_md() sets the message digest associated with the HKDF.
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EVP_PKEY_CTX_set1_hkdf_salt() sets the salt to B<saltlen> bytes of the
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buffer B<salt>. Any existing value is replaced.
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EVP_PKEY_CTX_set_hkdf_key() sets the key to B<keylen> bytes of the buffer
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B<key>. Any existing value is replaced.
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EVP_PKEY_CTX_add1_hkdf_info() sets the info value to B<infolen> bytes of the
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buffer B<info>. If a value is already set, it is appended to the existing
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value.
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=head1 STRING CTRLS
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HKDF also supports string based control operations via
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L<EVP_PKEY_CTX_ctrl_str(3)>.
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The B<type> parameter "md" uses the supplied B<value> as the name of the digest
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algorithm to use.
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The B<type> parameter "mode" uses the values "EXTRACT_AND_EXPAND",
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"EXTRACT_ONLY" and "EXPAND_ONLY" to determine the mode to use.
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The B<type> parameters "salt", "key" and "info" use the supplied B<value>
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parameter as a B<seed>, B<key> or B<info> value.
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The names "hexsalt", "hexkey" and "hexinfo" are similar except they take a hex
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string which is converted to binary.
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=head1 NOTES
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All these functions are implemented as macros.
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A context for HKDF can be obtained by calling:
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EVP_PKEY_CTX *pctx = EVP_PKEY_new_id(EVP_PKEY_HKDF, NULL);
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The total length of the info buffer cannot exceed 1024 bytes in length: this
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should be more than enough for any normal use of HKDF.
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The output length of an HKDF expand operation is specified via the length
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parameter to the L<EVP_PKEY_derive(3)> function.
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Since the HKDF output length is variable, passing a B<NULL> buffer as a means
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to obtain the requisite length is not meaningful with HKDF in any mode that
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performs an expand operation. Instead, the caller must allocate a buffer of the
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desired length, and pass that buffer to L<EVP_PKEY_derive(3)> along with (a
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pointer initialized to) the desired length. Passing a B<NULL> buffer to obtain
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the length is allowed when using EVP_PKEY_HKDEF_MODE_EXTRACT_ONLY.
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Optimised versions of HKDF can be implemented in an ENGINE.
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=head1 RETURN VALUES
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All these functions return 1 for success and 0 or a negative value for failure.
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In particular a return value of -2 indicates the operation is not supported by
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the public key algorithm.
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=head1 EXAMPLE
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This example derives 10 bytes using SHA-256 with the secret key "secret",
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salt value "salt" and info value "label":
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EVP_PKEY_CTX *pctx;
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unsigned char out[10];
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size_t outlen = sizeof(out);
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pctx = EVP_PKEY_CTX_new_id(EVP_PKEY_HKDF, NULL);
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if (EVP_PKEY_derive_init(pctx) <= 0)
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/* Error */
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if (EVP_PKEY_CTX_set_hkdf_md(pctx, EVP_sha256()) <= 0)
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/* Error */
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if (EVP_PKEY_CTX_set1_hkdf_salt(pctx, "salt", 4) <= 0)
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/* Error */
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if (EVP_PKEY_CTX_set1_hkdf_key(pctx, "secret", 6) <= 0)
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/* Error */
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if (EVP_PKEY_CTX_add1_hkdf_info(pctx, "label", 6) <= 0)
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/* Error */
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if (EVP_PKEY_derive(pctx, out, &outlen) <= 0)
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/* Error */
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=head1 CONFORMING TO
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RFC 5869
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=head1 SEE ALSO
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L<EVP_PKEY_CTX_new(3)>,
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L<EVP_PKEY_CTX_ctrl_str(3)>,
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L<EVP_PKEY_derive(3)>
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=head1 COPYRIGHT
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Copyright 2016 The OpenSSL Project Authors. All Rights Reserved.
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Licensed under the OpenSSL license (the "License"). You may not use
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this file except in compliance with the License. You can obtain a copy
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in the file LICENSE in the source distribution or at
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L<https://www.openssl.org/source/license.html>.
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=cut
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