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Add some documentation for the new HKDF modes

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Reviewed-by: Rich Salz <rsalz@openssl.org>
This commit is contained in:
Matt Caswell 2016-11-08 10:25:21 +00:00
parent ddd2c38917
commit 327c162792
1 changed files with 48 additions and 10 deletions
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58
doc/man3/EVP_PKEY_CTX_set_hkdf_md.pod
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@ -3,13 +3,16 @@
=head1 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_set1_hkdf_key, EVP_PKEY_CTX_add1_hkdf_info,
EVP_PKEY_CTX_hkdf_mode -
HMAC-based Extract-and-Expand key derivation algorithm
=head1 SYNOPSIS
#include <openssl/kdf.h>
int EVP_PKEY_CTX_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,
@ -30,6 +33,41 @@ 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_hkdf_mode() sets the mode for the HKDF operation. There are three
modes that are currently defined:
=over 4
=item EVP_PKEY_HKDEF_MODE_EXTRACT_AND_EXPAND
This is the default mode. Calling L<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.
=item EVP_PKEY_HKDEF_MODE_EXTRACT_ONLY
In this mode calling L<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.
=item EVP_PKEY_HKDEF_MODE_EXPAND_ONLY
In this mode calling L<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.
=back
EVP_PKEY_set_hkdf_md() sets the message digest associated with the HKDF.
EVP_PKEY_CTX_set1_hkdf_salt() sets the salt to B<saltlen> bytes of the
@ -48,6 +86,8 @@ HKDF also supports string based control operations via
L<EVP_PKEY_CTX_ctrl_str(3)>.
The B<type> parameter "md" uses the supplied B<value> as the name of the digest
algorithm to use.
The B<type> parameter "mode" uses the values "EXTRACT_AND_EXPAND",
"EXTRACT_ONLY" and "EXPAND_ONLY" to determine the mode to use.
The B<type> parameters "salt", "key" and "info" use the supplied B<value>
parameter as a B<seed>, B<key> or B<info> value.
The names "hexsalt", "hexkey" and "hexinfo" are similar except they take a hex
@ -61,19 +101,17 @@ 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
L<EVP_PKEY_derive(3)> function.
The output length of an HKDF expand operation is specified via the length
parameter to the L<EVP_PKEY_derive(3)> function.
Since the HKDF output length is variable, passing a B<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 L<EVP_PKEY_derive(3)> along with (a pointer initialized to) the
desired length.
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 L<EVP_PKEY_derive(3)> along with (a
pointer initialized to) the desired length. Passing a B<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.