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=pod
=head1 NAME
provider-keyexch - The keyexch library E<lt>-E<gt> provider functions
=head1 SYNOPSIS
=for comment multiple includes
#include <openssl/core_numbers.h>
#include <openssl/core_names.h>
/*
* None of these are actual functions, but are displayed like this for
* the function signatures for functions that are offered as function
* pointers in OSSL_DISPATCH arrays.
*/
/* Context management */
void *OP_keyexch_newctx(void *provctx);
void OP_keyexch_freectx(void *ctx);
void *OP_keyexch_dupctx(void *ctx);
/* Shared secret derivation */
int OP_keyexch_init(void *ctx, void *provkey);
int OP_keyexch_set_peer(void *ctx, void *provkey);
int OP_keyexch_derive(void *ctx, unsigned char *secret, size_t *secretlen,
size_t outlen);
/* Key Exchange parameters */
int OP_keyexch_set_ctx_params(void *ctx, const OSSL_PARAM params[]);
const OSSL_PARAM *OP_keyexch_settable_ctx_params(void);
=head1 DESCRIPTION
This documentation is primarily aimed at provider authors. See L<provider(7)>
for further information.
The key exchange (OSSL_OP_KEYEXCH) operation enables providers to implement key
exchange algorithms and make them available to applications via the API
functions L<EVP_PKEY_derive_init_ex(3)>, and L<EVP_PKEY_derive(3)> (as well as
other related functions).
All "functions" mentioned here are passed as function pointers between
F<libcrypto> and the provider in B<OSSL_DISPATCH> arrays via
B<OSSL_ALGORITHM> arrays that are returned by the provider's
provider_query_operation() function
(see L<provider-base(7)/Provider Functions>).
All these "functions" have a corresponding function type definition
named B<OSSL_{name}_fn>, and a helper function to retrieve the
function pointer from an B<OSSL_DISPATCH> element named
B<OSSL_get_{name}>.
For example, the "function" OP_keyexch_newctx() has these:
typedef void *(OSSL_OP_keyexch_newctx_fn)(void *provctx);
static ossl_inline OSSL_OP_keyexch_newctx_fn
OSSL_get_OP_keyexch_newctx(const OSSL_DISPATCH *opf);
B<OSSL_DISPATCH> arrays are indexed by numbers that are provided as
macros in L<openssl-core_numbers.h(7)>, as follows:
OP_keyexch_newctx OSSL_FUNC_KEYEXCH_NEWCTX
OP_keyexch_freectx OSSL_FUNC_KEYEXCH_FREECTX
OP_keyexch_dupctx OSSL_FUNC_KEYEXCH_DUPCTX
OP_keyexch_init OSSL_FUNC_KEYEXCH_INIT
OP_keyexch_set_peer OSSL_FUNC_KEYEXCH_SET_PEER
OP_keyexch_derive OSSL_FUNC_KEYEXCH_DERIVE
OP_keyexch_set_ctx_params OSSL_FUNC_KEYEXCH_SET_CTX_PARAMS
OP_keyexch_settable_ctx_params OSSL_FUNC_KEYEXCH_SETTABLE_CTX_PARAMS
A key exchange algorithm implementation may not implement all of these functions.
In order to be a consistent set of functions a provider must implement
OP_keyexch_newctx, OP_keyexch_freectx, OP_keyexch_init and OP_keyexch_derive.
All other functions are optional.
A key exchange algorithm must also implement some mechanism for generating,
loading or importing keys via the key management (OSSL_OP_KEYMGMT) operation.
See L<provider-keymgmt(7)> for further details.
=head2 Context Management Functions
OP_keyexch_newctx() should create and return a pointer to a provider side
structure for holding context information during a key exchange operation.
A pointer to this context will be passed back in a number of the other key
exchange operation function calls.
The paramater B<provctx> is the provider context generated during provider
initialisation (see L<provider(3)>).
OP_keyexch_freectx() is passed a pointer to the provider side key exchange
context in the B<ctx> parameter.
This function should free any resources associated with that context.
OP_keyexch_dupctx() should duplicate the provider side key exchange context in
the B<ctx> parameter and return the duplicate copy.
=head2 Shared Secret Derivation Functions
OP_keyexch_init() initialises a key exchange operation given a provider side key
exchange context in the B<ctx> paramter, and a pointer to a provider key object
in the B<provkey> parameter. The key object should have been previously
generated, loaded or imported into the provider using the key management
(OSSL_OP_KEYMGMT) operation (see provider-keymgmt(7)>.
OP_keyexch_set_peer() is called to supply the peer's public key (in the
B<provkey> parameter) to be used when deriving the shared secret.
It is also passed a previously initialised key exchange context in the B<ctx>
parameter.
The key object should have been previously generated, loaded or imported into
the provider using the key management (OSSL_OP_KEYMGMT) operation (see
provider-keymgmt(7)>.
OP_keyexch_derive() performs the actual key exchange itself by deriving a shared
secret.
A previously initialised key exchange context is passed in the B<ctx>
parameter.
The derived secret should be written to the location B<secret> which should not
exceed B<outlen> bytes.
The length of the shared secret should be written to B<*secretlen>.
If B<secret> is NULL then the maximum length of the shared secret should be
written to B<*secretlen>.
=head2 Key Exchange Parameters
See L<OSSL_PARAM(3)> for further details on the parameters structure used by
the OP_keyexch_set_params() function.
OP_keyexch_set_ctx_params() sets key exchange parameters associated with the
given provider side key exchange context B<ctx> to B<params>.
Any parameter settings are additional to any that were previously set.
Parameters currently recognised by built-in key exchange algorithms are as
follows.
Not all parameters are relevant to, or are understood by all key exchange
algorithms:
=over 4
=item "pad" (B<OSSL_EXCHANGE_PARAM_PAD>) <unsigned integer>
Sets the padding mode for the associated key exchange ctx.
Setting a value of 1 will turn padding on.
Setting a vlue of 0 will turn padding off.
If padding is off then the derived shared secret may be smaller than the largest
possible secret size.
If padding is on then the derived shared secret will have its first bytes filled
with 0s where necessary to make the shared secret the same size as the largest
possible secret size.
=back
OP_keyexch_settable_ctx_params() gets a constant B<OSSL_PARAM> array that
decribes the settable parameters, i.e. parameters that can be used with
OP_signature_set_ctx_params().
See L<OSSL_PARAM(3)> for the use of B<OSSL_PARAM> as parameter descriptor.
=head1 RETURN VALUES
OP_keyexch_newctx() and OP_keyexch_dupctx() should return the newly created
provider side key exchange context, or NULL on failure.
OP_keyexch_init(), OP_keyexch_set_peer(), OP_keyexch_derive() and
OP_keyexch_set_params() should return 1 for success or 0 on error.
=head1 SEE ALSO
L<provider(7)>
=head1 HISTORY
The provider KEYEXCH interface was introduced in OpenSSL 3.0.
=head1 COPYRIGHT
Copyright 2019 The OpenSSL Project Authors. All Rights Reserved.
Licensed under the Apache License 2.0 (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
L<https://www.openssl.org/source/license.html>.
=cut
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