PROVIDER-KEYEXCH(7ossl) OpenSSL PROVIDER-KEYEXCH(7ossl)
NAME
provider-keyexch - The keyexch library <-> provider functions
SYNOPSIS
#include <openssl/core_dispatch.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 *OSSL_FUNC_keyexch_newctx(void *provctx);
void OSSL_FUNC_keyexch_freectx(void *ctx);
void *OSSL_FUNC_keyexch_dupctx(void *ctx);
/* Shared secret derivation */
int OSSL_FUNC_keyexch_init(void *ctx, void *provkey,
const OSSL_PARAM params[]);
int OSSL_FUNC_keyexch_set_peer(void *ctx, void *provkey);
int OSSL_FUNC_keyexch_derive(void *ctx, unsigned char *secret, size_t *secretlen,
size_t outlen);
void *OSSL_FUNC_keyexch_derive_skey(void *ctx, const char *key_type,
void *provctx,
OSSL_FUNC_skeymgmt_import_fn *import,
size_t keylen, const OSSL_PARAM params[]);
/* Key Exchange parameters */
int OSSL_FUNC_keyexch_set_ctx_params(void *ctx, const OSSL_PARAM params[]);
const OSSL_PARAM *OSSL_FUNC_keyexch_settable_ctx_params(void *ctx,
void *provctx);
int OSSL_FUNC_keyexch_get_ctx_params(void *ctx, OSSL_PARAM params[]);
const OSSL_PARAM *OSSL_FUNC_keyexch_gettable_ctx_params(void *ctx,
void *provctx);
DESCRIPTION
This documentation is primarily aimed at provider authors. See
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 EVP_PKEY_derive(3) and other related functions).
All "functions" mentioned here are passed as function pointers between
libcrypto and the provider in OSSL_DISPATCH(3) arrays via
OSSL_ALGORITHM(3) arrays that are returned by the provider's
provider_query_operation() function (see "Provider Functions" in
provider-base(7)).
All these "functions" have a corresponding function type definition named
OSSL_FUNC_{name}_fn, and a helper function to retrieve the function
pointer from an OSSL_DISPATCH(3) element named OSSL_FUNC_{name}. For
example, the "function" OSSL_FUNC_keyexch_newctx() has these:
typedef void *(OSSL_FUNC_keyexch_newctx_fn)(void *provctx);
static ossl_inline OSSL_FUNC_keyexch_newctx_fn
OSSL_FUNC_keyexch_newctx(const OSSL_DISPATCH *opf);
OSSL_DISPATCH(3) arrays are indexed by numbers that are provided as
macros in openssl-core_dispatch.h(7), as follows:
OSSL_FUNC_keyexch_newctx OSSL_FUNC_KEYEXCH_NEWCTX
OSSL_FUNC_keyexch_freectx OSSL_FUNC_KEYEXCH_FREECTX
OSSL_FUNC_keyexch_dupctx OSSL_FUNC_KEYEXCH_DUPCTX
OSSL_FUNC_keyexch_init OSSL_FUNC_KEYEXCH_INIT
OSSL_FUNC_keyexch_set_peer OSSL_FUNC_KEYEXCH_SET_PEER
OSSL_FUNC_keyexch_derive OSSL_FUNC_KEYEXCH_DERIVE
OSSL_FUNC_keyexch_derive_skey OSSL_FUNC_KEYEXCH_DERIVE_SKEY
OSSL_FUNC_keyexch_set_ctx_params OSSL_FUNC_KEYEXCH_SET_CTX_PARAMS
OSSL_FUNC_keyexch_settable_ctx_params OSSL_FUNC_KEYEXCH_SETTABLE_CTX_PARAMS
OSSL_FUNC_keyexch_get_ctx_params OSSL_FUNC_KEYEXCH_GET_CTX_PARAMS
OSSL_FUNC_keyexch_gettable_ctx_params OSSL_FUNC_KEYEXCH_GETTABLE_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 OSSL_FUNC_keyexch_newctx, OSSL_FUNC_keyexch_freectx,
OSSL_FUNC_keyexch_init and at least one of OSSL_FUNC_keyexch_derive and
OSSL_FUNC_keyexch_derive_skey. 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 provider-keymgmt(7) for further
details.
Context Management Functions
OSSL_FUNC_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 parameter
provctx is the provider context generated during provider initialisation
(see provider(7)).
OSSL_FUNC_keyexch_freectx() is passed a pointer to the provider side key
exchange context in the ctx parameter. This function should free any
resources associated with that context.
OSSL_FUNC_keyexch_dupctx() should duplicate the provider side key
exchange context in the ctx parameter and return the duplicate copy.
Shared Secret Derivation Functions
OSSL_FUNC_keyexch_init() initialises a key exchange operation given a
provider side key exchange context in the ctx parameter, and a pointer to
a provider key object in the provkey parameter. The params, if not NULL,
should be set on the context in a manner similar to using
OSSL_FUNC_keyexch_set_ctx_params(). 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).
OSSL_FUNC_keyexch_set_peer() is called to supply the peer's public key
(in the provkey parameter) to be used when deriving the shared secret.
It is also passed a previously initialised key exchange context in the
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).
OSSL_FUNC_keyexch_derive() performs the actual key exchange itself by
deriving a shared secret. A previously initialised key exchange context
is passed in the ctx parameter. The derived secret should be written to
the location secret which should not exceed outlen bytes. The length of
the shared secret should be written to *secretlen. If secret is NULL
then the maximum length of the shared secret should be written to
*secretlen.
OSSL_FUNC_keyexch_derive_skey() is similar to OSSL_FUNC_keyexch_derive()
but uses an opaque object for storing the derived key. It accepts
key_type parameter to give a hint to the provider what type of the key
(e.g. generic or AES) would be generated and import function from the
EVP_SKEYMGMT object to be associated with the key. The EVP_SKEYMGMT
object comes from the same provider as the key exchange implementation.
Key Exchange Parameters Functions
OSSL_FUNC_keyexch_set_ctx_params() sets key exchange parameters
associated with the given provider side key exchange context ctx to
params, see "Common Key Exchange parameters". Any parameter settings are
additional to any that were previously set. Passing NULL for params
should return true.
OSSL_FUNC_keyexch_get_ctx_params() gets key exchange parameters
associated with the given provider side key exchange context ctx into
params, see "Common Key Exchange parameters". Passing NULL for params
should return true.
OSSL_FUNC_keyexch_settable_ctx_params() yields a constant OSSL_PARAM(3)
array that describes the settable parameters, i.e. parameters that can be
used with OP_signature_set_ctx_params(). If
OSSL_FUNC_keyexch_settable_ctx_params() is present,
OSSL_FUNC_keyexch_set_ctx_params() must also be present, and vice versa.
Similarly, OSSL_FUNC_keyexch_gettable_ctx_params() yields a constant
OSSL_PARAM(3) array that describes the gettable parameters, i.e.
parameters that can be handled by OP_signature_get_ctx_params(). If
OSSL_FUNC_keyexch_gettable_ctx_params() is present,
OSSL_FUNC_keyexch_get_ctx_params() must also be present, and vice versa.
Notice that not all settable parameters are also gettable, and vice
versa.
Common Key Exchange parameters
See OSSL_PARAM(3) for further details on the parameters structure used by
the OSSL_FUNC_keyexch_set_ctx_params() and
OSSL_FUNC_keyexch_get_ctx_params() functions.
Common parameters currently recognised by built-in key exchange
algorithms are as follows.
"kdf-type" (OSSL_EXCHANGE_PARAM_KDF_TYPE) <UTF8 string>
Sets or gets the Key Derivation Function type to apply within the
associated key exchange ctx.
"kdf-digest" (OSSL_EXCHANGE_PARAM_KDF_DIGEST) <UTF8 string>
Sets or gets the Digest algorithm to be used as part of the Key
Derivation Function associated with the given key exchange ctx.
"kdf-digest-props" (OSSL_EXCHANGE_PARAM_KDF_DIGEST_PROPS) <UTF8 string>
Sets properties to be used upon look up of the implementation for the
selected Digest algorithm for the Key Derivation Function associated
with the given key exchange ctx.
"kdf-outlen" (OSSL_EXCHANGE_PARAM_KDF_OUTLEN) <unsigned integer>
Sets or gets the desired size for the output of the chosen Key
Derivation Function associated with the given key exchange ctx. The
length of the "kdf-outlen" parameter should not exceed that of a
size_t.
"kdf-ukm" (OSSL_EXCHANGE_PARAM_KDF_UKM) <octet string>
Sets the User Key Material to be used as part of the selected Key
Derivation Function associated with the given key exchange ctx.
"kdf-ukm" (OSSL_EXCHANGE_PARAM_KDF_UKM) <octet string ptr>
Gets a pointer to the User Key Material to be used as part of the
selected Key Derivation Function associated with the given key
exchange ctx. Providers usually do not need to support this gettable
parameter as its sole purpose is to support functionality of the
deprecated EVP_PKEY_CTX_get0_ecdh_kdf_ukm() and
EVP_PKEY_CTX_get0_dh_kdf_ukm() functions.
The OpenSSL FIPS provider also supports the following parameters:
"fips-indicator" (OSSL_EXCHANGE_PARAM_FIPS_APPROVED_INDICATOR) <integer>
A getter that returns 1 if the operation is FIPS approved, or 0
otherwise. This may be used after calling
OSSL_FUNC_keyexch_derive(). It may return 0 if either the "digest-
check" or the "key-check" are set to 0.
"key-check" (OSSL_EXCHANGE_PARAM_FIPS_KEY_CHECK) <integer>
If required this parameter should be set using
OSSL_FUNC_keyexch_init(). The default value of 1 causes an error
during the init if the key is not FIPS approved (e.g. The key has a
security strength of less than 112 bits). Setting this to 0 will
ignore the error and set the approved "fips-indicator" to 0. This
option breaks FIPS compliance if it causes the approved "fips-
indicator" to return 0.
"digest-check" (OSSL_EXCHANGE_PARAM_FIPS_DIGEST_CHECK) <integer>
If required this parameter should be set before any optional digest
is set. The default value of 1 causes an error when the digest is
set if the digest is not FIPS approved. Setting this to 0 will ignore
the error and set the approved "fips-indicator" to 0. This option
breaks FIPS compliance if it causes the approved "fips-indicator" to
return 0.
RETURN VALUES
OSSL_FUNC_keyexch_newctx() and OSSL_FUNC_keyexch_dupctx() should return
the newly created provider side key exchange context, or NULL on failure.
OSSL_FUNC_keyexch_derive_skey() should return the newly created opaque
provider-side key object, or NULL on failure.
OSSL_FUNC_keyexch_init(), OSSL_FUNC_keyexch_set_peer(),
OSSL_FUNC_keyexch_derive(), OSSL_FUNC_keyexch_set_ctx_params(), and
OSSL_FUNC_keyexch_get_ctx_params() should return 1 for success or 0 on
error.
OSSL_FUNC_keyexch_settable_ctx_params() and
OSSL_FUNC_keyexch_gettable_ctx_params() should always return a constant
OSSL_PARAM(3) array.
SEE ALSO
provider(7)
HISTORY
The provider KEYEXCH interface was introduced in OpenSSL 3.0.
The Key Exchange Parameters "fips-indicator", "key-check" and "digest-
check" were added in OpenSSL 3.4.
The OSSL_FUNC_keyexch_derive_skey() function was added in OpenSSL 3.6.
COPYRIGHT
Copyright 2019-2026 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
<https://www.openssl.org/source/license.html>.
3.6.5 2026-09-29 PROVIDER-KEYEXCH(7ossl)
openssl 3.6.5 - Generated Sat Oct 3 13:06:21 CDT 2026