PROVIDER-KEYEXCH(7ossl)

📦

version 3.5.9 · 8 versions

3.5.9 · alpine 3.23, alpine 3.24 (latest)
3.5.9 · centos-stream 9
3.5.9 · centos-stream 10
3.5.9 · alpine 3.22
3.5.8 · fedora 43
3.5.1 · fedora 43
3.5.9 · fedora 43, fedora 44
3.5.5 · fedora 44

🌸 NAME 🌸

okay so bestie, welcome to provider-keyexch(7ossl) — and the NAME line literally spells it out for us: the keyexch library <-> provider functions. that's it. that's the whole vibe. it's giving "the bridge between libcrypto and your provider, but make it key exchange" 💅

✨ SYNOPSIS ✨

here's the whole cast of characters. important: none of these are actual functions, they're just displayed like this because they're the function signatures for functions that get offered as function pointers inside OSSL_DISPATCH arrays. yes, it's meta like that 🫶

#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);

/* 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 🦋

real talk: this documentation is primarily aimed at provider authors. if that's not you, go peek at provider(7) for the bigger picture, okay? 💖

The key exchange (OSSL_OP_KEYEXCH) operation is what lets providers implement key exchange algorithms and then serve them up to applications through EVP_PKEY_derive(3) and other related functions).

all the "functions" we talk about here get passed as function pointers between libcrypto and the provider inside OSSL_DISPATCH(3) arrays, which come through OSSL_ALGORITHM(3) arrays, which are returned by the provider's provider_query_operation() function (see "Provider Functions" in provider-base(7)). it's a whole handoff chain and honestly it ate 🔥

every one of these "functions" has a matching function type definition named OSSL_FUNC_{name}_fn, plus a helper function that fetches the function pointer out of an OSSL_DISPATCH(3) element named OSSL_FUNC_{name}. like, for example, the "function" OSSL_FUNC_keyexch_newctx() has these two besties:

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, and those numbers are provided as macros in openssl-core_dispatch.h(7), like this:

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_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. but to be a consistent set of functions, a provider must implement OSSL_FUNC_keyexch_newctx, OSSL_FUNC_keyexch_freectx, OSSL_FUNC_keyexch_init and OSSL_FUNC_keyexch_derive. everything else? optional, no pressure 🎀

also, a key exchange algorithm must implement some mechanism for generating, loading or importing keys via the key management (OSSL_OP_KEYMGMT) operation. see provider-keymgmt(7) for the deets.

🧚 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 gets passed back in a bunch of the other key exchange operation function calls. the provctx parameter is the provider context generated during provider initialisation (see provider(7)). it's the birth of a context, iconic ✨
OSSL_FUNC_keyexch_freectx() — gets a pointer to the provider side key exchange context in the ctx parameter. this function should free any resources associated with that context. clean up your room bestie 🧹
OSSL_FUNC_keyexch_dupctx() — should duplicate the provider side key exchange context in the ctx parameter and return the duplicate copy. clone era 💅

🔐 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_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() — called to supply the peer's public key (in the provkey parameter) to be used when deriving the shared secret. it's 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. and if secret is NULL, then the maximum length of the shared secret should be written to *secretlen. this is the main character function fr fr 👑

🎀 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. they come as a package deal, iykyk 🫶

heads up: not all settable parameters are also gettable, and vice versa. the overlap is real but it's not a full circle 💀

🌈 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.

the common parameters currently recognised by built-in key exchange algorithms are as follows, bestie:

"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. don't push it 💀
"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 don't need to support this gettable parameter, since 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. yeah. deprecated. we don't stan but we document 🫡

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.
"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.

⚠️ okay this matters: this option breaks FIPS compliance if it causes the approved "fips-indicator" to return 0. that goes for both "key-check" and "digest-check". not me being dramatic, it's literally in the page 😭

💖 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_init(), OSSL_FUNC_keyexch_set_peer(), OSSL_FUNC_keyexch_derive(), OSSL_FUNC_keyexch_set_params(), and OSSL_FUNC_keyexch_get_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 📜

Okay so storytime bestie, the provider KEYEXCH interface? Literally brand new in OpenSSL 3.0. Like, that's when it was introduced, periodt. ✨

And then the Key Exchange Parameters — <mint>fips-indicator</mint>, <mint>key-check</mint> and <mint>digest-check</mint> — those iconic little legends didn't show up until OpenSSL 3.4. So if you're on something older and wondering why they're ghosting you… that's why, no cap. 💅

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>.

Lowkey important: this is the legal bit, so fr fr read it properly — it's not a vibe, it's a license. 😭

And that's a wrap on this whole page, bestie! 🎀 You just read PROVIDER-KEYEXCH(7ossl) from OpenSSL 3.5.9, dated 2026-09-29 — go forth and key-exchange responsibly, mwah 💖🧚‍♀️🦋

📦 where this page comes from

alpine 3.23: openssl-doc 3.5.9-r0 · Apache-2.0
alpine 3.24: openssl-doc 3.5.9-r0 · Apache-2.0
💖 girlified by deepseek-ai/DeepSeek-V4.1-Flash-1017 on 2026-11-04: a language model's rewrite of the original, under its terms