--- name: "provider-keyexch" section: "7ssl" platform: "linux" package: "openssl" version: "3.6.5-1" release: "Arch Linux 2026-11-01" platforms: ["linux"] url: "https://new.carta.tech/provider-keyexch" --- # provider-keyexch(7ssl) The keyexch library \<-\> provider functions ## NAME provider-keyexch - The keyexch library \<-\> provider functions ## SYNOPSIS ``` #include #include /* * 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**) \ Sets or gets the Key Derivation Function type to apply within the associated key exchange ctx. - "kdf-digest" (**OSSL\_EXCHANGE\_PARAM\_KDF\_DIGEST**) \ 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**) \ 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**) \ 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**) \ 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**) \ 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**) \ 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**) \ 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**) \ 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 \.