PROVIDER-KEM(7ossl)

📦
version 3.5.9 · 8 versions
, in alpine 3.23, alpine 3.24

💅 PROVIDER-KEM (7ossl) — the OpenSSL kem library ↔ provider functions page

okay bestie so this is the PROVIDER-KEM(7ossl) manual page from OpenSSL, and it's basically the group chat rules for how the kem library talks to provider functions. grab a strawberry drink 🍓 and let's get into it.

✨ NAME ✨

provider-kem — the kem library ↔ provider functions. that's the whole vibe. 💫

🦋 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_kem_newctx(void *provctx);
void OSSL_FUNC_kem_freectx(void *ctx);
void *OSSL_FUNC_kem_dupctx(void *ctx);

/* Encapsulation */
int OSSL_FUNC_kem_encapsulate_init(void *ctx, void *provkey,
                                   const OSSL_PARAM params[]);
int OSSL_FUNC_kem_auth_encapsulate_init(void *ctx, void *provkey,
                                        void *provauthkey,
                                        const OSSL_PARAM params[]);
int OSSL_FUNC_kem_encapsulate(void *ctx, unsigned char *out, size_t *outlen,
                              unsigned char *secret, size_t *secretlen);

/* Decapsulation */
int OSSL_FUNC_kem_decapsulate_init(void *ctx, void *provkey,
                                   const OSSL_PARAM params[]);
int OSSL_FUNC_kem_auth_decapsulate_init(void *ctx, void *provkey,
                                        void *provauthkey,
                                        const OSSL_PARAM params[]);
int OSSL_FUNC_kem_decapsulate(void *ctx, unsigned char *out, size_t *outlen,
                              const unsigned char *in, size_t inlen);

/* KEM parameters */
int OSSL_FUNC_kem_get_ctx_params(void *ctx, OSSL_PARAM params[]);
const OSSL_PARAM *OSSL_FUNC_kem_gettable_ctx_params(void *ctx, void *provctx);
int OSSL_FUNC_kem_set_ctx_params(void *ctx, const OSSL_PARAM params[]);
const OSSL_PARAM *OSSL_FUNC_kem_settable_ctx_params(void *ctx, void *provctx);

like, read the comment bestie — none of these are actual functions, they're just displayed like this so you can see the signatures of the function pointers you'll be stuffing into OSSL_DISPATCH arrays. 🎀

💖 DESCRIPTION 💖

so first thing's first: this documentation is primarily aimed at provider authors. if that's you, slay. if you want the bigger picture, go see provider(7) for further information. 🫶

The asymmetric kem (that's OSSL_OP_KEM) operation is what lets providers implement asymmetric kem algorithms and make them available to applications via the API functions EVP_PKEY_encapsulate(3), EVP_PKEY_decapsulate(3) and other related functions. iconic behavior honestly. 👑

All the "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)). it's giving relay race. 🏃‍♀️

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_kem_newctx() has these:

typedef void *(OSSL_FUNC_kem_newctx_fn)(void *provctx);
static ossl_inline OSSL_FUNC_kem_newctx_fn
    OSSL_FUNC_kem_newctx(const OSSL_DISPATCH *opf);

OSSL_DISPATCH(3) arrays are indexed by numbers that are provided as macros in openssl-core_dispatch.h(7), and here's the whole matching set, no cap:

OSSL_FUNC_kem_newctx                OSSL_FUNC_KEM_NEWCTX
OSSL_FUNC_kem_freectx               OSSL_FUNC_KEM_FREECTX
OSSL_FUNC_kem_dupctx                OSSL_FUNC_KEM_DUPCTX

OSSL_FUNC_kem_encapsulate_init      OSSL_FUNC_KEM_ENCAPSULATE_INIT
OSSL_FUNC_kem_auth_encapsulate_init OSSL_FUNC_KEM_AUTH_ENCAPSULATE_INIT
OSSL_FUNC_kem_encapsulate           OSSL_FUNC_KEM_ENCAPSULATE

OSSL_FUNC_kem_decapsulate_init      OSSL_FUNC_KEM_DECAPSULATE_INIT
OSSL_FUNC_kem_auth_decapsulate_init OSSL_FUNC_KEM_AUTH_DECAPSULATE_INIT
OSSL_FUNC_kem_decapsulate           OSSL_FUNC_KEM_DECAPSULATE

OSSL_FUNC_kem_get_ctx_params        OSSL_FUNC_KEM_GET_CTX_PARAMS
OSSL_FUNC_kem_gettable_ctx_params   OSSL_FUNC_KEM_GETTABLE_CTX_PARAMS
OSSL_FUNC_kem_set_ctx_params        OSSL_FUNC_KEM_SET_CTX_PARAMS
OSSL_FUNC_kem_settable_ctx_params   OSSL_FUNC_KEM_SETTABLE_CTX_PARAMS

okay and here's the part you actually need to memorize for the vibe check ✨ — an asymmetric kem algorithm implementation may not implement all of these functions. to be a consistent set of functions, a provider must implement OSSL_FUNC_kem_newctx and OSSL_FUNC_kem_freectx. it must also implement both of OSSL_FUNC_kem_encapsulate_init and OSSL_FUNC_kem_encapsulate, or both of OSSL_FUNC_kem_decapsulate_init and OSSL_FUNC_kem_decapsulate. 😤

  • OSSL_FUNC_kem_auth_encapsulate_init is optional, but if it's present then OSSL_FUNC_kem_auth_decapsulate_init must be too.
  • OSSL_FUNC_kem_get_ctx_params is optional, but if it's present then OSSL_FUNC_kem_gettable_ctx_params must be too.
  • similarly, OSSL_FUNC_kem_set_ctx_params is optional, but if it's present then OSSL_FUNC_kem_settable_ctx_params must also be present.

they come as a package deal bestie, it's giving matching outfits. 🎀

also! an asymmetric kem algorithm must implement some mechanism for generating, loading or importing keys via the key management (OSSL_OP_KEYMGMT) operation. see provider-keymgmt(7) for further details. no skipping that step, fr fr. 🚨

🌸 Context Management Functions 🌸

  • OSSL_FUNC_kem_newctx() — should create and return a pointer to a provider side structure for holding context information during an asymmetric kem operation. a pointer to this context will be passed back in a number of the other asymmetric kem operation function calls. the parameter provctx is the provider context generated during provider initialisation (see provider(7)). it's the birth of the context, lowkey emotional. 😭
  • OSSL_FUNC_kem_freectx() — is passed a pointer to the provider side asymmetric kem context in the ctx parameter. this function should free any resources associated with that context. clean up your room bestie 🧹✨
  • OSSL_FUNC_kem_dupctx() — should duplicate the provider side asymmetric kem context in the ctx parameter and return the duplicate copy. twin energy 👯‍♀️

💜 Asymmetric Key Encapsulation Functions 💜

OSSL_FUNC_kem_encapsulate_init() initialises a context for an asymmetric encapsulation, given a provider side asymmetric kem 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_kem_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_kem_auth_encapsulate_init() is similar to OSSL_FUNC_kem_encapsulate_init(), but it also passes an additional authentication key provauthkey — which cannot be NULL. bestie do not test it. 🙈

OSSL_FUNC_kem_encapsulate() performs the actual encapsulation itself. a previously initialised asymmetric kem context is passed in the ctx parameter. unless out is NULL, the data to be encapsulated is internally generated, and returned into the buffer pointed to by the secret parameter — and the encapsulated data should also be written to the location pointed to by the out parameter. the length of the encapsulated data should be written to *outlen and the length of the generated secret should be written to *secretlen. 🦋✨

if out is NULL then the maximum length of the encapsulated data should be written to *outlen, and the maximum length of the generated secret should be written to *secretlen. that's the "just tell me how much room i need" era. 📏

🎀 Decapsulation Functions 🎀

OSSL_FUNC_kem_decapsulate_init() initialises a context for an asymmetric decapsulation, given a provider side asymmetric kem 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_kem_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)). same drill, opposite direction 💫

OSSL_FUNC_kem_auth_decapsulate_init() is similar to OSSL_FUNC_kem_decapsulate_init(), but it also passes an additional authentication key provauthkey — again, cannot be NULL. 💀

OSSL_FUNC_kem_decapsulate() performs the actual decapsulation itself. a previously initialised asymmetric kem context is passed in the ctx parameter. the data to be decapsulated is pointed to by the in parameter, which is inlen bytes long. unless out is NULL, the decapsulated data should be written to the location pointed to by the out parameter. the length of the decapsulated data should be written to *outlen. if out is NULL then the maximum length of the decapsulated data should be written to *outlen. 🫶

🧚 Asymmetric Key Encapsulation Parameters 🧚

see OSSL_PARAM(3) for further details on the parameters structure used by the OSSL_FUNC_kem_get_ctx_params() and OSSL_FUNC_kem_set_ctx_params() functions.

the OpenSSL FIPS provider also supports the following parameters, bestie these are the serious ones 👀:

  • "fips-indicator" (OSSL_KEM_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 either OSSL_FUNC_kem_encapsulate() or OSSL_FUNC_kem_decapsulate(). it may return 0 if the "key-check" is set to 0.
  • "key-check" (OSSL_KEM_PARAM_FIPS_KEY_CHECK) <integer> — if required, this parameter should be set using OSSL_FUNC_kem_encapsulate_init() or OSSL_FUNC_kem_decapsulate_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.
🚨 okay real talk, heads up: this option breaks FIPS compliance if it causes the approved "fips-indicator" to return 0. like it literally says that in the original. so don't come crying to me, i warned you 💅

🌈 Asymmetric Key Encapsulation Parameter Functions 🌈

  • OSSL_FUNC_kem_get_ctx_params() — gets asymmetric KEM parameters associated with the given provider side asymmetric kem context ctx and stores them in params. passing NULL for params should return true. ✅
  • OSSL_FUNC_kem_set_ctx_params() — sets the asymmetric KEM parameters associated with the given provider side asymmetric kem context ctx to params. any parameter settings are additional to any that were previously set. passing NULL for params should return true. (yes, it stacks. it's giving layered necklaces ✨)
  • OSSL_FUNC_kem_gettable_ctx_params() and OSSL_FUNC_kem_settable_ctx_params() — get a constant OSSL_PARAM(3) array that describes the gettable and settable parameters, i.e. parameters that can be used with OSSL_FUNC_kem_get_ctx_params() and OSSL_FUNC_kem_set_ctx_params() respectively.

and a fun lil fact: no parameters are currently recognised by built-in asymmetric kem algorithms. so if you were looking for a cheat sheet of built-in params, there isn't one, that's just the whole list. 🫠

🏆 RETURN VALUES 🏆

  • OSSL_FUNC_kem_newctx() and OSSL_FUNC_kem_dupctx() should return the newly created provider side asymmetric kem context, or NULL on failure.
  • all other functions should return 1 for success or 0 on error.

that's it. one or zero. the simplest glow-up check there is. ⭐

🔥 SEE ALSO 🔥

provider(7)

(go read it, it's the parent lore iykyk 📚)

👑 HISTORY 👑

  • the provider KEM interface was introduced in OpenSSL 3.0.
  • OSSL_FUNC_kem_auth_encapsulate_init() and OSSL_FUNC_kem_auth_decapsulate_init() were added in OpenSSL 3.2.
  • the Asymmetric Key Encapsulation Parameters "fips-indicator" and "key-check" were added in OpenSSL 3.4.

and that's part one bestie 💖 — don't go anywhere, there's more coming 🦋

okay so legal stuff, but make it cute — here's the whole vibe:

Copyright 2020-2026 The OpenSSL Project Authors. All Rights Reserved. 🫶

This file is licensed under the Apache License 2.0 (the "License"), fr fr. You may not use this file except in compliance with the License — like, that's the whole deal, bestie, no cap.

You can grab a copy of the License in the file LICENSE in the source distribution, or at <https://www.openssl.org/source/license.html> 💅

ngl, please actually read the license before you go doing stuff, okay? it's giving "respect the rules" energy 🙈

and that's the whole page, bestie — you made it to the end, iconic behavior honestly 👑✨

this lil masterpiece is from OpenSSL 3.5.9, dated 2026-09-29, and it lives at PROVIDER-KEM(7ossl) — go off, you Unix queen 🦋💜

📦 where this page comes from