PROVIDER-MAC(7SSL)

📦

version 4.0.3 · 13 versions

💖 PROVIDER-MAC 💖

hiii bestie, welcome to provider-mac — this is the OpenSSL manpage that explains how the mac library and a provider talk to each other. yes it's the crypto side of OpenSSL, no it's not about your MacBook, pls. it's message authentication codes. slay. 🎀

✨ NAME ✨

provider-mac — The mac library <-> provider functions

okay so that little <-> is doing a lot of work. it's giving "two-way street energy" between libcrypto and your provider. periodt. 💅

🌸 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_mac_newctx(void *provctx);
void OSSL_FUNC_mac_freectx(void *mctx);
void *OSSL_FUNC_mac_dupctx(void *src);

/* Encryption/decryption */
int OSSL_FUNC_mac_init(void *mctx, unsigned char *key, size_t keylen,
                       const OSSL_PARAM params[]);
int OSSL_FUNC_mac_init_skey(void *mctx, const void *key, const OSSL_PARAM params[]);
int OSSL_FUNC_mac_update(void *mctx, const unsigned char *in, size_t inl);
int OSSL_FUNC_mac_final(void *mctx, unsigned char *out, size_t *outl, size_t outsize);

/* MAC parameter descriptors */
const OSSL_PARAM *OSSL_FUNC_mac_gettable_params(void *provctx);
const OSSL_PARAM *OSSL_FUNC_mac_gettable_ctx_params(void *mctx, void *provctx);
const OSSL_PARAM *OSSL_FUNC_mac_settable_ctx_params(void *mctx, void *provctx);

/* MAC parameters */
int OSSL_FUNC_mac_get_params(OSSL_PARAM params[]);
int OSSL_FUNC_mac_get_ctx_params(void *mctx, OSSL_PARAM params[]);
int OSSL_FUNC_mac_set_ctx_params(void *mctx, const OSSL_PARAM params[]);

🦋 DESCRIPTION 🦋

This documentation is primarily aimed at provider authors. See provider(7) for further information.

so like… this is for the provider girlies specifically. if that's not you, go read provider(7) first, i'll wait. 🫶

The MAC operation enables providers to implement mac algorithms and make them available to applications via the API functions EVP_MAC_init(3), EVP_MAC_update(3) and EVP_MAC_final(3).

the way that your provider implements a mac algorithm and the apps just… use it?? iconic. that's the whole vibe.

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)).

💀 quoted-parently "functions" because none of these are real functions, bestie. they're function pointers riding around inside dispatch arrays. iykyk.

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_mac_newctx() has these:

typedef void *(OSSL_FUNC_mac_newctx_fn)(void *provctx);
static ossl_inline OSSL_FUNC_mac_newctx_fn
    OSSL_FUNC_mac_newctx(const OSSL_DISPATCH *opf);

see the pattern?? _fn suffix for the type, plain name for the getter. so clean. so consistent. we love a naming convention that eats. ✨

OSSL_DISPATCH(3) arrays are indexed by numbers that are provided as macros in openssl-core_dispatch.h(7), as follows:

OSSL_FUNC_mac_newctx               OSSL_FUNC_MAC_NEWCTX
OSSL_FUNC_mac_freectx              OSSL_FUNC_MAC_FREECTX
OSSL_FUNC_mac_dupctx               OSSL_FUNC_MAC_DUPCTX

OSSL_FUNC_mac_init                 OSSL_FUNC_MAC_INIT
OSSL_FUNC_mac_init_skey            OSSL_FUNC_MAC_INIT_SKEY
OSSL_FUNC_mac_update               OSSL_FUNC_MAC_UPDATE
OSSL_FUNC_mac_final                OSSL_FUNC_MAC_FINAL

OSSL_FUNC_mac_get_params           OSSL_FUNC_MAC_GET_PARAMS
OSSL_FUNC_mac_get_ctx_params       OSSL_FUNC_MAC_GET_CTX_PARAMS
OSSL_FUNC_mac_set_ctx_params       OSSL_FUNC_MAC_SET_CTX_PARAMS

OSSL_FUNC_mac_gettable_params      OSSL_FUNC_MAC_GETTABLE_PARAMS
OSSL_FUNC_mac_gettable_ctx_params  OSSL_FUNC_MAC_GETTABLE_CTX_PARAMS
OSSL_FUNC_mac_settable_ctx_params  OSSL_FUNC_MAC_SETTABLE_CTX_PARAMS

A mac algorithm implementation may not implement all of these functions. In order to be a consistent set of functions, at least the following functions must be implemented: OSSL_FUNC_mac_newctx(), OSSL_FUNC_mac_freectx(), at least one of OSSL_FUNC_mac_init() or OSSL_FUNC_mac_init_skey(), OSSL_FUNC_mac_update(), OSSL_FUNC_mac_final(). All other functions are optional.

⭐ the required core five (well, four-or-five, because init is a pick-your-fighter situation) are the non-negotiables. everything else is optional garnish. 🌈

🎀 Context Management Functions 🎀

OSSL_FUNC_mac_newctx() should create and return a pointer to a provider side structure for holding context information during a mac operation. A pointer to this context will be passed back in a number of the other mac operation function calls. The parameter provctx is the provider context generated during provider initialisation (see provider(7)).

like… birth of a context. you make the lil guy, you hand him back, and everybody else gets to hold him later. 👑

OSSL_FUNC_mac_freectx() is passed a pointer to the provider side mac context in the mctx parameter. If it receives NULL as mctx value, it should not do anything other than return. This function should free any resources associated with that context.

🚨 important: if mctx is NULL, just return. do not pass go. do not free. do not collect $200. and for anything else — free your resources, bestie, we're not leaking today.

OSSL_FUNC_mac_dupctx() should duplicate the provider side mac context in the mctx parameter and return the duplicate copy.

(okay yes the synopsis calls that parameter src, and the prose calls it mctx. both are here, both are real, moving on 💖)

🧚 Encryption/Decryption Functions 🧚

OSSL_FUNC_mac_init() initialises a mac operation given a newly created provider side mac context in the mctx parameter. The params are set before setting the MAC key of keylen bytes.

order matters: params first, then the key. no cap.

OSSL_FUNC_mac_init_skey() is similar but uses an opaque provider-specific object to initialize the MAC context.

opaque object core. provider-specific. mysterious. we love a lil mystery. 🙈

OSSL_FUNC_mac_update() is called to supply data for MAC computation of a previously initialised mac operation. The mctx parameter contains a pointer to a previously initialised provider side context. OSSL_FUNC_mac_update() may be called multiple times for a single mac operation.

multiple calls?? for ONE mac operation?? yes babe, it's a feed-it-as-you-go situation. stream the data in. 🔥

OSSL_FUNC_mac_final() completes the MAC computation started through previous OSSL_FUNC_mac_init() and OSSL_FUNC_mac_update() calls. The mctx parameter contains a pointer to the provider side context. The resulting MAC should be written to out and the amount of data written to *outl, which should not exceed outsize bytes. The same expectations apply to outsize as documented for EVP_MAC_final(3).

so: write the MAC to out, write the byte count to *outl, and do not blow past outsize. the outsize rules are the same ones as EVP_MAC_final(3) — go check those, fr fr. 🫶

💜 Mac Parameters 💜

See OSSL_PARAM(3) for further details on the parameters structure used by these functions.

OSSL_FUNC_mac_get_params() gets details of parameter values associated with the provider algorithm and stores them in params.

OSSL_FUNC_mac_set_ctx_params() sets mac parameters associated with the given provider side mac context mctx to params. Any parameter settings are additional to any that were previously set. Passing NULL for params should return true.

note the vibe: settings are additive, they stack on top of what was already set. and NULL params → return true. yes, even when it's nothing. 😌

OSSL_FUNC_mac_get_ctx_params() gets details of currently set parameter values associated with the given provider side mac context mctx and stores them in params. Passing NULL for params should return true.

OSSL_FUNC_mac_gettable_params(), OSSL_FUNC_mac_gettable_ctx_params(), and OSSL_FUNC_mac_settable_ctx_params() all return constant OSSL_PARAM(3) arrays as descriptors of the parameters that OSSL_FUNC_mac_get_params(), OSSL_FUNC_mac_get_ctx_params(), and OSSL_FUNC_mac_set_ctx_params() can handle, respectively. OSSL_FUNC_mac_gettable_ctx_params() and OSSL_FUNC_mac_settable_ctx_params() will return the parameters associated with the provider side context mctx in its current state if it is not NULL. Otherwise, they return the parameters associated with the provider side algorithm provctx.

so the gettable/settable trio are like the lil menus 📜 that say "here's what i can handle". and depending on whether mctx is NULL or not, you get either the context's current-state menu, or the algorithm-level (provctx) menu. got it? got it. 💅

All MAC implementations are expected to handle the following parameters:

with OSSL_FUNC_set_ctx_params():

"key" (OSSL_MAC_PARAM_KEY) <octet string> — Sets the key in the associated MAC ctx. This is identical to passing a key argument to the OSSL_FUNC_mac_init() function.

with OSSL_FUNC_get_params():

"size" (OSSL_MAC_PARAM_SIZE) <integer> — Can be used to get the default MAC size (which might be the only allowable MAC size for the implementation).
"size" (OSSL_MAC_PARAM_BLOCK_SIZE) <integer> — Can be used to get the MAC block size (if supported by the algorithm).

👀 yes both of those literally say "size" in the original. that's not a typo on my end bestie, that's just what it is. the macros are different, so it all works out. iykyk.

Also: Note that some implementations allow setting the size that the resulting MAC should have as well, see the documentation of the implementation.

The OpenSSL FIPS provider may support the following parameters:

"fips-indicator" (OSSL_MAC_PARAM_FIPS_APPROVED_INDICATOR) <int> — A getter that returns 1 if the operation is FIPS approved, or 0 otherwise. This may be used after calling the final function. It may return 0 if either "no-short-mac" or "key-check" are set to 0.
"no-short-mac" (OSSL_MAC_PARAM_FIPS_NO_SHORT_MAC) <integer> — If required this parameter should be set early via an init function. The default value of 1 causes an error when too short MAC output is asked for. 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.
"key-check" (OSSL_MAC_PARAM_FIPS_KEY_CHECK) <integer> — If required this parameter should be set before OSSL_FUNC_mac_init. The default value of 1 causes an error when small key sizes are asked for. 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.

🚨 dead serious time: setting "no-short-mac" or "key-check" to 0 will set the approved "fips-indicator" to 0, and that breaks FIPS compliance. it says it twice in the page and i'm saying it twice again. this is the "read the fine print" part. 💀

🌈 NOTES 🌈

The MAC life-cycle is described in life_cycle-rand(7). Providers should ensure that the various transitions listed there are supported. At some point the EVP layer will begin enforcing the listed transitions.

so yeah — support the transitions now, because eventually the EVP layer stops asking nicely and starts enforcing. ⏳ (and that's life_cycle-rand(7) exactly as it's written, bestie, i promise i'm not being weird about it.)

👑 RETURN VALUES 👑

OSSL_FUNC_mac_newctx() and OSSL_FUNC_mac_dupctx() should return the newly created provider side mac context, or NULL on failure.

OSSL_FUNC_mac_init(), OSSL_FUNC_mac_init_skey(), OSSL_FUNC_mac_update(), OSSL_FUNC_mac_final(), OSSL_FUNC_mac_get_params(), OSSL_FUNC_mac_get_ctx_params() and OSSL_FUNC_mac_set_ctx_params() should return 1 for success or 0 on error.

OSSL_FUNC_mac_gettable_params(), OSSL_FUNC_mac_gettable_ctx_params() and OSSL_FUNC_mac_settable_ctx_params() should return a constant OSSL_PARAM(3) array, or NULL if none is offered.

go read the mac algorithm girlies, bestie, they're all right there. 🫶

⭐ HISTORY ⭐

The provider MAC interface was introduced in OpenSSL 3.0. The parameters "no-short-mac" and "fips-indicator" were added in OpenSSL 3.4.

The function OSSL_FUNC_mac_init_skey() was introduced in OpenSSL 3.5.

a whole glow-up timeline. 💅

💖 COPYRIGHT 💖

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>.

okay that's the whole page, bestie!! you read it, you get it, you're a provider author now, main character energy fr fr. 🦋✨

this has been provider-mac from OpenSSL 4.0.3, dated 2026-09-29 — love u, go implement a mac, slay, periodt. 👑💖

📦 where this page comes from

ubuntu stonking: openssl 4.0.3-1ubuntu1
💖 girlified by deepseek-ai/DeepSeek-V4.1-Flash on 2026-11-03: a language model's rewrite of the original, under its terms