PROVIDER-BASE(7ossl)

💅 provider-base — the `7ossl` girlies' guide to the basic OpenSSL library ↔ provider functions 💅

okay so this is PROVIDER-BASE(7ossl) and it's literally the group chat between libcrypto and your provider. grab a strawberry drink 🍓 and let's get into it.

✨ NAME ✨

provider-base — the basic OpenSSL library ↔ provider functions. that's it, that's the vibe. 🫶

🌈 SYNOPSIS 🌈

bestie. there is So Much here. it all lives in one header, so:

#include <openssl/core_dispatch.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.
 */

/* Functions offered by libcrypto to the providers */
const OSSL_ITEM *core_gettable_params(const OSSL_CORE_HANDLE *handle);
int core_get_params(const OSSL_CORE_HANDLE *handle, OSSL_PARAM params[]);

typedef void (*OSSL_thread_stop_handler_fn)(void *arg);
int core_thread_start(const OSSL_CORE_HANDLE *handle,
                      OSSL_thread_stop_handler_fn handfn,
                      void *arg);

OPENSSL_CORE_CTX *core_get_libctx(const OSSL_CORE_HANDLE *handle);
void core_new_error(const OSSL_CORE_HANDLE *handle);
void core_set_error_debug(const OSSL_CORE_HANDLE *handle,
                          const char *file, int line, const char *func);
void core_vset_error(const OSSL_CORE_HANDLE *handle,
                     uint32_t reason, const char *fmt, va_list args);

int core_obj_add_sigid(const OSSL_CORE_HANDLE *prov, const char  *sign_name,
                       const char *digest_name, const char *pkey_name);
int core_obj_create(const OSSL_CORE_HANDLE *handle, const char *oid,
                    const char *sn, const char *ln);

/*
 * Some OpenSSL functionality is directly offered to providers via
 * dispatch
 */
void *CRYPTO_malloc(size_t num, const char *file, int line);
void *CRYPTO_zalloc(size_t num, const char *file, int line);
void CRYPTO_free(void *ptr, const char *file, int line);
void CRYPTO_clear_free(void *ptr, size_t num,
                       const char *file, int line);
void *CRYPTO_realloc(void *addr, size_t num,
                     const char *file, int line);
void *CRYPTO_clear_realloc(void *addr, size_t old_num, size_t num,
                           const char *file, int line);
void *CRYPTO_secure_malloc(size_t num, const char *file, int line);
void *CRYPTO_secure_zalloc(size_t num, const char *file, int line);
void CRYPTO_secure_free(void *ptr, const char *file, int line);
void CRYPTO_secure_clear_free(void *ptr, size_t num,
                              const char *file, int line);
int CRYPTO_secure_allocated(const void *ptr);
void OPENSSL_cleanse(void *ptr, size_t len);

unsigned char *OPENSSL_hexstr2buf(const char *str, long *buflen);

OSSL_CORE_BIO *BIO_new_file(const char *filename, const char *mode);
OSSL_CORE_BIO *BIO_new_membuf(const void *buf, int len);
int BIO_read_ex(OSSL_CORE_BIO *bio, void *data, size_t data_len,
                size_t *bytes_read);
int BIO_write_ex(OSSL_CORE_BIO *bio, const void *data, size_t data_len,
                 size_t *written);
int BIO_up_ref(OSSL_CORE_BIO *bio);
int BIO_free(OSSL_CORE_BIO *bio);
int BIO_vprintf(OSSL_CORE_BIO *bio, const char *format, va_list args);
int BIO_vsnprintf(char *buf, size_t n, const char *fmt, va_list args);

void OSSL_SELF_TEST_set_callback(OSSL_LIB_CTX *libctx, OSSL_CALLBACK *cb,
                                 void *cbarg);

size_t get_entropy(const OSSL_CORE_HANDLE *handle,
                   unsigned char **pout, int entropy,
                   size_t min_len, size_t max_len);
size_t get_user_entropy(const OSSL_CORE_HANDLE *handle,
                        unsigned char **pout, int entropy,
                        size_t min_len, size_t max_len);
void cleanup_entropy(const OSSL_CORE_HANDLE *handle,
                     unsigned char *buf, size_t len);
void cleanup_user_entropy(const OSSL_CORE_HANDLE *handle,
                          unsigned char *buf, size_t len);
size_t get_nonce(const OSSL_CORE_HANDLE *handle,
                 unsigned char **pout, size_t min_len, size_t max_len,
                 const void *salt, size_t salt_len);
size_t get_user_nonce(const OSSL_CORE_HANDLE *handle,
                      unsigned char **pout, size_t min_len, size_t max_len,
                      const void *salt, size_t salt_len);
void cleanup_nonce(const OSSL_CORE_HANDLE *handle,
                   unsigned char *buf, size_t len);
void cleanup_user_nonce(const OSSL_CORE_HANDLE *handle,
                        unsigned char *buf, size_t len);

/* Functions for querying the providers in the application library context */
int provider_register_child_cb(const OSSL_CORE_HANDLE *handle,
                    int (*create_cb)(const OSSL_CORE_HANDLE *provider,
                                     void *cbdata),
                    int (*remove_cb)(const OSSL_CORE_HANDLE *provider,
                                     void *cbdata),
                    int (*global_props_cb)(const char *props, void *cbdata),
                    void *cbdata);
void provider_deregister_child_cb(const OSSL_CORE_HANDLE *handle);
const char *provider_name(const OSSL_CORE_HANDLE *prov);
void *provider_get0_provider_ctx(const OSSL_CORE_HANDLE *prov);
const OSSL_DISPATCH *provider_get0_dispatch(const OSSL_CORE_HANDLE *prov);
int provider_up_ref(const OSSL_CORE_HANDLE *prov, int activate);
int provider_free(const OSSL_CORE_HANDLE *prov, int deactivate);

/* Functions offered by the provider to libcrypto */
void provider_teardown(void *provctx);
const OSSL_ITEM *provider_gettable_params(void *provctx);
int provider_get_params(void *provctx, OSSL_PARAM params[]);
const OSSL_ALGORITHM *provider_query_operation(void *provctx,
                                               int operation_id,
                                               const int *no_store);
void provider_unquery_operation(void *provctx, int operation_id,
                                const OSSL_ALGORITHM *algs);
const OSSL_ITEM *provider_get_reason_strings(void *provctx);
int provider_get_capabilities(void *provctx, const char *capability,
                              OSSL_CALLBACK *cb, void *arg);
int provider_self_test(void *provctx);

like, yes, it's a lot. breathe. we'll go through it. 💖

🌸 DESCRIPTION 🌸

so, all the "functions" mentioned here — and yes the scare quotes are doing heavy lifting — are passed as function pointers between libcrypto and the provider inside OSSL_DISPATCH(3) arrays, in the call of the provider initialization function. if you want the whole story on that init function, see "Provider" in provider(7). these little function pointers are known as "upcalls" (iconic name, no notes). 👑

every single one of these "functions" has a matching function-type definition named OSSL_FUNC_{name}_fn, plus a little helper to fetch the pointer out of an OSSL_DISPATCH(3) element, named OSSL_FUNC_{name}. for example, the "function" core_gettable_params() has these besties:

typedef OSSL_PARAM *
    (OSSL_FUNC_core_gettable_params_fn)(const OSSL_CORE_HANDLE *handle);
static ossl_inline OSSL_NAME_core_gettable_params_fn
    OSSL_FUNC_core_gettable_params(const OSSL_DISPATCH *opf);

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

⭐ for `in` (the `OSSL_DISPATCH(3)` array passed from libcrypto to the provider)

core_gettable_params → OSSL_FUNC_CORE_GETTABLE_PARAMS
core_get_params → OSSL_FUNC_CORE_GET_PARAMS
core_thread_start → OSSL_FUNC_CORE_THREAD_START
core_get_libctx → OSSL_FUNC_CORE_GET_LIBCTX
core_new_error → OSSL_FUNC_CORE_NEW_ERROR
core_set_error_debug → OSSL_FUNC_CORE_SET_ERROR_DEBUG
core_vset_error → OSSL_FUNC_CORE_VSET_ERROR
core_obj_add_sigid → OSSL_FUNC_CORE_OBJ_ADD_SIGID
core_obj_create → OSSL_FUNC_CORE_OBJ_CREATE
CRYPTO_malloc → OSSL_FUNC_CRYPTO_MALLOC
CRYPTO_zalloc → OSSL_FUNC_CRYPTO_ZALLOC
CRYPTO_free → OSSL_FUNC_CRYPTO_FREE
CRYPTO_clear_free → OSSL_FUNC_CRYPTO_CLEAR_FREE
CRYPTO_realloc → OSSL_FUNC_CRYPTO_REALLOC
CRYPTO_clear_realloc → OSSL_FUNC_CRYPTO_CLEAR_REALLOC
CRYPTO_secure_malloc → OSSL_FUNC_CRYPTO_SECURE_MALLOC
CRYPTO_secure_zalloc → OSSL_FUNC_CRYPTO_SECURE_ZALLOC
CRYPTO_secure_free → OSSL_FUNC_CRYPTO_SECURE_FREE
CRYPTO_secure_clear_free → OSSL_FUNC_CRYPTO_SECURE_CLEAR_FREE
CRYPTO_secure_allocated → OSSL_FUNC_CRYPTO_SECURE_ALLOCATED
BIO_new_file → OSSL_FUNC_BIO_NEW_FILE
BIO_new_mem_buf → OSSL_FUNC_BIO_NEW_MEMBUF
BIO_read_ex → OSSL_FUNC_BIO_READ_EX
BIO_write_ex → OSSL_FUNC_BIO_WRITE_EX
BIO_up_ref → OSSL_FUNC_BIO_UP_REF
BIO_free → OSSL_FUNC_BIO_FREE
BIO_vprintf → OSSL_FUNC_BIO_VPRINTF
BIO_vsnprintf → OSSL_FUNC_BIO_VSNPRINTF
BIO_puts → OSSL_FUNC_BIO_PUTS
BIO_gets → OSSL_FUNC_BIO_GETS
BIO_ctrl → OSSL_FUNC_BIO_CTRL
OPENSSL_cleanse → OSSL_FUNC_OPENSSL_CLEANSE
OSSL_SELF_TEST_set_callback → OSSL_FUNC_SELF_TEST_CB
ossl_rand_get_entropy → OSSL_FUNC_GET_ENTROPY
ossl_rand_get_user_entropy → OSSL_FUNC_GET_USER_ENTROPY
ossl_rand_cleanup_entropy → OSSL_FUNC_CLEANUP_ENTROPY
ossl_rand_cleanup_user_entropy → OSSL_FUNC_CLEANUP_USER_ENTROPY
ossl_rand_get_nonce → OSSL_FUNC_GET_NONCE
ossl_rand_get_user_nonce → OSSL_FUNC_GET_USER_NONCE
ossl_rand_cleanup_nonce → OSSL_FUNC_CLEANUP_NONCE
ossl_rand_cleanup_user_nonce → OSSL_FUNC_CLEANUP_USER_NONCE
provider_register_child_cb → OSSL_FUNC_PROVIDER_REGISTER_CHILD_CB
provider_deregister_child_cb → OSSL_FUNC_PROVIDER_DEREGISTER_CHILD_CB
provider_name → OSSL_FUNC_PROVIDER_NAME
provider_get0_provider_ctx → OSSL_FUNC_PROVIDER_GET0_PROVIDER_CTX
provider_get0_dispatch → OSSL_FUNC_PROVIDER_GET0_DISPATCH
provider_up_ref → OSSL_FUNC_PROVIDER_UP_REF
provider_free → OSSL_FUNC_PROVIDER_FREE

🦋 for `*out` (the `OSSL_DISPATCH(3)` array passed from the provider to libcrypto)

provider_teardown → OSSL_FUNC_PROVIDER_TEARDOWN
provider_gettable_params → OSSL_FUNC_PROVIDER_GETTABLE_PARAMS
provider_get_params → OSSL_FUNC_PROVIDER_GET_PARAMS
provider_query_operation → OSSL_FUNC_PROVIDER_QUERY_OPERATION
provider_unquery_operation → OSSL_FUNC_PROVIDER_UNQUERY_OPERATION
provider_get_reason_strings → OSSL_FUNC_PROVIDER_GET_REASON_STRINGS
provider_get_capabilities → OSSL_FUNC_PROVIDER_GET_CAPABILITIES
provider_self_test → OSSL_FUNC_PROVIDER_SELF_TEST

💜 Core functions 💜

core_gettable_params() returns a constant array of descriptor OSSL_PARAM(3), for parameters that core_get_params() can handle. it's giving menu-of-what's-available energy, and honestly? we stan. ✨

So core_get_params() retrieves parameters from the core for the given handle. Peep “Core parameters” below for a description of currently known parameters. 💖

core_thread_start() informs the core that the provider has stated an interest in the current thread. The core will inform the provider when the thread eventually stops. It must be passed the handle for this provider, plus a callback handfn which will be called when the thread stops. The callback will subsequently be called, with the supplied argument arg, from the thread that is stopping and gets passed the provider context as an argument. This may be useful to perform thread specific clean up such as freeing thread local variables. ✨

core_get_libctx() retrieves the core context in which the library object for the current provider is stored, accessible through the handle. This function is useful only for built-in providers such as the default provider. Never cast this to OSSL_LIB_CTX in a provider that is not built-in, because the OSSL_LIB_CTX of the library loading the provider might be a completely different structure than the OSSL_LIB_CTX of the library the provider is linked to. Use OSSL_LIB_CTX_new_child(3) instead to obtain a proper library context that is linked to the application library context. 🫶

core_new_error(), core_set_error_debug() and core_vset_error() are building blocks for reporting an error back to the core, with reference to the handle.

core_new_error() — allocates a new thread specific error record. This corresponds to the OpenSSL function ERR_new(3).
core_set_error_debug() — sets debugging information in the current thread specific error record. The debugging information includes the name of the file file, the line line and the function name func where the error occurred. This corresponds to the OpenSSL function ERR_set_debug(3).
core_vset_error() — sets the reason for the error, along with any additional data. The reason is a number defined by the provider and used to index the reason strings table that’s returned by provider_get_reason_strings(). The additional data is given as a format string fmt and a set of arguments args, which are treated in the same manner as with BIO_vsnprintf(). file and line may also be passed to indicate exactly where the error occurred or was reported. This corresponds to the OpenSSL function ERR_vset_error(3).

The core_obj_create() function registers a new OID and associated short name sn and long name ln for the given handle. It is similar to the OpenSSL function OBJ_create(3) except that it returns 1 on success or 0 on failure. It will treat as success the case where the OID already exists (even if the short name sn or long name ln provided as arguments differ from those associated with the existing OID, in which case the new names are not associated). 🌸

The core_obj_add_sigid() function registers a new composite signature algorithm (sign_name) consisting of an underlying signature algorithm (pkey_name) and digest algorithm (digest_name) for the given handle. It assumes that the OIDs for the composite signature algorithm as well as for the underlying signature and digest algorithms are either already known to OpenSSL or have been registered via a call to core_obj_create(). It corresponds to the OpenSSL function OBJ_add_sigid(3), except that the objects are identified by name rather than a numeric NID. Any name (OID, short name or long name) can be used to identify the object. It will treat as success the case where the composite signature algorithm already exists (even if registered against a different underlying signature or digest algorithm). For digest_name, NULL or an empty string is permissible for signature algorithms that do not need a digest to operate correctly. The function returns 1 on success or 0 on failure. 💅

CRYPTO_malloc(), CRYPTO_zalloc(), CRYPTO_free(), CRYPTO_clear_free(), CRYPTO_realloc(), CRYPTO_clear_realloc(), CRYPTO_secure_malloc(), CRYPTO_secure_zalloc(), CRYPTO_secure_free(), CRYPTO_secure_clear_free(), CRYPTO_secure_allocated(), BIO_new_file(), BIO_new_mem_buf(), BIO_read_ex(), BIO_write_ex(), BIO_up_ref(), BIO_free(), BIO_vprintf(), BIO_vsnprintf(), BIO_gets(), BIO_puts(), BIO_ctrl(), OPENSSL_cleanse() and OPENSSL_hexstr2buf() correspond exactly to the public functions with the same name. As a matter of fact, the pointers in the OSSL_DISPATCH(3) array are typically direct pointers to those public functions. Note that the BIO functions take an OSSL_CORE_BIO type rather than the standard BIO type. This is to ensure that a provider does not mix BIOs from the core with BIOs used on the provider side (the two are not compatible). OSSL_SELF_TEST_set_callback() is used to set an optional callback that can be passed into a provider. This may be ignored by a provider. 🦋

get_entropy() — retrieves seeding material from the operating system. The seeding material will have at least entropy bytes of randomness and the output will have at least min_len and at most max_len bytes. The buffer address is stored in *pout and the buffer length is returned to the caller. On error, zero is returned.
get_user_entropy() — the same as get_entropy() except that it will attempt to gather seed material via the seed source specified by a call to RAND_set_seed_source_type(3) or via “Random Configuration” in config(5).
cleanup_entropy() — used to clean up and free the buffer returned by get_entropy(). The entropy pointer returned by get_entropy() is passed in buf and its length in len.
cleanup_user_entropy() — used to clean up and free the buffer returned by get_user_entropy(). The entropy pointer returned by get_user_entropy() is passed in buf and its length in len.
get_nonce() — retrieves a nonce using the passed salt parameter of length salt_len and operating system specific information. The salt should contain uniquely identifying information and this is included, in an unspecified manner, as part of the output. The output is stored in a buffer which contains at least min_len and at most max_len bytes. The buffer address is stored in *pout and the buffer length returned to the caller. On error, zero is returned.
get_user_nonce() — the same as get_nonce() except that it will attempt to gather seed material via the seed source specified by a call to RAND_set_seed_source_type(3) or via “Random Configuration” in config(5).
cleanup_nonce() — used to clean up and free the buffer returned by get_nonce(). The nonce pointer returned by get_nonce() is passed in buf and its length in len.
cleanup_user_nonce() — used to clean up and free the buffer returned by get_user_nonce(). The nonce pointer returned by get_user_nonce() is passed in buf and its length in len.

provider_register_child_cb() registers callbacks for being informed about the loading and unloading of providers in the application’s library context. handle is this provider’s handle and cbdata is this provider’s data that will be passed back to the callbacks. It returns 1 on success or 0 otherwise.

These callbacks may be called while holding locks in libcrypto. In order to avoid deadlocks the callback implementation must not be long running and must not call other OpenSSL API functions or upcalls. 💀

create_cb is a callback that will be called when a new provider is loaded into the application’s library context. It is also called for any providers that are already loaded at the point that this callback is registered. The callback is passed the handle being used for the new provider being loaded and this provider’s data in cbdata. It should return 1 on success or 0 on failure.

remove_cb is a callback that will be called when a new provider is unloaded from the application’s library context. It is passed the handle being used for the provider being unloaded and this provider’s data in cbdata. It should return 1 on success or 0 on failure.

global_props_cb is a callback that will be called when the global properties from the parent library context are changed. It should return 1 on success or 0 on failure.

provider_deregister_child_cb() unregisters callbacks previously registered via provider_register_child_cb(). If provider_register_child_cb() has been called then provider_deregister_child_cb() should be called at or before the point that this provider’s teardown function is called.

provider_name() returns a string giving the name of the provider identified by handle.

provider_get0_provider_ctx() returns the provider context that is associated with the provider identified by prov.

provider_get0_dispatch() gets the dispatch table registered by the provider identified by prov when it initialised.

provider_up_ref() increments the reference count on the provider prov. If activate is nonzero then the provider is also loaded if it is not already loaded. It returns 1 on success or 0 on failure.

provider_free() decrements the reference count on the provider prov. If deactivate is nonzero then the provider is also unloaded if it is not already loaded. It returns 1 on success or 0 on failure.

🧚 Provider functions 🧚

provider_teardown() is called when a provider is shut down and removed from the core’s provider store. It must free the passed provctx.

provider_gettable_params() should return a constant array of descriptor OSSL_PARAM(3), for parameters that provider_get_params() can handle.

provider_get_params() should process the OSSL_PARAM(3) array params, setting the values of the parameters it understands.

provider_query_operation() should return a constant OSSL_ALGORITHM(3) that corresponds to the given operation_id. It should indicate if the core may store a reference to this array by setting *no_store to 0 (core may store a reference) or 1 (core may not store a reference).

provider_unquery_operation() informs the provider that the result of a provider_query_operation() is no longer directly required and that the function pointers have been copied. The operation_id should match that passed to provider_query_operation() and algs should be its return value.

provider_get_reason_strings() should return a constant OSSL_ITEM(3) array that provides reason strings for reason codes the provider may use when reporting errors using core_put_error(). 🔥

Like, provider_get_capabilities() is the function that calls the callback cb and hands it a bundle of OSSL_PARAM(3)s plus the caller-supplied argument arg 🫶. Those OSSL_PARAM(3)s should spill all the deets about the capability whose name is passed in capability, and all of that is relevant to the provider context provctx. And bestie, if a provider serves up multiple capabilities under the same name? It can just call the callback multiple times, one per capability — no cap 💅. Capabilities are literally how a provider describes the services it can offer. (Full lore lives in CAPABILITIES below, iykyk.) This one returns 1 on success or 0 on error, periodt.

Then there's provider_self_test(), which should run known answer tests on a subset of the algorithms it uses, and it may also verify the integrity of the provider module ✨. Same deal: 1 on success, 0 on error. And if this function isn't used at all, it just returns 1 — cool, cool, no stress.

Ngl none of these functions are mandatory, but a provider is kind of useless without at least provider_query_operation(), and provider_gettable_params() is also fairly useless if it's not accompanied by provider_get_params(). Like, they're a package deal 💀.

🌸 Provider parameters 🌸

provider_get_params() can hand the following provider parameters back to the core:

"name" (OSSL_PROV_PARAM_NAME) <UTF8 ptr> — points to a string that should give a unique name for the provider 👑.
"version" (OSSL_PROV_PARAM_VERSION) <UTF8 ptr> — points to a string that's a version number associated with this provider. OpenSSL in-built providers use OPENSSL_VERSION_STR, but a third party provider can totally do its own thing. Heads up: this string is for informational purposes only 💅.
"buildinfo" (OSSL_PROV_PARAM_BUILDINFO) <UTF8 ptr> — points to a string that's build information associated with this provider. OpenSSL in-built providers use OPENSSL_FULL_VERSION_STR, but again, third party providers may differ.
"status" (OSSL_PROV_PARAM_STATUS) <unsigned integer> — returns 0 if the provider has entered an error state, otherwise it returns 1 🩺.

provider_gettable_params() should return the parameters listed above, fr fr.

🎀 Core parameters 🎀

core_get_params() can retrieve the following core parameters for each provider:

"openssl-version" (OSSL_PROV_PARAM_CORE_VERSION) <UTF8 string ptr> — points to the OpenSSL libraries' full version string, i.e. the string expanded from the macro OPENSSL_VERSION_STR.
"provider-name" (OSSL_PROV_PARAM_CORE_PROV_NAME) <UTF8 string ptr> — points to the OpenSSL libraries' idea of what the calling provider is named.
"module-filename" (OSSL_PROV_PARAM_CORE_MODULE_FILENAME) <UTF8 string ptr> — points to a string containing the full filename of the providers module file 🗂️.

And on top of that, provider-specific configuration parameters from the config file are available too — in dotted name form. The dotted name form is just a concatenation of section names and the final config command name, all separated by periods ✨.

For example, let's say we have this config example:

config_diagnostics = 1
openssl_conf = openssl_init

[openssl_init]
providers = providers_sect

[providers_sect]
foo = foo_sect

[foo_sect]
activate = 1
data1 = 2
data2 = str
more = foo_more

[foo_more]
data3 = foo,bar

Then the provider will have these additional parameters available:

"activate" — pointing at the string "1"
"data1" — pointing at the string "2"
"data2" — pointing at the string "str"
"more.data3" — pointing at the string "foo,bar"

For more information on handling parameters, see OSSL_PARAM(3) as OSSL_PARAM_int(3) 📚.

🔥 CAPABILITIES 🔥

Capabilities describe some of the services that a provider can offer. Applications can query the capabilities to discover those services 👀.

🌈 "TLS-GROUP" Capability 🌈

The "TLS-GROUP" capability can be queried by libssl to discover the list of TLS groups a provider can support. Each supported group can be used for key exchange (KEX) or key encapsulation method (KEM) during a TLS handshake 🤝. TLS clients can advertise the list of TLS groups they support in the supported_groups extension, and TLS servers can select a group from the offered list that they also support. So yeah — that's how a provider can add to the list of groups libssl already supports with extra ones. Iconic.

Each TLS group a provider supports should be described via the callback passed in through the provider_get_capabilities function. Every group should supply the following details — and all of them are mandatory except OSSL_CAPABILITY_TLS_GROUP_IS_KEM 👑:

"tls-group-name" (OSSL_CAPABILITY_TLS_GROUP_NAME) <UTF8 string> — the name of the group as given in the IANA TLS Supported Groups registry at https://www.iana.org/assignments/tls-parameters/tls-parameters.xhtml#tls-parameters-8.
"tls-group-name-internal" (OSSL_CAPABILITY_TLS_GROUP_NAME_INTERNAL) <UTF8 string> — the name of the group as known by the provider. Could be the same as "tls-group-name", but it doesn't have to be 👀.
"tls-group-id" (OSSL_CAPABILITY_TLS_GROUP_ID) <unsigned integer> — the TLS group id value as given in the IANA TLS Supported Groups registry.

Fr fr heads up: it is possible to register the same group id from within different providers. Users should note that if no property query is specified, or if more than one implementation matches the property query, then it is unspecified which implementation for a particular group id will be used 😭.

"tls-group-alg" (OSSL_CAPABILITY_TLS_GROUP_ALG) <UTF8 string> — the name of a Key Management algorithm that the provider offers and that should be used with this group. Keys created should be able to support key exchange or key encapsulation method (KEM), as implied by the optional OSSL_CAPABILITY_TLS_GROUP_IS_KEM flag. The algorithm must support key and parameter generation, as well as the key/parameter generation parameter OSSL_PKEY_PARAM_GROUP_NAME. The group name given via "tls-group-name-internal" above will be passed via OSSL_PKEY_PARAM_GROUP_NAME when libssl wishes to generate keys/parameters 🔑.
"tls-group-sec-bits" (OSSL_CAPABILITY_TLS_GROUP_SECURITY_BITS) <unsigned integer> — the number of bits of security offered by keys in this group. That number should be comparable with the ones given in table 2 and 3 of the NIST SP800-57 document 📊.
"tls-group-is-kem" (OSSL_CAPABILITY_TLS_GROUP_IS_KEM) <unsigned integer> — a boolean flag describing if the group should be used in key exchange (KEX) mode (0, the default) or in key encapsulation method (KEM) mode (1).

This parameter is optional: if it's not specified, KEX mode is assumed as the default mode for the group 💅.

In KEX mode, in a typical Diffie-Hellman fashion, both sides execute keygen then derive against the peer public key. To operate in KEX mode, the group implementation must support the provider functions as described in provider-keyexch(7).

In KEM mode, the client executes keygen and sends its public key, the server executes encapsulate using the client's public key and sends back the resulting ciphertext, and finally the client executes decapsulate to retrieve the same shared secret generated by the server's encapsulate 🔐. To operate in KEM mode, the group implementation must support the provider functions as described in provider-kem(7).

Both in KEX and KEM mode, the resulting shared secret is then used according to the protocol specification. Slay.

"tls-min-tls" (OSSL_CAPABILITY_TLS_GROUP_MIN_TLS) <integer>
"tls-max-tls" (OSSL_CAPABILITY_TLS_GROUP_MAX_TLS) <integer>
"tls-min-dtls" (OSSL_CAPABILITY_TLS_GROUP_MIN_DTLS) <integer>
"tls-max-dtls" (OSSL_CAPABILITY_TLS_GROUP_MAX_DTLS) <integer>

These parameters can be used to describe the minimum and maximum TLS and DTLS versions supported by the group 📶. The values equate to the on-the-wire encoding of the various TLS versions. For example TLSv1.3 is 0x0304 (772 decimal), and TLSv1.2 is 0x0303 (771 decimal). A 0 indicates that there is no defined minimum or maximum. A -1 indicates that the group should not be used in that protocol 🙅.

🦋 "TLS-SIGALG" Capability 🦋

The "TLS-SIGALG" capability can be queried by libssl to discover the list of TLS signature algorithms that a provider can support. Each supported signature can be used for client- or server-authentication in addition to the built-in signature algorithms ✍️. TLS1.3 clients can advertise the list of TLS signature algorithms they support in the signature_algorithms extension, and TLS servers can select an algorithm from the offered list that they also support. So a provider can add to the list of signature algorithms libssl already supports with additional ones. Chef's kiss.

Each TLS signature algorithm a provider supports should be described via the callback passed in through the provider_get_capabilities function. Every algorithm can have the following details supplied:

"iana-name" (OSSL_CAPABILITY_TLS_SIGALG_IANA_NAME) <UTF8 string> — the name of the signature algorithm as given in the IANA TLS Signature Scheme registry as "Description": https://www.iana.org/assignments/tls-parameters/tls-parameters.xhtml#tls-signaturescheme. This value must be supplied ⭐.
"iana-code-point" (OSSL_CAPABILITY_TLS_SIGALG_CODE_POINT) <unsigned integer> — the TLS algorithm ID value as given in the IANA TLS SignatureScheme registry. This value must be supplied ⭐.

Same energy as before, bestie: it is possible to register the same code point from within different providers. Users should note that if no property query is specified, or if more than one implementation matches the property query, then it is unspecified which implementation for a particular code point will be used 😭.

"sigalg-name" (OSSL_CAPABILITY_TLS_SIGALG_NAME) <UTF8 string> — a name for the full (possibly composite hash-and-signature) signature algorithm. The provider may, but is not obligated to, provide a signature implementation with this name; if it doesn't, this is assumed to be a composite of a pure signature algorithm and a hash algorithm, which must be given with the parameters "sig-name" and "hash-name". This value must be supplied ⭐.
"sigalg-oid" (OSSL_CAPABILITY_TLS_SIGALG_OID) <UTF8 string> — the OID of the "sigalg-name" algorithm, written in canonical numeric text form. Drop this param in and OBJ_create() gets called to make an OBJ + a NID for that OID, using the "sigalg-name" param as its (short) name. Leave it out and we just assume it already lives in the object database — maybe the provider put it there with the core_obj_create() upcall. Optional, bestie. 💅
"sig-name" (OSSL_CAPABILITY_TLS_SIGALG_SIG_NAME) <UTF8 string> — the name of the pure signature algorithm that's part of a composite "sigalg-name". ⚠️ If the provider itself implements "sigalg-name", then this param is redundant and must not be given. Optional.
"sig-oid" (OSSL_CAPABILITY_TLS_SIGALG_SIG_OID) <UTF8 string> — the OID of the "sig-name" algorithm in canonical numeric text form. Give it and OBJ_create() makes an OBJ + NID for this OID, with "sig-name" as its (short) name. Don't give it and it's assumed to already be in the object database — the provider can do that with the core_obj_create() upcall. Optional. 🦋
"hash-name" (OSSL_CAPABILITY_TLS_SIGALG_HASH_NAME) <UTF8 string> — the name of the hash algorithm that's part of a composite "sigalg-name". Same deal: if "sigalg-name" is implemented by the provider, this is redundant and must not be given. Optional.
"hash-oid" (OSSL_CAPABILITY_TLS_SIGALG_HASH_OID) <UTF8 string> — the OID of "hash-name" in canonical numeric text form. Provide it and OBJ_create() builds an OBJ + NID for the OID using "hash-name" as the (short) name. Otherwise we assume it's already in the object database, possibly courtesy of the provider's core_obj_create() upcall. Optional. 🎀
"key-type" (OSSL_CAPABILITY_TLS_SIGALG_KEYTYPE) <UTF8 string> — the key type of the public key in the applicable certificates. If this isn't present, it's assumed to be the same as "sig-name" when that's present, otherwise "sigalg-name". Optional.
"key-type-oid" (OSSL_CAPABILITY_TLS_SIGALG_KEYTYPE_OID) <UTF8 string> — the OID of the "key-type" in canonical numeric text form. Give it and OBJ_create() makes an OBJ + NID for this OID, using "key-type" for its (short) name. Otherwise it's assumed to already exist in the object database, possibly done by the provider with the core_obj_create() upcall. Optional.
"sec-bits" (OSSL_CAPABILITY_TLS_SIGALG_SECURITY_BITS) <unsigned integer> — how many bits of security keys of this algorithm offer. This number should be comparable with the ones in table 2 and 3 of the NIST SP800-57 document, no cap. 🔥 It's used to figure out the security strength of the algorithm when no digest algorithm has been registered that would otherwise define that strength. And if the signature algorithm implements its own digest internally, you need to set this to properly reflect the overall security strength. This value must be supplied — not optional, bestie, it's giving mandatory.
"tls-min-tls" (OSSL_CAPABILITY_TLS_SIGALG_MIN_TLS) <integer>
"tls-max-tls" (OSSL_CAPABILITY_TLS_SIGALG_MAX_TLS) <integer>
"tls-min-dtls" (OSSL_CAPABILITY_TLS_SIGALG_MIN_DTLS) <integer>
"tls-max-dtls" (OSSL_CAPABILITY_TLS_SIGALG_MAX_DTLS) <integer> — these four describe the min and max TLS and DTLS versions the signature algorithm supports. The values are literally the on-the-wire encoding of the TLS versions: for example TLSv1.3 is 0x0304 (772 decimal) and TLSv1.2 is 0x0303 (771 decimal). A 0 means there's no defined minimum or maximum. A -1 in either the min or max field means the signature algorithm shouldn't be used in that protocol at all, tyvm. 🚫 Right now, provider signature algorithms are only used with TLS 1.3, if that version falls inside the range you specified.

🌸 NOTES 🌸

The core_obj_create() and core_obj_add_sigid() functions were not thread safe in OpenSSL 3.0. Just so you know. 😭

✨ EXAMPLES ✨

Okay so here's an example of a simple provider, made available as a dynamically loadable module. It implements the completely fictitious algorithm "FOO" for the equally fictitious operation "BAR". Iconic behaviour, honestly.

#include <malloc.h>
#include <openssl/core.h>
#include <openssl/core_dispatch.h>

/* Errors used in this provider */
#define E_MALLOC       1

static const OSSL_ITEM reasons[] = {
    { E_MALLOC, "memory allocation failure" }.
    OSSL_DISPATCH_END
};

/*
 * To ensure we get the function signature right, forward declare
 * them using function types provided by openssl/core_dispatch.h
 */
OSSL_FUNC_bar_newctx_fn foo_newctx;
OSSL_FUNC_bar_freectx_fn foo_freectx;
OSSL_FUNC_bar_init_fn foo_init;
OSSL_FUNC_bar_update_fn foo_update;
OSSL_FUNC_bar_final_fn foo_final;

OSSL_FUNC_provider_query_operation_fn p_query;
OSSL_FUNC_provider_get_reason_strings_fn p_reasons;
OSSL_FUNC_provider_teardown_fn p_teardown;

OSSL_provider_init_fn OSSL_provider_init;

OSSL_FUNC_core_put_error *c_put_error = NULL;

/* Provider context */
struct prov_ctx_st {
    OSSL_CORE_HANDLE *handle;
}

/* operation context for the algorithm FOO */
struct foo_ctx_st {
    struct prov_ctx_st *provctx;
    int b;
};

static void *foo_newctx(void *provctx)
{
    struct foo_ctx_st *fooctx = malloc(sizeof(*fooctx));

    if (fooctx != NULL)
        fooctx->provctx = provctx;
    else
        c_put_error(provctx->handle, E_MALLOC, __FILE__, __LINE__);
    return fooctx;
}

static void foo_freectx(void *fooctx)
{
    free(fooctx);
}

static int foo_init(void *vfooctx)
{
    struct foo_ctx_st *fooctx = vfooctx;

    fooctx->b = 0x33;
}

static int foo_update(void *vfooctx, unsigned char *in, size_t inl)
{
    struct foo_ctx_st *fooctx = vfooctx;

    /* did you expect something serious? */
    if (inl == 0)
        return 1;
    for (; inl-- > 0; in++)
        *in ^= fooctx->b;
    return 1;
}

static int foo_final(void *vfooctx)
{
    struct foo_ctx_st *fooctx = vfooctx;

    fooctx->b = 0x66;
}

static const OSSL_DISPATCH foo_fns[] = {
    { OSSL_FUNC_BAR_NEWCTX, (void (*)(void))foo_newctx },
    { OSSL_FUNC_BAR_FREECTX, (void (*)(void))foo_freectx },
    { OSSL_FUNC_BAR_INIT, (void (*)(void))foo_init },
    { OSSL_FUNC_BAR_UPDATE, (void (*)(void))foo_update },
    { OSSL_FUNC_BAR_FINAL, (void (*)(void))foo_final },
    OSSL_DISPATCH_END
};

static const OSSL_ALGORITHM bars[] = {
    { "FOO", "provider=chumbawamba", foo_fns },
    { NULL, NULL, NULL }
};

static const OSSL_ALGORITHM *p_query(void *provctx, int operation_id,
                                     int *no_store)
{
    switch (operation_id) {
    case OSSL_OP_BAR:
        return bars;
    }
    return NULL;
}

static const OSSL_ITEM *p_reasons(void *provctx)
{
    return reasons;
}

static void p_teardown(void *provctx)
{
    free(provctx);
}

static const OSSL_DISPATCH prov_fns[] = {
    { OSSL_FUNC_PROVIDER_TEARDOWN, (void (*)(void))p_teardown },
    { OSSL_FUNC_PROVIDER_QUERY_OPERATION, (void (*)(void))p_query },
    { OSSL_FUNC_PROVIDER_GET_REASON_STRINGS, (void (*)(void))p_reasons },
    OSSL_DISPATCH_END
};

int OSSL_provider_init(const OSSL_CORE_HANDLE *handle,
                       const OSSL_DISPATCH *in,
                       const OSSL_DISPATCH **out,
                       void **provctx)
{
    struct prov_ctx_st *pctx = NULL;

    for (; in->function_id != 0; in++)
        switch (in->function_id) {
        case OSSL_FUNC_CORE_PUT_ERROR:
            c_put_error = OSSL_FUNC_core_put_error(in);
            break;
        }

    *out = prov_fns;

    if ((pctx = malloc(sizeof(*pctx))) == NULL) {
        /*
         * ALEA IACTA EST, if the core retrieves the reason table
         * regardless, that string will be displayed, otherwise not.
         */
        c_put_error(handle, E_MALLOC, __FILE__, __LINE__);
        return 0;
    }
    pctx->handle = handle;
    return 1;
}

This whole thing relies on a few things existing in openssl/core_dispatch.h, so here they are 🫶:

#define OSSL_OP_BAR            4711

#define OSSL_FUNC_BAR_NEWCTX      1
typedef void *(OSSL_FUNC_bar_newctx_fn)(void *provctx);
static ossl_inline OSSL_FUNC_bar_newctx(const OSSL_DISPATCH *opf)
{ return (OSSL_FUNC_bar_newctx_fn *)opf->function; }

#define OSSL_FUNC_BAR_FREECTX     2
typedef void (OSSL_FUNC_bar_freectx_fn)(void *ctx);
static ossl_inline OSSL_FUNC_bar_freectx(const OSSL_DISPATCH *opf)
{ return (OSSL_FUNC_bar_freectx_fn *)opf->function; }

#define OSSL_FUNC_BAR_INIT        3
typedef void *(OSSL_FUNC_bar_init_fn)(void *ctx);
static ossl_inline OSSL_FUNC_bar_init(const OSSL_DISPATCH *opf)
{ return (OSSL_FUNC_bar_init_fn *)opf->function; }

#define OSSL_FUNC_BAR_UPDATE      4
typedef void *(OSSL_FUNC_bar_update_fn)(void *ctx,
                                      unsigned char *in, size_t inl);
static ossl_inline OSSL_FUNC_bar_update(const OSSL_DISPATCH *opf)
{ return (OSSL_FUNC_bar_update_fn *)opf->function; }

#define OSSL_FUNC_BAR_FINAL       5
typedef void *(OSSL_FUNC_bar_final_fn)(void *ctx);
static ossl_inline OSSL_FUNC_bar_final(const OSSL_DISPATCH *opf)
{ return (OSSL_FUNC_bar_final_fn *)opf->function; }

👑 SEE ALSO 👑

provider(7)

🦋 HISTORY 🦋

The whole concept of providers and everything that comes with them? Introduced in OpenSSL 3.0. Periodt.

The definitions for OSSL_CAPABILITY_TLS_SIGALG_MIN_DTLS and OSSL_CAPABILITY_TLS_SIGALG_MAX_DTLS were added in OpenSSL 3.5.

💖 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>.

And that's the whole page, bestie, you ate that 🍓✨ This one's from OpenSSL 3.5.0, dated 2026-09-25 — go forth and implement providers, mwah 💜🧚

📦 where this page comes from

opensuse-leap 16.0: openssl-3 3.5.0-160000.11.1 · Apache-2.0
opensuse-leap 16.0: openssl-3 3.5.0-160000.11.1 · Apache-2.0
opensuse-leap 16.0: openssl-3 3.5.0-160000.11.1 · Apache-2.0
opensuse-leap 16.0: openssl-3 3.5.0-160000.11.1 · 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