openssl/doc/man7/provider-base.pod

=pod

=head1 NAME

provider-base
- The basic OpenSSL library E<lt>-E<gt> provider functions

=head1 SYNOPSIS

 #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);

 /*
  * 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_memdup(const void *str, size_t siz,
                     const char *file, int line);
 char *CRYPTO_strdup(const char *str, const char *file, int line);
 char *CRYPTO_strndup(const char *str, size_t s,
                      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);
 void cleanup_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);
 void cleanup_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);

=head1 DESCRIPTION

All "functions" mentioned here are passed as function pointers between
F<libcrypto> and the provider in B<OSSL_DISPATCH> arrays, in the call
of the provider initialization function.  See L<provider(7)/Provider>
for a description of the initialization function.

All these "functions" have a corresponding function type definition
named B<OSSL_FUNC_{name}_fn>, and a helper function to retrieve the
function pointer from a B<OSSL_DISPATCH> element named
B<OSSL_FUNC_{name}>.
For example, the "function" core_gettable_params() has these:

 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);

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

For I<in> (the B<OSSL_DISPATCH> array passed from F<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
 CRYPTO_malloc                  OSSL_FUNC_CRYPTO_MALLOC
 CRYPTO_zalloc                  OSSL_FUNC_CRYPTO_ZALLOC
 CRYPTO_memdup                  OSSL_FUNC_CRYPTO_MEMDUP
 CRYPTO_strdup                  OSSL_FUNC_CRYPTO_STRDUP
 CRYPTO_strndup                 OSSL_FUNC_CRYPTO_STRNDUP
 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_up_ref                     OSSL_FUNC_BIO_UP_REF
 BIO_free                       OSSL_FUNC_BIO_FREE
 BIO_vprintf                    OSSL_FUNC_BIO_VPRINTF
 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_cleanup_entropy      OSSL_FUNC_CLEANUP_ENTROPY
 ossl_rand_get_nonce            OSSL_FUNC_GET_NONCE
 ossl_rand_cleanup_nonce        OSSL_FUNC_CLEANUP_NONCE

For I<*out> (the B<OSSL_DISPATCH> array passed from the provider to
F<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

=head2 Core functions

core_gettable_params() returns a constant array of descriptor
B<OSSL_PARAM>, for parameters that core_get_params() can handle.

core_get_params() retrieves parameters from the core for the given I<handle>.
See L</Core parameters> below for a description of currently known
parameters.

The core_thread_start() function 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 I<handle> for this provider, as
well as a callback I<handfn> which will be called when the thread stops. The
callback will subsequently be called, with the supplied argument I<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 library context in which the library
object for the current provider is stored, accessible through the I<handle>.
This may sometimes be useful if the provider wishes to store a
reference to its context in the same 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 I<handle>.

=over 4

=item core_new_error()

allocates a new thread specific error record.

This corresponds to the OpenSSL function L<ERR_new(3)>.

=item core_set_error_debug()

sets debugging information in the current thread specific error
record.
The debugging information includes the name of the file I<file>, the
line I<line> and the function name I<func> where the error occurred.

This corresponds to the OpenSSL function L<ERR_set_debug(3)>.

=item core_vset_error()

sets the I<reason> for the error, along with any addition data.
The I<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 I<fmt> and a set of
arguments I<args>, which are treated in the same manner as with
BIO_vsnprintf().
I<file> and I<line> may also be passed to indicate exactly where the
error occurred or was reported.

This corresponds to the OpenSSL function L<ERR_vset_error(3)>.

=back

CRYPTO_malloc(), CRYPTO_zalloc(), CRYPTO_memdup(), CRYPTO_strdup(),
CRYPTO_strndup(), 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(), OPENSSL_cleanse()
and OPENSSL_hexstr2buf() correspond exactly to the public functions with
the same name.  As a matter of fact, the pointers in the B<OSSL_DISPATCH>
array are direct pointers to those public functions. Note that the BIO
functions take an B<OSSL_CORE_BIO> type rather than the standard B<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 I<entropy> bytes of randomness and the
output will have at least I<min_len> and at most I<max_len> bytes.
The buffer address is stored in I<*pout> and the buffer length is
returned to the caller.  On error, zero is returned.

cleanup_entropy() is used to clean up and free the buffer returned by
get_entropy().  The entropy pointer returned by get_entropy() is passed in
B<buf> and its length in B<len>.

get_nonce() retrieves a nonce using the passed I<salt> parameter
of length I<salt_len> and operating system specific information.
The I<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 contrains at least I<min_len> and at
most I<max_len> bytes.  The buffer address is stored in I<*pout> and the
buffer length returned to the caller.  On error, zero is returned.

cleanup_nonce() is used to clean up and free the buffer returned by
get_nonce().  The nonce pointer returned by get_nonce() is passed in
B<buf> and its length in B<len>.

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

I<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 loadded and this
provider's data in I<cbdata>. It should return 1 on success or 0 on failure.

I<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 I<cbdata>. It should
return 1 on success or 0 on failure.

I<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
I<handle>.

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

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

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

=head2 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 I<provctx>.

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

provider_get_params() should process the B<OSSL_PARAM> array
I<params>, setting the values of the parameters it understands.

provider_query_operation() should return a constant B<OSSL_ALGORITHM>
that corresponds to the given I<operation_id>.
It should indicate if the core may store a reference to this array by
setting I<*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 I<operation_id> should match that passed to
provider_query_operation() and I<algs> should be its return value.

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

The provider_get_capabilities() function should call the callback I<cb> passing
it a set of B<OSSL_PARAM>s and the caller supplied argument I<arg>. The
B<OSSL_PARAM>s should provide details about the capability with the name given
in the I<capability> argument relevant for the provider context I<provctx>. If a
provider supports multiple capabilities with the given name then it may call the
callback multiple times (one for each capability). Capabilities can be useful for
describing the services that a provider can offer. For further details see the
L</CAPABILITIES> section below. It should return 1 on success or 0 on error.

The provider_self_test() function should perform known answer tests on a subset
of the algorithms that it uses, and may also verify the integrity of the
provider module. It should return 1 on success or 0 on error. It will return 1
if this function is not used.

None of these functions are mandatory, but a provider is fairly
useless without at least provider_query_operation(), and
provider_gettable_params() is fairly useless if not accompanied by
provider_get_params().

=head2 Provider parameters

provider_get_params() can return the following provider parameters to the core:

=over 4

=item "name" (B<OSSL_PROV_PARAM_NAME>) <UTF8 string ptr>

This points to a string that should give a unique name for the provider.

=item "version" (B<OSSL_PROV_PARAM_VERSION>) <UTF8 string ptr>

This points to a string that is a version number associated with this provider.
OpenSSL in-built providers use OPENSSL_VERSION_STR, but this may be different
for any third party provider. This string is for informational purposes only.

=item "buildinfo" (B<OSSL_PROV_PARAM_BUILDINFO>) <UTF8 string ptr>

This points to a string that is a build information associated with this provider.
OpenSSL in-built providers use OPENSSL_FULL_VERSION_STR, but this may be
different for any third party provider.

=item "status" (B<OSSL_PROV_PARAM_STATUS>) <unsigned integer>

This returns 0 if the provider has entered an error state, otherwise it returns
1.

=back 

provider_gettable_params() should return the above parameters.


=head2 Core parameters

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

=over 4

=item "openssl-version" (B<OSSL_PROV_PARAM_CORE_VERSION>) <UTF8 string ptr>

This points to the OpenSSL libraries' full version string, i.e. the string
expanded from the macro B<OPENSSL_VERSION_STR>.

=item "provider-name" (B<OSSL_PROV_PARAM_CORE_PROV_NAME>) <UTF8 string ptr>

This points to the OpenSSL libraries' idea of what the calling provider is named.

=item "module-filename" (B<OSSL_PROV_PARAM_CORE_MODULE_FILENAME>) <UTF8 string ptr>

This points to a string containing the full filename of the providers
module file.

=back

Additionally, provider specific configuration parameters from the
config file are available, in dotted name form.
The dotted name form is a concatenation of section names and final
config command name separated by periods.

For example, let's say we have the following config example:

 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

The provider will have these additional parameters available:

=over 4

=item "activate"

pointing at the string "1"

=item "data1"

pointing at the string "2"

=item "data2"

pointing at the string "str"

=item "more.data3"

pointing at the string "foo,bar"

=back

For more information on handling parameters, see L<OSSL_PARAM(3)> as
L<OSSL_PARAM_int(3)>.

=head1 CAPABILITIES

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

=head3 "TLS-GROUP" Capability

The "TLS-GROUP" capability can be queried by libssl to discover the list of
TLS groups that a provider can support. Each group supported can be used for
I<key exchange> (KEX) or I<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. In this way a provider can add to the list of
groups that libssl already supports with additional ones.

Each TLS group that a provider supports should be described via the callback
passed in through the provider_get_capabilities function. Each group should have
the following details supplied (all are mandatory, except
B<OSSL_CAPABILITY_TLS_GROUP_IS_KEM>):

=over 4

=item "tls-group-name" (B<OSSL_CAPABILITY_TLS_GROUP_NAME>) <UTF8 string>

The name of the group as given in the IANA TLS Supported Groups registry
L<https://www.iana.org/assignments/tls-parameters/tls-parameters.xhtml#tls-parameters-8>.

=item "tls-group-name-internal" (B<OSSL_CAPABILITY_TLS_GROUP_NAME_INTERNAL>) <UTF8 string>

The name of the group as known by the provider. This could be the same as the
"tls-group-name", but does not have to be.

=item "tls-group-id" (B<OSSL_CAPABILITY_TLS_GROUP_ID>) <unsigned integer>

The TLS group id value as given in the IANA TLS Supported Groups registry.

=item "tls-group-alg" (B<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 I<key exchange>
or I<key encapsulation method> (KEM), as implied by the optional
B<OSSL_CAPABILITY_TLS_GROUP_IS_KEM> flag.
The algorithm must support key and parameter generation as well as the
key/parameter generation parameter, B<OSSL_PKEY_PARAM_GROUP_NAME>. The group
name given via "tls-group-name-internal" above will be passed via
B<OSSL_PKEY_PARAM_GROUP_NAME> when libssl wishes to generate keys/parameters.

=item "tls-group-sec-bits" (B<OSSL_CAPABILITY_TLS_GROUP_SECURITY_BITS>) <unsigned integer>

The number of bits of security offered by keys in this group. The number of bits
should be comparable with the ones given in table 2 and 3 of the NIST SP800-57
document.

=item "tls-group-is-kem" (B<OSSL_CAPABILITY_TLS_GROUP_IS_KEM>) <unsigned integer>

Boolean flag to describe if the group should be used in I<key exchange> (KEX)
mode (0, default) or in I<key encapsulation method> (KEM) mode (1).

This parameter is optional: if 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 I<keygen>
then I<derive> against the peer public key. To operate in KEX mode, the group
implementation must support the provider functions as described in
L<provider-keyexch(7)>.

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

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

=item "tls-min-tls" (B<OSSL_CAPABILITY_TLS_GROUP_MIN_TLS>) <integer>

=item "tls-max-tls" (B<OSSL_CAPABILITY_TLS_GROUP_MAX_TLS>) <integer>

=item "tls-min-dtls" (B<OSSL_CAPABILITY_TLS_GROUP_MIN_DTLS>) <integer>

=item "tls-max-dtls" (B<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.

=back

=head1 EXAMPLES

This is an example of a simple provider made available as a
dynamically loadable module.
It implements the fictitious algorithm C<FOO> for the fictitious
operation C<BAR>.

 #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" }.
     { 0, NULL } /* Termination */
 };

 /*
  * 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 },
     { 0, NULL }
 };

 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 },
     { 0, NULL }
 };

 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 relies on a few things existing in F<openssl/core_dispatch.h>:

 #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_newctx(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; }

=head1 SEE ALSO

L<provider(7)>

=head1 HISTORY

The concept of providers and everything surrounding them was
introduced in OpenSSL 3.0.

=head1 COPYRIGHT

Copyright 2019-2021 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
L<https://www.openssl.org/source/license.html>.

=cut