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127 .TH engine 3 "2012-03-14" "1.0.1" "OpenSSL"
128 .\" For nroff, turn off justification. Always turn off hyphenation; it makes
129 .\" way too many mistakes in technical documents.
133 engine \- ENGINE cryptographic module support
135 .IX Header "SYNOPSIS"
137 \& #include <openssl/engine.h>
139 \& ENGINE *ENGINE_get_first(void);
140 \& ENGINE *ENGINE_get_last(void);
141 \& ENGINE *ENGINE_get_next(ENGINE *e);
142 \& ENGINE *ENGINE_get_prev(ENGINE *e);
144 \& int ENGINE_add(ENGINE *e);
145 \& int ENGINE_remove(ENGINE *e);
147 \& ENGINE *ENGINE_by_id(const char *id);
149 \& int ENGINE_init(ENGINE *e);
150 \& int ENGINE_finish(ENGINE *e);
152 \& void ENGINE_load_openssl(void);
153 \& void ENGINE_load_dynamic(void);
154 \& #ifndef OPENSSL_NO_STATIC_ENGINE
155 \& void ENGINE_load_4758cca(void);
156 \& void ENGINE_load_aep(void);
157 \& void ENGINE_load_atalla(void);
158 \& void ENGINE_load_chil(void);
159 \& void ENGINE_load_cswift(void);
160 \& void ENGINE_load_gmp(void);
161 \& void ENGINE_load_nuron(void);
162 \& void ENGINE_load_sureware(void);
163 \& void ENGINE_load_ubsec(void);
165 \& void ENGINE_load_cryptodev(void);
166 \& void ENGINE_load_builtin_engines(void);
168 \& void ENGINE_cleanup(void);
170 \& ENGINE *ENGINE_get_default_RSA(void);
171 \& ENGINE *ENGINE_get_default_DSA(void);
172 \& ENGINE *ENGINE_get_default_ECDH(void);
173 \& ENGINE *ENGINE_get_default_ECDSA(void);
174 \& ENGINE *ENGINE_get_default_DH(void);
175 \& ENGINE *ENGINE_get_default_RAND(void);
176 \& ENGINE *ENGINE_get_cipher_engine(int nid);
177 \& ENGINE *ENGINE_get_digest_engine(int nid);
179 \& int ENGINE_set_default_RSA(ENGINE *e);
180 \& int ENGINE_set_default_DSA(ENGINE *e);
181 \& int ENGINE_set_default_ECDH(ENGINE *e);
182 \& int ENGINE_set_default_ECDSA(ENGINE *e);
183 \& int ENGINE_set_default_DH(ENGINE *e);
184 \& int ENGINE_set_default_RAND(ENGINE *e);
185 \& int ENGINE_set_default_ciphers(ENGINE *e);
186 \& int ENGINE_set_default_digests(ENGINE *e);
187 \& int ENGINE_set_default_string(ENGINE *e, const char *list);
189 \& int ENGINE_set_default(ENGINE *e, unsigned int flags);
191 \& unsigned int ENGINE_get_table_flags(void);
192 \& void ENGINE_set_table_flags(unsigned int flags);
194 \& int ENGINE_register_RSA(ENGINE *e);
195 \& void ENGINE_unregister_RSA(ENGINE *e);
196 \& void ENGINE_register_all_RSA(void);
197 \& int ENGINE_register_DSA(ENGINE *e);
198 \& void ENGINE_unregister_DSA(ENGINE *e);
199 \& void ENGINE_register_all_DSA(void);
200 \& int ENGINE_register_ECDH(ENGINE *e);
201 \& void ENGINE_unregister_ECDH(ENGINE *e);
202 \& void ENGINE_register_all_ECDH(void);
203 \& int ENGINE_register_ECDSA(ENGINE *e);
204 \& void ENGINE_unregister_ECDSA(ENGINE *e);
205 \& void ENGINE_register_all_ECDSA(void);
206 \& int ENGINE_register_DH(ENGINE *e);
207 \& void ENGINE_unregister_DH(ENGINE *e);
208 \& void ENGINE_register_all_DH(void);
209 \& int ENGINE_register_RAND(ENGINE *e);
210 \& void ENGINE_unregister_RAND(ENGINE *e);
211 \& void ENGINE_register_all_RAND(void);
212 \& int ENGINE_register_STORE(ENGINE *e);
213 \& void ENGINE_unregister_STORE(ENGINE *e);
214 \& void ENGINE_register_all_STORE(void);
215 \& int ENGINE_register_ciphers(ENGINE *e);
216 \& void ENGINE_unregister_ciphers(ENGINE *e);
217 \& void ENGINE_register_all_ciphers(void);
218 \& int ENGINE_register_digests(ENGINE *e);
219 \& void ENGINE_unregister_digests(ENGINE *e);
220 \& void ENGINE_register_all_digests(void);
221 \& int ENGINE_register_complete(ENGINE *e);
222 \& int ENGINE_register_all_complete(void);
224 \& int ENGINE_ctrl(ENGINE *e, int cmd, long i, void *p, void (*f)(void));
225 \& int ENGINE_cmd_is_executable(ENGINE *e, int cmd);
226 \& int ENGINE_ctrl_cmd(ENGINE *e, const char *cmd_name,
227 \& long i, void *p, void (*f)(void), int cmd_optional);
228 \& int ENGINE_ctrl_cmd_string(ENGINE *e, const char *cmd_name, const char *arg,
229 \& int cmd_optional);
231 \& int ENGINE_set_ex_data(ENGINE *e, int idx, void *arg);
232 \& void *ENGINE_get_ex_data(const ENGINE *e, int idx);
234 \& int ENGINE_get_ex_new_index(long argl, void *argp, CRYPTO_EX_new *new_func,
235 \& CRYPTO_EX_dup *dup_func, CRYPTO_EX_free *free_func);
237 \& ENGINE *ENGINE_new(void);
238 \& int ENGINE_free(ENGINE *e);
239 \& int ENGINE_up_ref(ENGINE *e);
241 \& int ENGINE_set_id(ENGINE *e, const char *id);
242 \& int ENGINE_set_name(ENGINE *e, const char *name);
243 \& int ENGINE_set_RSA(ENGINE *e, const RSA_METHOD *rsa_meth);
244 \& int ENGINE_set_DSA(ENGINE *e, const DSA_METHOD *dsa_meth);
245 \& int ENGINE_set_ECDH(ENGINE *e, const ECDH_METHOD *dh_meth);
246 \& int ENGINE_set_ECDSA(ENGINE *e, const ECDSA_METHOD *dh_meth);
247 \& int ENGINE_set_DH(ENGINE *e, const DH_METHOD *dh_meth);
248 \& int ENGINE_set_RAND(ENGINE *e, const RAND_METHOD *rand_meth);
249 \& int ENGINE_set_STORE(ENGINE *e, const STORE_METHOD *rand_meth);
250 \& int ENGINE_set_destroy_function(ENGINE *e, ENGINE_GEN_INT_FUNC_PTR destroy_f);
251 \& int ENGINE_set_init_function(ENGINE *e, ENGINE_GEN_INT_FUNC_PTR init_f);
252 \& int ENGINE_set_finish_function(ENGINE *e, ENGINE_GEN_INT_FUNC_PTR finish_f);
253 \& int ENGINE_set_ctrl_function(ENGINE *e, ENGINE_CTRL_FUNC_PTR ctrl_f);
254 \& int ENGINE_set_load_privkey_function(ENGINE *e, ENGINE_LOAD_KEY_PTR loadpriv_f);
255 \& int ENGINE_set_load_pubkey_function(ENGINE *e, ENGINE_LOAD_KEY_PTR loadpub_f);
256 \& int ENGINE_set_ciphers(ENGINE *e, ENGINE_CIPHERS_PTR f);
257 \& int ENGINE_set_digests(ENGINE *e, ENGINE_DIGESTS_PTR f);
258 \& int ENGINE_set_flags(ENGINE *e, int flags);
259 \& int ENGINE_set_cmd_defns(ENGINE *e, const ENGINE_CMD_DEFN *defns);
261 \& const char *ENGINE_get_id(const ENGINE *e);
262 \& const char *ENGINE_get_name(const ENGINE *e);
263 \& const RSA_METHOD *ENGINE_get_RSA(const ENGINE *e);
264 \& const DSA_METHOD *ENGINE_get_DSA(const ENGINE *e);
265 \& const ECDH_METHOD *ENGINE_get_ECDH(const ENGINE *e);
266 \& const ECDSA_METHOD *ENGINE_get_ECDSA(const ENGINE *e);
267 \& const DH_METHOD *ENGINE_get_DH(const ENGINE *e);
268 \& const RAND_METHOD *ENGINE_get_RAND(const ENGINE *e);
269 \& const STORE_METHOD *ENGINE_get_STORE(const ENGINE *e);
270 \& ENGINE_GEN_INT_FUNC_PTR ENGINE_get_destroy_function(const ENGINE *e);
271 \& ENGINE_GEN_INT_FUNC_PTR ENGINE_get_init_function(const ENGINE *e);
272 \& ENGINE_GEN_INT_FUNC_PTR ENGINE_get_finish_function(const ENGINE *e);
273 \& ENGINE_CTRL_FUNC_PTR ENGINE_get_ctrl_function(const ENGINE *e);
274 \& ENGINE_LOAD_KEY_PTR ENGINE_get_load_privkey_function(const ENGINE *e);
275 \& ENGINE_LOAD_KEY_PTR ENGINE_get_load_pubkey_function(const ENGINE *e);
276 \& ENGINE_CIPHERS_PTR ENGINE_get_ciphers(const ENGINE *e);
277 \& ENGINE_DIGESTS_PTR ENGINE_get_digests(const ENGINE *e);
278 \& const EVP_CIPHER *ENGINE_get_cipher(ENGINE *e, int nid);
279 \& const EVP_MD *ENGINE_get_digest(ENGINE *e, int nid);
280 \& int ENGINE_get_flags(const ENGINE *e);
281 \& const ENGINE_CMD_DEFN *ENGINE_get_cmd_defns(const ENGINE *e);
283 \& EVP_PKEY *ENGINE_load_private_key(ENGINE *e, const char *key_id,
284 \& UI_METHOD *ui_method, void *callback_data);
285 \& EVP_PKEY *ENGINE_load_public_key(ENGINE *e, const char *key_id,
286 \& UI_METHOD *ui_method, void *callback_data);
288 \& void ENGINE_add_conf_module(void);
291 .IX Header "DESCRIPTION"
292 These functions create, manipulate, and use cryptographic modules in the
293 form of \fB\s-1ENGINE\s0\fR objects. These objects act as containers for
294 implementations of cryptographic algorithms, and support a
295 reference-counted mechanism to allow them to be dynamically loaded in and
296 out of the running application.
298 The cryptographic functionality that can be provided by an \fB\s-1ENGINE\s0\fR
299 implementation includes the following abstractions;
302 \& RSA_METHOD \- for providing alternative RSA implementations
303 \& DSA_METHOD, DH_METHOD, RAND_METHOD, ECDH_METHOD, ECDSA_METHOD,
304 \& STORE_METHOD \- similarly for other OpenSSL APIs
305 \& EVP_CIPHER \- potentially multiple cipher algorithms (indexed by \*(Aqnid\*(Aq)
306 \& EVP_DIGEST \- potentially multiple hash algorithms (indexed by \*(Aqnid\*(Aq)
307 \& key\-loading \- loading public and/or private EVP_PKEY keys
309 .SS "Reference counting and handles"
310 .IX Subsection "Reference counting and handles"
311 Due to the modular nature of the \s-1ENGINE\s0 \s-1API\s0, pointers to ENGINEs need to be
312 treated as handles \- ie. not only as pointers, but also as references to
313 the underlying \s-1ENGINE\s0 object. Ie. one should obtain a new reference when
314 making copies of an \s-1ENGINE\s0 pointer if the copies will be used (and
315 released) independently.
317 \&\s-1ENGINE\s0 objects have two levels of reference-counting to match the way in
318 which the objects are used. At the most basic level, each \s-1ENGINE\s0 pointer is
319 inherently a \fBstructural\fR reference \- a structural reference is required
320 to use the pointer value at all, as this kind of reference is a guarantee
321 that the structure can not be deallocated until the reference is released.
323 However, a structural reference provides no guarantee that the \s-1ENGINE\s0 is
324 initiliased and able to use any of its cryptographic
325 implementations. Indeed it's quite possible that most ENGINEs will not
326 initialise at all in typical environments, as ENGINEs are typically used to
327 support specialised hardware. To use an \s-1ENGINE\s0's functionality, you need a
328 \&\fBfunctional\fR reference. This kind of reference can be considered a
329 specialised form of structural reference, because each functional reference
330 implicitly contains a structural reference as well \- however to avoid
331 difficult-to-find programming bugs, it is recommended to treat the two
332 kinds of reference independently. If you have a functional reference to an
333 \&\s-1ENGINE\s0, you have a guarantee that the \s-1ENGINE\s0 has been initialised ready to
334 perform cryptographic operations and will remain uninitialised
335 until after you have released your reference.
337 \&\fIStructural references\fR
339 This basic type of reference is used for instantiating new ENGINEs,
340 iterating across OpenSSL's internal linked-list of loaded
341 ENGINEs, reading information about an \s-1ENGINE\s0, etc. Essentially a structural
342 reference is sufficient if you only need to query or manipulate the data of
343 an \s-1ENGINE\s0 implementation rather than use its functionality.
345 The \fIENGINE_new()\fR function returns a structural reference to a new (empty)
346 \&\s-1ENGINE\s0 object. There are other \s-1ENGINE\s0 \s-1API\s0 functions that return structural
347 references such as; \fIENGINE_by_id()\fR, \fIENGINE_get_first()\fR, \fIENGINE_get_last()\fR,
348 \&\fIENGINE_get_next()\fR, \fIENGINE_get_prev()\fR. All structural references should be
349 released by a corresponding to call to the \fIENGINE_free()\fR function \- the
350 \&\s-1ENGINE\s0 object itself will only actually be cleaned up and deallocated when
351 the last structural reference is released.
353 It should also be noted that many \s-1ENGINE\s0 \s-1API\s0 function calls that accept a
354 structural reference will internally obtain another reference \- typically
355 this happens whenever the supplied \s-1ENGINE\s0 will be needed by OpenSSL after
356 the function has returned. Eg. the function to add a new \s-1ENGINE\s0 to
357 OpenSSL's internal list is \fIENGINE_add()\fR \- if this function returns success,
358 then OpenSSL will have stored a new structural reference internally so the
359 caller is still responsible for freeing their own reference with
360 \&\fIENGINE_free()\fR when they are finished with it. In a similar way, some
361 functions will automatically release the structural reference passed to it
362 if part of the function's job is to do so. Eg. the \fIENGINE_get_next()\fR and
363 \&\fIENGINE_get_prev()\fR functions are used for iterating across the internal
364 \&\s-1ENGINE\s0 list \- they will return a new structural reference to the next (or
365 previous) \s-1ENGINE\s0 in the list or \s-1NULL\s0 if at the end (or beginning) of the
366 list, but in either case the structural reference passed to the function is
367 released on behalf of the caller.
369 To clarify a particular function's handling of references, one should
370 always consult that function's documentation \*(L"man\*(R" page, or failing that
371 the openssl/engine.h header file includes some hints.
373 \&\fIFunctional references\fR
375 As mentioned, functional references exist when the cryptographic
376 functionality of an \s-1ENGINE\s0 is required to be available. A functional
377 reference can be obtained in one of two ways; from an existing structural
378 reference to the required \s-1ENGINE\s0, or by asking OpenSSL for the default
379 operational \s-1ENGINE\s0 for a given cryptographic purpose.
381 To obtain a functional reference from an existing structural reference,
382 call the \fIENGINE_init()\fR function. This returns zero if the \s-1ENGINE\s0 was not
383 already operational and couldn't be successfully initialised (eg. lack of
384 system drivers, no special hardware attached, etc), otherwise it will
385 return non-zero to indicate that the \s-1ENGINE\s0 is now operational and will
386 have allocated a new \fBfunctional\fR reference to the \s-1ENGINE\s0. All functional
387 references are released by calling \fIENGINE_finish()\fR (which removes the
388 implicit structural reference as well).
390 The second way to get a functional reference is by asking OpenSSL for a
391 default implementation for a given task, eg. by \fIENGINE_get_default_RSA()\fR,
392 \&\fIENGINE_get_default_cipher_engine()\fR, etc. These are discussed in the next
393 section, though they are not usually required by application programmers as
394 they are used automatically when creating and using the relevant
395 algorithm-specific types in OpenSSL, such as \s-1RSA\s0, \s-1DSA\s0, \s-1EVP_CIPHER_CTX\s0, etc.
396 .SS "Default implementations"
397 .IX Subsection "Default implementations"
398 For each supported abstraction, the \s-1ENGINE\s0 code maintains an internal table
399 of state to control which implementations are available for a given
400 abstraction and which should be used by default. These implementations are
401 registered in the tables and indexed by an 'nid' value, because
402 abstractions like \s-1EVP_CIPHER\s0 and \s-1EVP_DIGEST\s0 support many distinct
403 algorithms and modes, and ENGINEs can support arbitrarily many of them.
404 In the case of other abstractions like \s-1RSA\s0, \s-1DSA\s0, etc, there is only one
405 \&\*(L"algorithm\*(R" so all implementations implicitly register using the same 'nid'
408 When a default \s-1ENGINE\s0 is requested for a given abstraction/algorithm/mode, (eg.
409 when calling RSA_new_method(\s-1NULL\s0)), a \*(L"get_default\*(R" call will be made to the
410 \&\s-1ENGINE\s0 subsystem to process the corresponding state table and return a
411 functional reference to an initialised \s-1ENGINE\s0 whose implementation should be
412 used. If no \s-1ENGINE\s0 should (or can) be used, it will return \s-1NULL\s0 and the caller
413 will operate with a \s-1NULL\s0 \s-1ENGINE\s0 handle \- this usually equates to using the
414 conventional software implementation. In the latter case, OpenSSL will from
415 then on behave the way it used to before the \s-1ENGINE\s0 \s-1API\s0 existed.
417 Each state table has a flag to note whether it has processed this
418 \&\*(L"get_default\*(R" query since the table was last modified, because to process
419 this question it must iterate across all the registered ENGINEs in the
420 table trying to initialise each of them in turn, in case one of them is
421 operational. If it returns a functional reference to an \s-1ENGINE\s0, it will
422 also cache another reference to speed up processing future queries (without
423 needing to iterate across the table). Likewise, it will cache a \s-1NULL\s0
424 response if no \s-1ENGINE\s0 was available so that future queries won't repeat the
425 same iteration unless the state table changes. This behaviour can also be
426 changed; if the \s-1ENGINE_TABLE_FLAG_NOINIT\s0 flag is set (using
427 \&\fIENGINE_set_table_flags()\fR), no attempted initialisations will take place,
428 instead the only way for the state table to return a non-NULL \s-1ENGINE\s0 to the
429 \&\*(L"get_default\*(R" query will be if one is expressly set in the table. Eg.
430 \&\fIENGINE_set_default_RSA()\fR does the same job as \fIENGINE_register_RSA()\fR except
431 that it also sets the state table's cached response for the \*(L"get_default\*(R"
432 query. In the case of abstractions like \s-1EVP_CIPHER\s0, where implementations are
433 indexed by 'nid', these flags and cached-responses are distinct for each 'nid'
435 .SS "Application requirements"
436 .IX Subsection "Application requirements"
437 This section will explain the basic things an application programmer should
438 support to make the most useful elements of the \s-1ENGINE\s0 functionality
439 available to the user. The first thing to consider is whether the
440 programmer wishes to make alternative \s-1ENGINE\s0 modules available to the
441 application and user. OpenSSL maintains an internal linked list of
442 \&\*(L"visible\*(R" ENGINEs from which it has to operate \- at start-up, this list is
443 empty and in fact if an application does not call any \s-1ENGINE\s0 \s-1API\s0 calls and
444 it uses static linking against openssl, then the resulting application
445 binary will not contain any alternative \s-1ENGINE\s0 code at all. So the first
446 consideration is whether any/all available \s-1ENGINE\s0 implementations should be
447 made visible to OpenSSL \- this is controlled by calling the various \*(L"load\*(R"
451 \& /* Make the "dynamic" ENGINE available */
452 \& void ENGINE_load_dynamic(void);
453 \& /* Make the CryptoSwift hardware acceleration support available */
454 \& void ENGINE_load_cswift(void);
455 \& /* Make support for nCipher\*(Aqs "CHIL" hardware available */
456 \& void ENGINE_load_chil(void);
458 \& /* Make ALL ENGINE implementations bundled with OpenSSL available */
459 \& void ENGINE_load_builtin_engines(void);
462 Having called any of these functions, \s-1ENGINE\s0 objects would have been
463 dynamically allocated and populated with these implementations and linked
464 into OpenSSL's internal linked list. At this point it is important to
465 mention an important \s-1API\s0 function;
468 \& void ENGINE_cleanup(void);
471 If no \s-1ENGINE\s0 \s-1API\s0 functions are called at all in an application, then there
472 are no inherent memory leaks to worry about from the \s-1ENGINE\s0 functionality,
473 however if any ENGINEs are loaded, even if they are never registered or
474 used, it is necessary to use the \fIENGINE_cleanup()\fR function to
475 correspondingly cleanup before program exit, if the caller wishes to avoid
476 memory leaks. This mechanism uses an internal callback registration table
477 so that any \s-1ENGINE\s0 \s-1API\s0 functionality that knows it requires cleanup can
478 register its cleanup details to be called during \fIENGINE_cleanup()\fR. This
479 approach allows \fIENGINE_cleanup()\fR to clean up after any \s-1ENGINE\s0 functionality
480 at all that your program uses, yet doesn't automatically create linker
481 dependencies to all possible \s-1ENGINE\s0 functionality \- only the cleanup
482 callbacks required by the functionality you do use will be required by the
485 The fact that ENGINEs are made visible to OpenSSL (and thus are linked into
486 the program and loaded into memory at run-time) does not mean they are
487 \&\*(L"registered\*(R" or called into use by OpenSSL automatically \- that behaviour
488 is something for the application to control. Some applications
489 will want to allow the user to specify exactly which \s-1ENGINE\s0 they want used
490 if any is to be used at all. Others may prefer to load all support and have
491 OpenSSL automatically use at run-time any \s-1ENGINE\s0 that is able to
492 successfully initialise \- ie. to assume that this corresponds to
493 acceleration hardware attached to the machine or some such thing. There are
494 probably numerous other ways in which applications may prefer to handle
495 things, so we will simply illustrate the consequences as they apply to a
496 couple of simple cases and leave developers to consider these and the
497 source code to openssl's builtin utilities as guides.
499 \&\fIUsing a specific \s-1ENGINE\s0 implementation\fR
501 Here we'll assume an application has been configured by its user or admin
502 to want to use the \*(L"\s-1ACME\s0\*(R" \s-1ENGINE\s0 if it is available in the version of
503 OpenSSL the application was compiled with. If it is available, it should be
504 used by default for all \s-1RSA\s0, \s-1DSA\s0, and symmetric cipher operation, otherwise
505 OpenSSL should use its builtin software as per usual. The following code
506 illustrates how to approach this;
510 \& const char *engine_id = "ACME";
511 \& ENGINE_load_builtin_engines();
512 \& e = ENGINE_by_id(engine_id);
514 \& /* the engine isn\*(Aqt available */
516 \& if(!ENGINE_init(e)) {
517 \& /* the engine couldn\*(Aqt initialise, release \*(Aqe\*(Aq */
521 \& if(!ENGINE_set_default_RSA(e))
522 \& /* This should only happen when \*(Aqe\*(Aq can\*(Aqt initialise, but the previous
523 \& * statement suggests it did. */
525 \& ENGINE_set_default_DSA(e);
526 \& ENGINE_set_default_ciphers(e);
527 \& /* Release the functional reference from ENGINE_init() */
529 \& /* Release the structural reference from ENGINE_by_id() */
533 \&\fIAutomatically using builtin \s-1ENGINE\s0 implementations\fR
535 Here we'll assume we want to load and register all \s-1ENGINE\s0 implementations
536 bundled with OpenSSL, such that for any cryptographic algorithm required by
537 OpenSSL \- if there is an \s-1ENGINE\s0 that implements it and can be initialise,
538 it should be used. The following code illustrates how this can work;
541 \& /* Load all bundled ENGINEs into memory and make them visible */
542 \& ENGINE_load_builtin_engines();
543 \& /* Register all of them for every algorithm they collectively implement */
544 \& ENGINE_register_all_complete();
547 That's all that's required. Eg. the next time OpenSSL tries to set up an
548 \&\s-1RSA\s0 key, any bundled ENGINEs that implement \s-1RSA_METHOD\s0 will be passed to
549 \&\fIENGINE_init()\fR and if any of those succeed, that \s-1ENGINE\s0 will be set as the
550 default for \s-1RSA\s0 use from then on.
551 .SS "Advanced configuration support"
552 .IX Subsection "Advanced configuration support"
553 There is a mechanism supported by the \s-1ENGINE\s0 framework that allows each
554 \&\s-1ENGINE\s0 implementation to define an arbitrary set of configuration
555 \&\*(L"commands\*(R" and expose them to OpenSSL and any applications based on
556 OpenSSL. This mechanism is entirely based on the use of name-value pairs
557 and assumes \s-1ASCII\s0 input (no unicode or \s-1UTF\s0 for now!), so it is ideal if
558 applications want to provide a transparent way for users to provide
559 arbitrary configuration \*(L"directives\*(R" directly to such ENGINEs. It is also
560 possible for the application to dynamically interrogate the loaded \s-1ENGINE\s0
561 implementations for the names, descriptions, and input flags of their
562 available \*(L"control commands\*(R", providing a more flexible configuration
563 scheme. However, if the user is expected to know which \s-1ENGINE\s0 device he/she
564 is using (in the case of specialised hardware, this goes without saying)
565 then applications may not need to concern themselves with discovering the
566 supported control commands and simply prefer to pass settings into ENGINEs
567 exactly as they are provided by the user.
569 Before illustrating how control commands work, it is worth mentioning what
570 they are typically used for. Broadly speaking there are two uses for
571 control commands; the first is to provide the necessary details to the
572 implementation (which may know nothing at all specific to the host system)
573 so that it can be initialised for use. This could include the path to any
574 driver or config files it needs to load, required network addresses,
575 smart-card identifiers, passwords to initialise protected devices,
576 logging information, etc etc. This class of commands typically needs to be
577 passed to an \s-1ENGINE\s0 \fBbefore\fR attempting to initialise it, ie. before
578 calling \fIENGINE_init()\fR. The other class of commands consist of settings or
579 operations that tweak certain behaviour or cause certain operations to take
580 place, and these commands may work either before or after \fIENGINE_init()\fR, or
581 in some cases both. \s-1ENGINE\s0 implementations should provide indications of
582 this in the descriptions attached to builtin control commands and/or in
583 external product documentation.
585 \&\fIIssuing control commands to an \s-1ENGINE\s0\fR
587 Let's illustrate by example; a function for which the caller supplies the
588 name of the \s-1ENGINE\s0 it wishes to use, a table of string-pairs for use before
589 initialisation, and another table for use after initialisation. Note that
590 the string-pairs used for control commands consist of a command \*(L"name\*(R"
591 followed by the command \*(L"parameter\*(R" \- the parameter could be \s-1NULL\s0 in some
592 cases but the name can not. This function should initialise the \s-1ENGINE\s0
593 (issuing the \*(L"pre\*(R" commands beforehand and the \*(L"post\*(R" commands afterwards)
594 and set it as the default for everything except \s-1RAND\s0 and then return a
595 boolean success or failure.
598 \& int generic_load_engine_fn(const char *engine_id,
599 \& const char **pre_cmds, int pre_num,
600 \& const char **post_cmds, int post_num)
602 \& ENGINE *e = ENGINE_by_id(engine_id);
604 \& while(pre_num\-\-) {
605 \& if(!ENGINE_ctrl_cmd_string(e, pre_cmds[0], pre_cmds[1], 0)) {
606 \& fprintf(stderr, "Failed command (%s \- %s:%s)\en", engine_id,
607 \& pre_cmds[0], pre_cmds[1] ? pre_cmds[1] : "(NULL)");
613 \& if(!ENGINE_init(e)) {
614 \& fprintf(stderr, "Failed initialisation\en");
618 \& /* ENGINE_init() returned a functional reference, so free the structural
619 \& * reference from ENGINE_by_id(). */
621 \& while(post_num\-\-) {
622 \& if(!ENGINE_ctrl_cmd_string(e, post_cmds[0], post_cmds[1], 0)) {
623 \& fprintf(stderr, "Failed command (%s \- %s:%s)\en", engine_id,
624 \& post_cmds[0], post_cmds[1] ? post_cmds[1] : "(NULL)");
630 \& ENGINE_set_default(e, ENGINE_METHOD_ALL & ~ENGINE_METHOD_RAND);
636 Note that \fIENGINE_ctrl_cmd_string()\fR accepts a boolean argument that can
637 relax the semantics of the function \- if set non-zero it will only return
638 failure if the \s-1ENGINE\s0 supported the given command name but failed while
639 executing it, if the \s-1ENGINE\s0 doesn't support the command name it will simply
640 return success without doing anything. In this case we assume the user is
641 only supplying commands specific to the given \s-1ENGINE\s0 so we set this to
644 \&\fIDiscovering supported control commands\fR
646 It is possible to discover at run-time the names, numerical-ids, descriptions
647 and input parameters of the control commands supported by an \s-1ENGINE\s0 using a
648 structural reference. Note that some control commands are defined by OpenSSL
649 itself and it will intercept and handle these control commands on behalf of the
650 \&\s-1ENGINE\s0, ie. the \s-1ENGINE\s0's \fIctrl()\fR handler is not used for the control command.
651 openssl/engine.h defines an index, \s-1ENGINE_CMD_BASE\s0, that all control commands
652 implemented by ENGINEs should be numbered from. Any command value lower than
653 this symbol is considered a \*(L"generic\*(R" command is handled directly by the
654 OpenSSL core routines.
656 It is using these \*(L"core\*(R" control commands that one can discover the the control
657 commands implemented by a given \s-1ENGINE\s0, specifically the commands;
660 \& #define ENGINE_HAS_CTRL_FUNCTION 10
661 \& #define ENGINE_CTRL_GET_FIRST_CMD_TYPE 11
662 \& #define ENGINE_CTRL_GET_NEXT_CMD_TYPE 12
663 \& #define ENGINE_CTRL_GET_CMD_FROM_NAME 13
664 \& #define ENGINE_CTRL_GET_NAME_LEN_FROM_CMD 14
665 \& #define ENGINE_CTRL_GET_NAME_FROM_CMD 15
666 \& #define ENGINE_CTRL_GET_DESC_LEN_FROM_CMD 16
667 \& #define ENGINE_CTRL_GET_DESC_FROM_CMD 17
668 \& #define ENGINE_CTRL_GET_CMD_FLAGS 18
671 Whilst these commands are automatically processed by the OpenSSL framework code,
672 they use various properties exposed by each \s-1ENGINE\s0 to process these
673 queries. An \s-1ENGINE\s0 has 3 properties it exposes that can affect how this behaves;
674 it can supply a \fIctrl()\fR handler, it can specify \s-1ENGINE_FLAGS_MANUAL_CMD_CTRL\s0 in
675 the \s-1ENGINE\s0's flags, and it can expose an array of control command descriptions.
676 If an \s-1ENGINE\s0 specifies the \s-1ENGINE_FLAGS_MANUAL_CMD_CTRL\s0 flag, then it will
677 simply pass all these \*(L"core\*(R" control commands directly to the \s-1ENGINE\s0's \fIctrl()\fR
678 handler (and thus, it must have supplied one), so it is up to the \s-1ENGINE\s0 to
679 reply to these \*(L"discovery\*(R" commands itself. If that flag is not set, then the
680 OpenSSL framework code will work with the following rules;
683 \& if no ctrl() handler supplied;
684 \& ENGINE_HAS_CTRL_FUNCTION returns FALSE (zero),
685 \& all other commands fail.
686 \& if a ctrl() handler was supplied but no array of control commands;
687 \& ENGINE_HAS_CTRL_FUNCTION returns TRUE,
688 \& all other commands fail.
689 \& if a ctrl() handler and array of control commands was supplied;
690 \& ENGINE_HAS_CTRL_FUNCTION returns TRUE,
691 \& all other commands proceed processing ...
694 If the \s-1ENGINE\s0's array of control commands is empty then all other commands will
695 fail, otherwise; \s-1ENGINE_CTRL_GET_FIRST_CMD_TYPE\s0 returns the identifier of
696 the first command supported by the \s-1ENGINE\s0, \s-1ENGINE_GET_NEXT_CMD_TYPE\s0 takes the
697 identifier of a command supported by the \s-1ENGINE\s0 and returns the next command
698 identifier or fails if there are no more, \s-1ENGINE_CMD_FROM_NAME\s0 takes a string
699 name for a command and returns the corresponding identifier or fails if no such
700 command name exists, and the remaining commands take a command identifier and
701 return properties of the corresponding commands. All except
702 \&\s-1ENGINE_CTRL_GET_FLAGS\s0 return the string length of a command name or description,
703 or populate a supplied character buffer with a copy of the command name or
704 description. \s-1ENGINE_CTRL_GET_FLAGS\s0 returns a bitwise-OR'd mask of the following
708 \& #define ENGINE_CMD_FLAG_NUMERIC (unsigned int)0x0001
709 \& #define ENGINE_CMD_FLAG_STRING (unsigned int)0x0002
710 \& #define ENGINE_CMD_FLAG_NO_INPUT (unsigned int)0x0004
711 \& #define ENGINE_CMD_FLAG_INTERNAL (unsigned int)0x0008
714 If the \s-1ENGINE_CMD_FLAG_INTERNAL\s0 flag is set, then any other flags are purely
715 informational to the caller \- this flag will prevent the command being usable
716 for any higher-level \s-1ENGINE\s0 functions such as \fIENGINE_ctrl_cmd_string()\fR.
717 \&\*(L"\s-1INTERNAL\s0\*(R" commands are not intended to be exposed to text-based configuration
718 by applications, administrations, users, etc. These can support arbitrary
719 operations via \fIENGINE_ctrl()\fR, including passing to and/or from the control
720 commands data of any arbitrary type. These commands are supported in the
721 discovery mechanisms simply to allow applications determinie if an \s-1ENGINE\s0
722 supports certain specific commands it might want to use (eg. application \*(L"foo\*(R"
723 might query various ENGINEs to see if they implement \*(L"\s-1FOO_GET_VENDOR_LOGO_GIF\s0\*(R" \-
724 and \s-1ENGINE\s0 could therefore decide whether or not to support this \*(L"foo\*(R"\-specific
726 .SS "Future developments"
727 .IX Subsection "Future developments"
728 The \s-1ENGINE\s0 \s-1API\s0 and internal architecture is currently being reviewed. Slated for
729 possible release in 0.9.8 is support for transparent loading of \*(L"dynamic\*(R"
730 ENGINEs (built as self-contained shared-libraries). This would allow \s-1ENGINE\s0
731 implementations to be provided independently of OpenSSL libraries and/or
732 OpenSSL-based applications, and would also remove any requirement for
733 applications to explicitly use the \*(L"dynamic\*(R" \s-1ENGINE\s0 to bind to shared-library
736 .IX Header "SEE ALSO"
737 \&\fIrsa\fR\|(3), \fIdsa\fR\|(3), \fIdh\fR\|(3), \fIrand\fR\|(3)
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