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132 .TH engine 3 "2017-05-25" "1.0.2l" "OpenSSL"
133 .\" For nroff, turn off justification. Always turn off hyphenation; it makes
134 .\" way too many mistakes in technical documents.
138 engine \- ENGINE cryptographic module support
140 .IX Header "SYNOPSIS"
142 \& #include <openssl/engine.h>
144 \& ENGINE *ENGINE_get_first(void);
145 \& ENGINE *ENGINE_get_last(void);
146 \& ENGINE *ENGINE_get_next(ENGINE *e);
147 \& ENGINE *ENGINE_get_prev(ENGINE *e);
149 \& int ENGINE_add(ENGINE *e);
150 \& int ENGINE_remove(ENGINE *e);
152 \& ENGINE *ENGINE_by_id(const char *id);
154 \& int ENGINE_init(ENGINE *e);
155 \& int ENGINE_finish(ENGINE *e);
157 \& void ENGINE_load_openssl(void);
158 \& void ENGINE_load_dynamic(void);
159 \& #ifndef OPENSSL_NO_STATIC_ENGINE
160 \& void ENGINE_load_4758cca(void);
161 \& void ENGINE_load_aep(void);
162 \& void ENGINE_load_atalla(void);
163 \& void ENGINE_load_chil(void);
164 \& void ENGINE_load_cswift(void);
165 \& void ENGINE_load_gmp(void);
166 \& void ENGINE_load_nuron(void);
167 \& void ENGINE_load_sureware(void);
168 \& void ENGINE_load_ubsec(void);
170 \& void ENGINE_load_cryptodev(void);
171 \& void ENGINE_load_builtin_engines(void);
173 \& void ENGINE_cleanup(void);
175 \& ENGINE *ENGINE_get_default_RSA(void);
176 \& ENGINE *ENGINE_get_default_DSA(void);
177 \& ENGINE *ENGINE_get_default_ECDH(void);
178 \& ENGINE *ENGINE_get_default_ECDSA(void);
179 \& ENGINE *ENGINE_get_default_DH(void);
180 \& ENGINE *ENGINE_get_default_RAND(void);
181 \& ENGINE *ENGINE_get_cipher_engine(int nid);
182 \& ENGINE *ENGINE_get_digest_engine(int nid);
184 \& int ENGINE_set_default_RSA(ENGINE *e);
185 \& int ENGINE_set_default_DSA(ENGINE *e);
186 \& int ENGINE_set_default_ECDH(ENGINE *e);
187 \& int ENGINE_set_default_ECDSA(ENGINE *e);
188 \& int ENGINE_set_default_DH(ENGINE *e);
189 \& int ENGINE_set_default_RAND(ENGINE *e);
190 \& int ENGINE_set_default_ciphers(ENGINE *e);
191 \& int ENGINE_set_default_digests(ENGINE *e);
192 \& int ENGINE_set_default_string(ENGINE *e, const char *list);
194 \& int ENGINE_set_default(ENGINE *e, unsigned int flags);
196 \& unsigned int ENGINE_get_table_flags(void);
197 \& void ENGINE_set_table_flags(unsigned int flags);
199 \& int ENGINE_register_RSA(ENGINE *e);
200 \& void ENGINE_unregister_RSA(ENGINE *e);
201 \& void ENGINE_register_all_RSA(void);
202 \& int ENGINE_register_DSA(ENGINE *e);
203 \& void ENGINE_unregister_DSA(ENGINE *e);
204 \& void ENGINE_register_all_DSA(void);
205 \& int ENGINE_register_ECDH(ENGINE *e);
206 \& void ENGINE_unregister_ECDH(ENGINE *e);
207 \& void ENGINE_register_all_ECDH(void);
208 \& int ENGINE_register_ECDSA(ENGINE *e);
209 \& void ENGINE_unregister_ECDSA(ENGINE *e);
210 \& void ENGINE_register_all_ECDSA(void);
211 \& int ENGINE_register_DH(ENGINE *e);
212 \& void ENGINE_unregister_DH(ENGINE *e);
213 \& void ENGINE_register_all_DH(void);
214 \& int ENGINE_register_RAND(ENGINE *e);
215 \& void ENGINE_unregister_RAND(ENGINE *e);
216 \& void ENGINE_register_all_RAND(void);
217 \& int ENGINE_register_STORE(ENGINE *e);
218 \& void ENGINE_unregister_STORE(ENGINE *e);
219 \& void ENGINE_register_all_STORE(void);
220 \& int ENGINE_register_ciphers(ENGINE *e);
221 \& void ENGINE_unregister_ciphers(ENGINE *e);
222 \& void ENGINE_register_all_ciphers(void);
223 \& int ENGINE_register_digests(ENGINE *e);
224 \& void ENGINE_unregister_digests(ENGINE *e);
225 \& void ENGINE_register_all_digests(void);
226 \& int ENGINE_register_complete(ENGINE *e);
227 \& int ENGINE_register_all_complete(void);
229 \& int ENGINE_ctrl(ENGINE *e, int cmd, long i, void *p, void (*f)(void));
230 \& int ENGINE_cmd_is_executable(ENGINE *e, int cmd);
231 \& int ENGINE_ctrl_cmd(ENGINE *e, const char *cmd_name,
232 \& long i, void *p, void (*f)(void), int cmd_optional);
233 \& int ENGINE_ctrl_cmd_string(ENGINE *e, const char *cmd_name, const char *arg,
234 \& int cmd_optional);
236 \& int ENGINE_set_ex_data(ENGINE *e, int idx, void *arg);
237 \& void *ENGINE_get_ex_data(const ENGINE *e, int idx);
239 \& int ENGINE_get_ex_new_index(long argl, void *argp, CRYPTO_EX_new *new_func,
240 \& CRYPTO_EX_dup *dup_func, CRYPTO_EX_free *free_func);
242 \& ENGINE *ENGINE_new(void);
243 \& int ENGINE_free(ENGINE *e);
244 \& int ENGINE_up_ref(ENGINE *e);
246 \& int ENGINE_set_id(ENGINE *e, const char *id);
247 \& int ENGINE_set_name(ENGINE *e, const char *name);
248 \& int ENGINE_set_RSA(ENGINE *e, const RSA_METHOD *rsa_meth);
249 \& int ENGINE_set_DSA(ENGINE *e, const DSA_METHOD *dsa_meth);
250 \& int ENGINE_set_ECDH(ENGINE *e, const ECDH_METHOD *dh_meth);
251 \& int ENGINE_set_ECDSA(ENGINE *e, const ECDSA_METHOD *dh_meth);
252 \& int ENGINE_set_DH(ENGINE *e, const DH_METHOD *dh_meth);
253 \& int ENGINE_set_RAND(ENGINE *e, const RAND_METHOD *rand_meth);
254 \& int ENGINE_set_STORE(ENGINE *e, const STORE_METHOD *rand_meth);
255 \& int ENGINE_set_destroy_function(ENGINE *e, ENGINE_GEN_INT_FUNC_PTR destroy_f);
256 \& int ENGINE_set_init_function(ENGINE *e, ENGINE_GEN_INT_FUNC_PTR init_f);
257 \& int ENGINE_set_finish_function(ENGINE *e, ENGINE_GEN_INT_FUNC_PTR finish_f);
258 \& int ENGINE_set_ctrl_function(ENGINE *e, ENGINE_CTRL_FUNC_PTR ctrl_f);
259 \& int ENGINE_set_load_privkey_function(ENGINE *e, ENGINE_LOAD_KEY_PTR loadpriv_f);
260 \& int ENGINE_set_load_pubkey_function(ENGINE *e, ENGINE_LOAD_KEY_PTR loadpub_f);
261 \& int ENGINE_set_ciphers(ENGINE *e, ENGINE_CIPHERS_PTR f);
262 \& int ENGINE_set_digests(ENGINE *e, ENGINE_DIGESTS_PTR f);
263 \& int ENGINE_set_flags(ENGINE *e, int flags);
264 \& int ENGINE_set_cmd_defns(ENGINE *e, const ENGINE_CMD_DEFN *defns);
266 \& const char *ENGINE_get_id(const ENGINE *e);
267 \& const char *ENGINE_get_name(const ENGINE *e);
268 \& const RSA_METHOD *ENGINE_get_RSA(const ENGINE *e);
269 \& const DSA_METHOD *ENGINE_get_DSA(const ENGINE *e);
270 \& const ECDH_METHOD *ENGINE_get_ECDH(const ENGINE *e);
271 \& const ECDSA_METHOD *ENGINE_get_ECDSA(const ENGINE *e);
272 \& const DH_METHOD *ENGINE_get_DH(const ENGINE *e);
273 \& const RAND_METHOD *ENGINE_get_RAND(const ENGINE *e);
274 \& const STORE_METHOD *ENGINE_get_STORE(const ENGINE *e);
275 \& ENGINE_GEN_INT_FUNC_PTR ENGINE_get_destroy_function(const ENGINE *e);
276 \& ENGINE_GEN_INT_FUNC_PTR ENGINE_get_init_function(const ENGINE *e);
277 \& ENGINE_GEN_INT_FUNC_PTR ENGINE_get_finish_function(const ENGINE *e);
278 \& ENGINE_CTRL_FUNC_PTR ENGINE_get_ctrl_function(const ENGINE *e);
279 \& ENGINE_LOAD_KEY_PTR ENGINE_get_load_privkey_function(const ENGINE *e);
280 \& ENGINE_LOAD_KEY_PTR ENGINE_get_load_pubkey_function(const ENGINE *e);
281 \& ENGINE_CIPHERS_PTR ENGINE_get_ciphers(const ENGINE *e);
282 \& ENGINE_DIGESTS_PTR ENGINE_get_digests(const ENGINE *e);
283 \& const EVP_CIPHER *ENGINE_get_cipher(ENGINE *e, int nid);
284 \& const EVP_MD *ENGINE_get_digest(ENGINE *e, int nid);
285 \& int ENGINE_get_flags(const ENGINE *e);
286 \& const ENGINE_CMD_DEFN *ENGINE_get_cmd_defns(const ENGINE *e);
288 \& EVP_PKEY *ENGINE_load_private_key(ENGINE *e, const char *key_id,
289 \& UI_METHOD *ui_method, void *callback_data);
290 \& EVP_PKEY *ENGINE_load_public_key(ENGINE *e, const char *key_id,
291 \& UI_METHOD *ui_method, void *callback_data);
293 \& void ENGINE_add_conf_module(void);
296 .IX Header "DESCRIPTION"
297 These functions create, manipulate, and use cryptographic modules in the
298 form of \fB\s-1ENGINE\s0\fR objects. These objects act as containers for
299 implementations of cryptographic algorithms, and support a
300 reference-counted mechanism to allow them to be dynamically loaded in and
301 out of the running application.
303 The cryptographic functionality that can be provided by an \fB\s-1ENGINE\s0\fR
304 implementation includes the following abstractions;
307 \& RSA_METHOD \- for providing alternative RSA implementations
308 \& DSA_METHOD, DH_METHOD, RAND_METHOD, ECDH_METHOD, ECDSA_METHOD,
309 \& STORE_METHOD \- similarly for other OpenSSL APIs
310 \& EVP_CIPHER \- potentially multiple cipher algorithms (indexed by \*(Aqnid\*(Aq)
311 \& EVP_DIGEST \- potentially multiple hash algorithms (indexed by \*(Aqnid\*(Aq)
312 \& key\-loading \- loading public and/or private EVP_PKEY keys
314 .SS "Reference counting and handles"
315 .IX Subsection "Reference counting and handles"
316 Due to the modular nature of the \s-1ENGINE API,\s0 pointers to ENGINEs need to be
317 treated as handles \- ie. not only as pointers, but also as references to
318 the underlying \s-1ENGINE\s0 object. Ie. one should obtain a new reference when
319 making copies of an \s-1ENGINE\s0 pointer if the copies will be used (and
320 released) independently.
322 \&\s-1ENGINE\s0 objects have two levels of reference-counting to match the way in
323 which the objects are used. At the most basic level, each \s-1ENGINE\s0 pointer is
324 inherently a \fBstructural\fR reference \- a structural reference is required
325 to use the pointer value at all, as this kind of reference is a guarantee
326 that the structure can not be deallocated until the reference is released.
328 However, a structural reference provides no guarantee that the \s-1ENGINE\s0 is
329 initialised and able to use any of its cryptographic
330 implementations. Indeed it's quite possible that most ENGINEs will not
331 initialise at all in typical environments, as ENGINEs are typically used to
332 support specialised hardware. To use an \s-1ENGINE\s0's functionality, you need a
333 \&\fBfunctional\fR reference. This kind of reference can be considered a
334 specialised form of structural reference, because each functional reference
335 implicitly contains a structural reference as well \- however to avoid
336 difficult-to-find programming bugs, it is recommended to treat the two
337 kinds of reference independently. If you have a functional reference to an
338 \&\s-1ENGINE,\s0 you have a guarantee that the \s-1ENGINE\s0 has been initialised and
339 is ready to perform cryptographic operations, and will remain initialised
340 until after you have released your reference.
342 \&\fIStructural references\fR
344 This basic type of reference is used for instantiating new ENGINEs,
345 iterating across OpenSSL's internal linked-list of loaded
346 ENGINEs, reading information about an \s-1ENGINE,\s0 etc. Essentially a structural
347 reference is sufficient if you only need to query or manipulate the data of
348 an \s-1ENGINE\s0 implementation rather than use its functionality.
350 The \fIENGINE_new()\fR function returns a structural reference to a new (empty)
351 \&\s-1ENGINE\s0 object. There are other \s-1ENGINE API\s0 functions that return structural
352 references such as; \fIENGINE_by_id()\fR, \fIENGINE_get_first()\fR, \fIENGINE_get_last()\fR,
353 \&\fIENGINE_get_next()\fR, \fIENGINE_get_prev()\fR. All structural references should be
354 released by a corresponding to call to the \fIENGINE_free()\fR function \- the
355 \&\s-1ENGINE\s0 object itself will only actually be cleaned up and deallocated when
356 the last structural reference is released.
358 It should also be noted that many \s-1ENGINE API\s0 function calls that accept a
359 structural reference will internally obtain another reference \- typically
360 this happens whenever the supplied \s-1ENGINE\s0 will be needed by OpenSSL after
361 the function has returned. Eg. the function to add a new \s-1ENGINE\s0 to
362 OpenSSL's internal list is \fIENGINE_add()\fR \- if this function returns success,
363 then OpenSSL will have stored a new structural reference internally so the
364 caller is still responsible for freeing their own reference with
365 \&\fIENGINE_free()\fR when they are finished with it. In a similar way, some
366 functions will automatically release the structural reference passed to it
367 if part of the function's job is to do so. Eg. the \fIENGINE_get_next()\fR and
368 \&\fIENGINE_get_prev()\fR functions are used for iterating across the internal
369 \&\s-1ENGINE\s0 list \- they will return a new structural reference to the next (or
370 previous) \s-1ENGINE\s0 in the list or \s-1NULL\s0 if at the end (or beginning) of the
371 list, but in either case the structural reference passed to the function is
372 released on behalf of the caller.
374 To clarify a particular function's handling of references, one should
375 always consult that function's documentation \*(L"man\*(R" page, or failing that
376 the openssl/engine.h header file includes some hints.
378 \&\fIFunctional references\fR
380 As mentioned, functional references exist when the cryptographic
381 functionality of an \s-1ENGINE\s0 is required to be available. A functional
382 reference can be obtained in one of two ways; from an existing structural
383 reference to the required \s-1ENGINE,\s0 or by asking OpenSSL for the default
384 operational \s-1ENGINE\s0 for a given cryptographic purpose.
386 To obtain a functional reference from an existing structural reference,
387 call the \fIENGINE_init()\fR function. This returns zero if the \s-1ENGINE\s0 was not
388 already operational and couldn't be successfully initialised (eg. lack of
389 system drivers, no special hardware attached, etc), otherwise it will
390 return non-zero to indicate that the \s-1ENGINE\s0 is now operational and will
391 have allocated a new \fBfunctional\fR reference to the \s-1ENGINE.\s0 All functional
392 references are released by calling \fIENGINE_finish()\fR (which removes the
393 implicit structural reference as well).
395 The second way to get a functional reference is by asking OpenSSL for a
396 default implementation for a given task, eg. by \fIENGINE_get_default_RSA()\fR,
397 \&\fIENGINE_get_default_cipher_engine()\fR, etc. These are discussed in the next
398 section, though they are not usually required by application programmers as
399 they are used automatically when creating and using the relevant
400 algorithm-specific types in OpenSSL, such as \s-1RSA, DSA, EVP_CIPHER_CTX,\s0 etc.
401 .SS "Default implementations"
402 .IX Subsection "Default implementations"
403 For each supported abstraction, the \s-1ENGINE\s0 code maintains an internal table
404 of state to control which implementations are available for a given
405 abstraction and which should be used by default. These implementations are
406 registered in the tables and indexed by an 'nid' value, because
407 abstractions like \s-1EVP_CIPHER\s0 and \s-1EVP_DIGEST\s0 support many distinct
408 algorithms and modes, and ENGINEs can support arbitrarily many of them.
409 In the case of other abstractions like \s-1RSA, DSA,\s0 etc, there is only one
410 \&\*(L"algorithm\*(R" so all implementations implicitly register using the same 'nid'
413 When a default \s-1ENGINE\s0 is requested for a given abstraction/algorithm/mode, (eg.
414 when calling RSA_new_method(\s-1NULL\s0)), a \*(L"get_default\*(R" call will be made to the
415 \&\s-1ENGINE\s0 subsystem to process the corresponding state table and return a
416 functional reference to an initialised \s-1ENGINE\s0 whose implementation should be
417 used. If no \s-1ENGINE\s0 should (or can) be used, it will return \s-1NULL\s0 and the caller
418 will operate with a \s-1NULL ENGINE\s0 handle \- this usually equates to using the
419 conventional software implementation. In the latter case, OpenSSL will from
420 then on behave the way it used to before the \s-1ENGINE API\s0 existed.
422 Each state table has a flag to note whether it has processed this
423 \&\*(L"get_default\*(R" query since the table was last modified, because to process
424 this question it must iterate across all the registered ENGINEs in the
425 table trying to initialise each of them in turn, in case one of them is
426 operational. If it returns a functional reference to an \s-1ENGINE,\s0 it will
427 also cache another reference to speed up processing future queries (without
428 needing to iterate across the table). Likewise, it will cache a \s-1NULL\s0
429 response if no \s-1ENGINE\s0 was available so that future queries won't repeat the
430 same iteration unless the state table changes. This behaviour can also be
431 changed; if the \s-1ENGINE_TABLE_FLAG_NOINIT\s0 flag is set (using
432 \&\fIENGINE_set_table_flags()\fR), no attempted initialisations will take place,
433 instead the only way for the state table to return a non-NULL \s-1ENGINE\s0 to the
434 \&\*(L"get_default\*(R" query will be if one is expressly set in the table. Eg.
435 \&\fIENGINE_set_default_RSA()\fR does the same job as \fIENGINE_register_RSA()\fR except
436 that it also sets the state table's cached response for the \*(L"get_default\*(R"
437 query. In the case of abstractions like \s-1EVP_CIPHER,\s0 where implementations are
438 indexed by 'nid', these flags and cached-responses are distinct for each 'nid'
440 .SS "Application requirements"
441 .IX Subsection "Application requirements"
442 This section will explain the basic things an application programmer should
443 support to make the most useful elements of the \s-1ENGINE\s0 functionality
444 available to the user. The first thing to consider is whether the
445 programmer wishes to make alternative \s-1ENGINE\s0 modules available to the
446 application and user. OpenSSL maintains an internal linked list of
447 \&\*(L"visible\*(R" ENGINEs from which it has to operate \- at start-up, this list is
448 empty and in fact if an application does not call any \s-1ENGINE API\s0 calls and
449 it uses static linking against openssl, then the resulting application
450 binary will not contain any alternative \s-1ENGINE\s0 code at all. So the first
451 consideration is whether any/all available \s-1ENGINE\s0 implementations should be
452 made visible to OpenSSL \- this is controlled by calling the various \*(L"load\*(R"
456 \& /* Make the "dynamic" ENGINE available */
457 \& void ENGINE_load_dynamic(void);
458 \& /* Make the CryptoSwift hardware acceleration support available */
459 \& void ENGINE_load_cswift(void);
460 \& /* Make support for nCipher\*(Aqs "CHIL" hardware available */
461 \& void ENGINE_load_chil(void);
463 \& /* Make ALL ENGINE implementations bundled with OpenSSL available */
464 \& void ENGINE_load_builtin_engines(void);
467 Having called any of these functions, \s-1ENGINE\s0 objects would have been
468 dynamically allocated and populated with these implementations and linked
469 into OpenSSL's internal linked list. At this point it is important to
470 mention an important \s-1API\s0 function;
473 \& void ENGINE_cleanup(void);
476 If no \s-1ENGINE API\s0 functions are called at all in an application, then there
477 are no inherent memory leaks to worry about from the \s-1ENGINE\s0 functionality,
478 however if any ENGINEs are loaded, even if they are never registered or
479 used, it is necessary to use the \fIENGINE_cleanup()\fR function to
480 correspondingly cleanup before program exit, if the caller wishes to avoid
481 memory leaks. This mechanism uses an internal callback registration table
482 so that any \s-1ENGINE API\s0 functionality that knows it requires cleanup can
483 register its cleanup details to be called during \fIENGINE_cleanup()\fR. This
484 approach allows \fIENGINE_cleanup()\fR to clean up after any \s-1ENGINE\s0 functionality
485 at all that your program uses, yet doesn't automatically create linker
486 dependencies to all possible \s-1ENGINE\s0 functionality \- only the cleanup
487 callbacks required by the functionality you do use will be required by the
490 The fact that ENGINEs are made visible to OpenSSL (and thus are linked into
491 the program and loaded into memory at run-time) does not mean they are
492 \&\*(L"registered\*(R" or called into use by OpenSSL automatically \- that behaviour
493 is something for the application to control. Some applications
494 will want to allow the user to specify exactly which \s-1ENGINE\s0 they want used
495 if any is to be used at all. Others may prefer to load all support and have
496 OpenSSL automatically use at run-time any \s-1ENGINE\s0 that is able to
497 successfully initialise \- ie. to assume that this corresponds to
498 acceleration hardware attached to the machine or some such thing. There are
499 probably numerous other ways in which applications may prefer to handle
500 things, so we will simply illustrate the consequences as they apply to a
501 couple of simple cases and leave developers to consider these and the
502 source code to openssl's builtin utilities as guides.
504 \&\fIUsing a specific \s-1ENGINE\s0 implementation\fR
506 Here we'll assume an application has been configured by its user or admin
507 to want to use the \*(L"\s-1ACME\*(R" ENGINE\s0 if it is available in the version of
508 OpenSSL the application was compiled with. If it is available, it should be
509 used by default for all \s-1RSA, DSA,\s0 and symmetric cipher operations, otherwise
510 OpenSSL should use its builtin software as per usual. The following code
511 illustrates how to approach this;
515 \& const char *engine_id = "ACME";
516 \& ENGINE_load_builtin_engines();
517 \& e = ENGINE_by_id(engine_id);
519 \& /* the engine isn\*(Aqt available */
521 \& if(!ENGINE_init(e)) {
522 \& /* the engine couldn\*(Aqt initialise, release \*(Aqe\*(Aq */
526 \& if(!ENGINE_set_default_RSA(e))
527 \& /* This should only happen when \*(Aqe\*(Aq can\*(Aqt initialise, but the previous
528 \& * statement suggests it did. */
530 \& ENGINE_set_default_DSA(e);
531 \& ENGINE_set_default_ciphers(e);
532 \& /* Release the functional reference from ENGINE_init() */
534 \& /* Release the structural reference from ENGINE_by_id() */
538 \&\fIAutomatically using builtin \s-1ENGINE\s0 implementations\fR
540 Here we'll assume we want to load and register all \s-1ENGINE\s0 implementations
541 bundled with OpenSSL, such that for any cryptographic algorithm required by
542 OpenSSL \- if there is an \s-1ENGINE\s0 that implements it and can be initialised,
543 it should be used. The following code illustrates how this can work;
546 \& /* Load all bundled ENGINEs into memory and make them visible */
547 \& ENGINE_load_builtin_engines();
548 \& /* Register all of them for every algorithm they collectively implement */
549 \& ENGINE_register_all_complete();
552 That's all that's required. Eg. the next time OpenSSL tries to set up an
553 \&\s-1RSA\s0 key, any bundled ENGINEs that implement \s-1RSA_METHOD\s0 will be passed to
554 \&\fIENGINE_init()\fR and if any of those succeed, that \s-1ENGINE\s0 will be set as the
555 default for \s-1RSA\s0 use from then on.
556 .SS "Advanced configuration support"
557 .IX Subsection "Advanced configuration support"
558 There is a mechanism supported by the \s-1ENGINE\s0 framework that allows each
559 \&\s-1ENGINE\s0 implementation to define an arbitrary set of configuration
560 \&\*(L"commands\*(R" and expose them to OpenSSL and any applications based on
561 OpenSSL. This mechanism is entirely based on the use of name-value pairs
562 and assumes \s-1ASCII\s0 input (no unicode or \s-1UTF\s0 for now!), so it is ideal if
563 applications want to provide a transparent way for users to provide
564 arbitrary configuration \*(L"directives\*(R" directly to such ENGINEs. It is also
565 possible for the application to dynamically interrogate the loaded \s-1ENGINE\s0
566 implementations for the names, descriptions, and input flags of their
567 available \*(L"control commands\*(R", providing a more flexible configuration
568 scheme. However, if the user is expected to know which \s-1ENGINE\s0 device he/she
569 is using (in the case of specialised hardware, this goes without saying)
570 then applications may not need to concern themselves with discovering the
571 supported control commands and simply prefer to pass settings into ENGINEs
572 exactly as they are provided by the user.
574 Before illustrating how control commands work, it is worth mentioning what
575 they are typically used for. Broadly speaking there are two uses for
576 control commands; the first is to provide the necessary details to the
577 implementation (which may know nothing at all specific to the host system)
578 so that it can be initialised for use. This could include the path to any
579 driver or config files it needs to load, required network addresses,
580 smart-card identifiers, passwords to initialise protected devices,
581 logging information, etc etc. This class of commands typically needs to be
582 passed to an \s-1ENGINE \s0\fBbefore\fR attempting to initialise it, ie. before
583 calling \fIENGINE_init()\fR. The other class of commands consist of settings or
584 operations that tweak certain behaviour or cause certain operations to take
585 place, and these commands may work either before or after \fIENGINE_init()\fR, or
586 in some cases both. \s-1ENGINE\s0 implementations should provide indications of
587 this in the descriptions attached to builtin control commands and/or in
588 external product documentation.
590 \&\fIIssuing control commands to an \s-1ENGINE\s0\fR
592 Let's illustrate by example; a function for which the caller supplies the
593 name of the \s-1ENGINE\s0 it wishes to use, a table of string-pairs for use before
594 initialisation, and another table for use after initialisation. Note that
595 the string-pairs used for control commands consist of a command \*(L"name\*(R"
596 followed by the command \*(L"parameter\*(R" \- the parameter could be \s-1NULL\s0 in some
597 cases but the name can not. This function should initialise the \s-1ENGINE
598 \&\s0(issuing the \*(L"pre\*(R" commands beforehand and the \*(L"post\*(R" commands afterwards)
599 and set it as the default for everything except \s-1RAND\s0 and then return a
600 boolean success or failure.
603 \& int generic_load_engine_fn(const char *engine_id,
604 \& const char **pre_cmds, int pre_num,
605 \& const char **post_cmds, int post_num)
607 \& ENGINE *e = ENGINE_by_id(engine_id);
609 \& while(pre_num\-\-) {
610 \& if(!ENGINE_ctrl_cmd_string(e, pre_cmds[0], pre_cmds[1], 0)) {
611 \& fprintf(stderr, "Failed command (%s \- %s:%s)\en", engine_id,
612 \& pre_cmds[0], pre_cmds[1] ? pre_cmds[1] : "(NULL)");
618 \& if(!ENGINE_init(e)) {
619 \& fprintf(stderr, "Failed initialisation\en");
623 \& /* ENGINE_init() returned a functional reference, so free the structural
624 \& * reference from ENGINE_by_id(). */
626 \& while(post_num\-\-) {
627 \& if(!ENGINE_ctrl_cmd_string(e, post_cmds[0], post_cmds[1], 0)) {
628 \& fprintf(stderr, "Failed command (%s \- %s:%s)\en", engine_id,
629 \& post_cmds[0], post_cmds[1] ? post_cmds[1] : "(NULL)");
635 \& ENGINE_set_default(e, ENGINE_METHOD_ALL & ~ENGINE_METHOD_RAND);
641 Note that \fIENGINE_ctrl_cmd_string()\fR accepts a boolean argument that can
642 relax the semantics of the function \- if set non-zero it will only return
643 failure if the \s-1ENGINE\s0 supported the given command name but failed while
644 executing it, if the \s-1ENGINE\s0 doesn't support the command name it will simply
645 return success without doing anything. In this case we assume the user is
646 only supplying commands specific to the given \s-1ENGINE\s0 so we set this to
649 \&\fIDiscovering supported control commands\fR
651 It is possible to discover at run-time the names, numerical-ids, descriptions
652 and input parameters of the control commands supported by an \s-1ENGINE\s0 using a
653 structural reference. Note that some control commands are defined by OpenSSL
654 itself and it will intercept and handle these control commands on behalf of the
655 \&\s-1ENGINE,\s0 ie. the \s-1ENGINE\s0's \fIctrl()\fR handler is not used for the control command.
656 openssl/engine.h defines an index, \s-1ENGINE_CMD_BASE,\s0 that all control commands
657 implemented by ENGINEs should be numbered from. Any command value lower than
658 this symbol is considered a \*(L"generic\*(R" command is handled directly by the
659 OpenSSL core routines.
661 It is using these \*(L"core\*(R" control commands that one can discover the control
662 commands implemented by a given \s-1ENGINE,\s0 specifically the commands;
665 \& #define ENGINE_HAS_CTRL_FUNCTION 10
666 \& #define ENGINE_CTRL_GET_FIRST_CMD_TYPE 11
667 \& #define ENGINE_CTRL_GET_NEXT_CMD_TYPE 12
668 \& #define ENGINE_CTRL_GET_CMD_FROM_NAME 13
669 \& #define ENGINE_CTRL_GET_NAME_LEN_FROM_CMD 14
670 \& #define ENGINE_CTRL_GET_NAME_FROM_CMD 15
671 \& #define ENGINE_CTRL_GET_DESC_LEN_FROM_CMD 16
672 \& #define ENGINE_CTRL_GET_DESC_FROM_CMD 17
673 \& #define ENGINE_CTRL_GET_CMD_FLAGS 18
676 Whilst these commands are automatically processed by the OpenSSL framework code,
677 they use various properties exposed by each \s-1ENGINE\s0 to process these
678 queries. An \s-1ENGINE\s0 has 3 properties it exposes that can affect how this behaves;
679 it can supply a \fIctrl()\fR handler, it can specify \s-1ENGINE_FLAGS_MANUAL_CMD_CTRL\s0 in
680 the \s-1ENGINE\s0's flags, and it can expose an array of control command descriptions.
681 If an \s-1ENGINE\s0 specifies the \s-1ENGINE_FLAGS_MANUAL_CMD_CTRL\s0 flag, then it will
682 simply pass all these \*(L"core\*(R" control commands directly to the \s-1ENGINE\s0's \fIctrl()\fR
683 handler (and thus, it must have supplied one), so it is up to the \s-1ENGINE\s0 to
684 reply to these \*(L"discovery\*(R" commands itself. If that flag is not set, then the
685 OpenSSL framework code will work with the following rules;
688 \& if no ctrl() handler supplied;
689 \& ENGINE_HAS_CTRL_FUNCTION returns FALSE (zero),
690 \& all other commands fail.
691 \& if a ctrl() handler was supplied but no array of control commands;
692 \& ENGINE_HAS_CTRL_FUNCTION returns TRUE,
693 \& all other commands fail.
694 \& if a ctrl() handler and array of control commands was supplied;
695 \& ENGINE_HAS_CTRL_FUNCTION returns TRUE,
696 \& all other commands proceed processing ...
699 If the \s-1ENGINE\s0's array of control commands is empty then all other commands will
700 fail, otherwise; \s-1ENGINE_CTRL_GET_FIRST_CMD_TYPE\s0 returns the identifier of
701 the first command supported by the \s-1ENGINE, ENGINE_GET_NEXT_CMD_TYPE\s0 takes the
702 identifier of a command supported by the \s-1ENGINE\s0 and returns the next command
703 identifier or fails if there are no more, \s-1ENGINE_CMD_FROM_NAME\s0 takes a string
704 name for a command and returns the corresponding identifier or fails if no such
705 command name exists, and the remaining commands take a command identifier and
706 return properties of the corresponding commands. All except
707 \&\s-1ENGINE_CTRL_GET_FLAGS\s0 return the string length of a command name or description,
708 or populate a supplied character buffer with a copy of the command name or
709 description. \s-1ENGINE_CTRL_GET_FLAGS\s0 returns a bitwise-OR'd mask of the following
713 \& #define ENGINE_CMD_FLAG_NUMERIC (unsigned int)0x0001
714 \& #define ENGINE_CMD_FLAG_STRING (unsigned int)0x0002
715 \& #define ENGINE_CMD_FLAG_NO_INPUT (unsigned int)0x0004
716 \& #define ENGINE_CMD_FLAG_INTERNAL (unsigned int)0x0008
719 If the \s-1ENGINE_CMD_FLAG_INTERNAL\s0 flag is set, then any other flags are purely
720 informational to the caller \- this flag will prevent the command being usable
721 for any higher-level \s-1ENGINE\s0 functions such as \fIENGINE_ctrl_cmd_string()\fR.
722 \&\*(L"\s-1INTERNAL\*(R"\s0 commands are not intended to be exposed to text-based configuration
723 by applications, administrations, users, etc. These can support arbitrary
724 operations via \fIENGINE_ctrl()\fR, including passing to and/or from the control
725 commands data of any arbitrary type. These commands are supported in the
726 discovery mechanisms simply to allow applications determinie if an \s-1ENGINE\s0
727 supports certain specific commands it might want to use (eg. application \*(L"foo\*(R"
728 might query various ENGINEs to see if they implement \*(L"\s-1FOO_GET_VENDOR_LOGO_GIF\*(R" \-\s0
729 and \s-1ENGINE\s0 could therefore decide whether or not to support this \*(L"foo\*(R"\-specific
731 .SS "Future developments"
732 .IX Subsection "Future developments"
733 The \s-1ENGINE API\s0 and internal architecture is currently being reviewed. Slated for
734 possible release in 0.9.8 is support for transparent loading of \*(L"dynamic\*(R"
735 ENGINEs (built as self-contained shared-libraries). This would allow \s-1ENGINE\s0
736 implementations to be provided independently of OpenSSL libraries and/or
737 OpenSSL-based applications, and would also remove any requirement for
738 applications to explicitly use the \*(L"dynamic\*(R" \s-1ENGINE\s0 to bind to shared-library
741 .IX Header "SEE ALSO"
742 \&\fIrsa\fR\|(3), \fIdsa\fR\|(3), \fIdh\fR\|(3), \fIrand\fR\|(3)