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141 .TH engine 3 "0.9.7a" "2003-02-19" "OpenSSL"
144 engine \- \s-1ENGINE\s0 cryptographic module support
146 .IX Header "SYNOPSIS"
148 \& #include <openssl/engine.h>
151 \& ENGINE *ENGINE_get_first(void);
152 \& ENGINE *ENGINE_get_last(void);
153 \& ENGINE *ENGINE_get_next(ENGINE *e);
154 \& ENGINE *ENGINE_get_prev(ENGINE *e);
157 \& int ENGINE_add(ENGINE *e);
158 \& int ENGINE_remove(ENGINE *e);
161 \& ENGINE *ENGINE_by_id(const char *id);
164 \& int ENGINE_init(ENGINE *e);
165 \& int ENGINE_finish(ENGINE *e);
168 \& void ENGINE_load_openssl(void);
169 \& void ENGINE_load_dynamic(void);
170 \& void ENGINE_load_cswift(void);
171 \& void ENGINE_load_chil(void);
172 \& void ENGINE_load_atalla(void);
173 \& void ENGINE_load_nuron(void);
174 \& void ENGINE_load_ubsec(void);
175 \& void ENGINE_load_aep(void);
176 \& void ENGINE_load_sureware(void);
177 \& void ENGINE_load_4758cca(void);
178 \& void ENGINE_load_openbsd_dev_crypto(void);
179 \& void ENGINE_load_builtin_engines(void);
182 \& void ENGINE_cleanup(void);
185 \& ENGINE *ENGINE_get_default_RSA(void);
186 \& ENGINE *ENGINE_get_default_DSA(void);
187 \& ENGINE *ENGINE_get_default_DH(void);
188 \& ENGINE *ENGINE_get_default_RAND(void);
189 \& ENGINE *ENGINE_get_cipher_engine(int nid);
190 \& ENGINE *ENGINE_get_digest_engine(int nid);
193 \& int ENGINE_set_default_RSA(ENGINE *e);
194 \& int ENGINE_set_default_DSA(ENGINE *e);
195 \& int ENGINE_set_default_DH(ENGINE *e);
196 \& int ENGINE_set_default_RAND(ENGINE *e);
197 \& int ENGINE_set_default_ciphers(ENGINE *e);
198 \& int ENGINE_set_default_digests(ENGINE *e);
199 \& int ENGINE_set_default_string(ENGINE *e, const char *list);
202 \& int ENGINE_set_default(ENGINE *e, unsigned int flags);
205 \& unsigned int ENGINE_get_table_flags(void);
206 \& void ENGINE_set_table_flags(unsigned int flags);
209 \& int ENGINE_register_RSA(ENGINE *e);
210 \& void ENGINE_unregister_RSA(ENGINE *e);
211 \& void ENGINE_register_all_RSA(void);
212 \& int ENGINE_register_DSA(ENGINE *e);
213 \& void ENGINE_unregister_DSA(ENGINE *e);
214 \& void ENGINE_register_all_DSA(void);
215 \& int ENGINE_register_DH(ENGINE *e);
216 \& void ENGINE_unregister_DH(ENGINE *e);
217 \& void ENGINE_register_all_DH(void);
218 \& int ENGINE_register_RAND(ENGINE *e);
219 \& void ENGINE_unregister_RAND(ENGINE *e);
220 \& void ENGINE_register_all_RAND(void);
221 \& int ENGINE_register_ciphers(ENGINE *e);
222 \& void ENGINE_unregister_ciphers(ENGINE *e);
223 \& void ENGINE_register_all_ciphers(void);
224 \& int ENGINE_register_digests(ENGINE *e);
225 \& void ENGINE_unregister_digests(ENGINE *e);
226 \& void ENGINE_register_all_digests(void);
227 \& int ENGINE_register_complete(ENGINE *e);
228 \& int ENGINE_register_all_complete(void);
231 \& int ENGINE_ctrl(ENGINE *e, int cmd, long i, void *p, void (*f)());
232 \& int ENGINE_cmd_is_executable(ENGINE *e, int cmd);
233 \& int ENGINE_ctrl_cmd(ENGINE *e, const char *cmd_name,
234 \& long i, void *p, void (*f)(), int cmd_optional);
235 \& int ENGINE_ctrl_cmd_string(ENGINE *e, const char *cmd_name, const char *arg,
236 \& int cmd_optional);
239 \& int ENGINE_set_ex_data(ENGINE *e, int idx, void *arg);
240 \& void *ENGINE_get_ex_data(const ENGINE *e, int idx);
243 \& int ENGINE_get_ex_new_index(long argl, void *argp, CRYPTO_EX_new *new_func,
244 \& CRYPTO_EX_dup *dup_func, CRYPTO_EX_free *free_func);
247 \& ENGINE *ENGINE_new(void);
248 \& int ENGINE_free(ENGINE *e);
251 \& int ENGINE_set_id(ENGINE *e, const char *id);
252 \& int ENGINE_set_name(ENGINE *e, const char *name);
253 \& int ENGINE_set_RSA(ENGINE *e, const RSA_METHOD *rsa_meth);
254 \& int ENGINE_set_DSA(ENGINE *e, const DSA_METHOD *dsa_meth);
255 \& int ENGINE_set_DH(ENGINE *e, const DH_METHOD *dh_meth);
256 \& int ENGINE_set_RAND(ENGINE *e, const RAND_METHOD *rand_meth);
257 \& int ENGINE_set_destroy_function(ENGINE *e, ENGINE_GEN_INT_FUNC_PTR destroy_f);
258 \& int ENGINE_set_init_function(ENGINE *e, ENGINE_GEN_INT_FUNC_PTR init_f);
259 \& int ENGINE_set_finish_function(ENGINE *e, ENGINE_GEN_INT_FUNC_PTR finish_f);
260 \& int ENGINE_set_ctrl_function(ENGINE *e, ENGINE_CTRL_FUNC_PTR ctrl_f);
261 \& int ENGINE_set_load_privkey_function(ENGINE *e, ENGINE_LOAD_KEY_PTR loadpriv_f);
262 \& int ENGINE_set_load_pubkey_function(ENGINE *e, ENGINE_LOAD_KEY_PTR loadpub_f);
263 \& int ENGINE_set_ciphers(ENGINE *e, ENGINE_CIPHERS_PTR f);
264 \& int ENGINE_set_digests(ENGINE *e, ENGINE_DIGESTS_PTR f);
265 \& int ENGINE_set_flags(ENGINE *e, int flags);
266 \& int ENGINE_set_cmd_defns(ENGINE *e, const ENGINE_CMD_DEFN *defns);
269 \& const char *ENGINE_get_id(const ENGINE *e);
270 \& const char *ENGINE_get_name(const ENGINE *e);
271 \& const RSA_METHOD *ENGINE_get_RSA(const ENGINE *e);
272 \& const DSA_METHOD *ENGINE_get_DSA(const ENGINE *e);
273 \& const DH_METHOD *ENGINE_get_DH(const ENGINE *e);
274 \& const RAND_METHOD *ENGINE_get_RAND(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);
289 \& EVP_PKEY *ENGINE_load_private_key(ENGINE *e, const char *key_id,
290 \& UI_METHOD *ui_method, void *callback_data);
291 \& EVP_PKEY *ENGINE_load_public_key(ENGINE *e, const char *key_id,
292 \& UI_METHOD *ui_method, void *callback_data);
295 \& void ENGINE_add_conf_module(void);
298 .IX Header "DESCRIPTION"
299 These functions create, manipulate, and use cryptographic modules in the
300 form of \fB\s-1ENGINE\s0\fR objects. These objects act as containers for
301 implementations of cryptographic algorithms, and support a
302 reference-counted mechanism to allow them to be dynamically loaded in and
303 out of the running application.
305 The cryptographic functionality that can be provided by an \fB\s-1ENGINE\s0\fR
306 implementation includes the following abstractions;
309 \& RSA_METHOD - for providing alternative RSA implementations
310 \& DSA_METHOD, DH_METHOD, RAND_METHOD - alternative DSA, DH, and RAND
311 \& EVP_CIPHER - potentially multiple cipher algorithms (indexed by 'nid')
312 \& EVP_DIGEST - potentially multiple hash algorithms (indexed by 'nid')
313 \& key-loading - loading public and/or private EVP_PKEY keys
315 .Sh "Reference counting and handles"
316 .IX Subsection "Reference counting and handles"
317 Due to the modular nature of the \s-1ENGINE\s0 \s-1API\s0, pointers to ENGINEs need to be
318 treated as handles \- ie. not only as pointers, but also as references to
319 the underlying \s-1ENGINE\s0 object. Ie. you should obtain a new reference when
320 making copies of an \s-1ENGINE\s0 pointer if the copies will be used (and
321 released) independantly.
323 \&\s-1ENGINE\s0 objects have two levels of reference-counting to match the way in
324 which the objects are used. At the most basic level, each \s-1ENGINE\s0 pointer is
325 inherently a \fBstructural\fR reference \- you need a structural reference
326 simply to refer to the pointer value at all, as this kind of reference is
327 your guarantee that the structure can not be deallocated until you release
330 However, a structural reference provides no guarantee that the \s-1ENGINE\s0 has
331 been initiliased to be usable to perform any of its cryptographic
332 implementations \- and indeed it's quite possible that most ENGINEs will not
333 initialised at all on standard setups, as ENGINEs are typically used to
334 support specialised hardware. To use an \s-1ENGINE\s0's functionality, you need a
335 \&\fBfunctional\fR reference. This kind of reference can be considered a
336 specialised form of structural reference, because each functional reference
337 implicitly contains a structural reference as well \- however to avoid
338 difficult-to-find programming bugs, it is recommended to treat the two
339 kinds of reference independantly. If you have a functional reference to an
340 \&\s-1ENGINE\s0, you have a guarantee that the \s-1ENGINE\s0 has been initialised ready to
341 perform cryptographic operations and will not be uninitialised or cleaned
342 up until after you have released your reference.
344 We will discuss the two kinds of reference separately, including how to
345 tell which one you are dealing with at any given point in time (after all
346 they are both simply (\s-1ENGINE\s0 *) pointers, the difference is in the way they
349 \&\fIStructural references\fR
351 This basic type of reference is typically used for creating new ENGINEs
352 dynamically, iterating across OpenSSL's internal linked-list of loaded
353 ENGINEs, reading information about an \s-1ENGINE\s0, etc. Essentially a structural
354 reference is sufficient if you only need to query or manipulate the data of
355 an \s-1ENGINE\s0 implementation rather than use its functionality.
357 The \fIENGINE_new()\fR function returns a structural reference to a new (empty)
358 \&\s-1ENGINE\s0 object. Other than that, structural references come from return
359 values to various \s-1ENGINE\s0 \s-1API\s0 functions such as; \fIENGINE_by_id()\fR,
360 \&\fIENGINE_get_first()\fR, \fIENGINE_get_last()\fR, \fIENGINE_get_next()\fR,
361 \&\fIENGINE_get_prev()\fR. All structural references should be released by a
362 corresponding to call to the \fIENGINE_free()\fR function \- the \s-1ENGINE\s0 object
363 itself will only actually be cleaned up and deallocated when the last
364 structural reference is released.
366 It should also be noted that many \s-1ENGINE\s0 \s-1API\s0 function calls that accept a
367 structural reference will internally obtain another reference \- typically
368 this happens whenever the supplied \s-1ENGINE\s0 will be needed by OpenSSL after
369 the function has returned. Eg. the function to add a new \s-1ENGINE\s0 to
370 OpenSSL's internal list is \fIENGINE_add()\fR \- if this function returns success,
371 then OpenSSL will have stored a new structural reference internally so the
372 caller is still responsible for freeing their own reference with
373 \&\fIENGINE_free()\fR when they are finished with it. In a similar way, some
374 functions will automatically release the structural reference passed to it
375 if part of the function's job is to do so. Eg. the \fIENGINE_get_next()\fR and
376 \&\fIENGINE_get_prev()\fR functions are used for iterating across the internal
377 \&\s-1ENGINE\s0 list \- they will return a new structural reference to the next (or
378 previous) \s-1ENGINE\s0 in the list or \s-1NULL\s0 if at the end (or beginning) of the
379 list, but in either case the structural reference passed to the function is
380 released on behalf of the caller.
382 To clarify a particular function's handling of references, one should
383 always consult that function's documentation \*(L"man\*(R" page, or failing that
384 the openssl/engine.h header file includes some hints.
386 \&\fIFunctional references\fR
388 As mentioned, functional references exist when the cryptographic
389 functionality of an \s-1ENGINE\s0 is required to be available. A functional
390 reference can be obtained in one of two ways; from an existing structural
391 reference to the required \s-1ENGINE\s0, or by asking OpenSSL for the default
392 operational \s-1ENGINE\s0 for a given cryptographic purpose.
394 To obtain a functional reference from an existing structural reference,
395 call the \fIENGINE_init()\fR function. This returns zero if the \s-1ENGINE\s0 was not
396 already operational and couldn't be successfully initialised (eg. lack of
397 system drivers, no special hardware attached, etc), otherwise it will
398 return non-zero to indicate that the \s-1ENGINE\s0 is now operational and will
399 have allocated a new \fBfunctional\fR reference to the \s-1ENGINE\s0. In this case,
400 the supplied \s-1ENGINE\s0 pointer is, from the point of the view of the caller,
401 both a structural reference and a functional reference \- so if the caller
402 intends to use it as a functional reference it should free the structural
403 reference with \fIENGINE_free()\fR first. If the caller wishes to use it only as
404 a structural reference (eg. if the \fIENGINE_init()\fR call was simply to test if
405 the \s-1ENGINE\s0 seems available/online), then it should free the functional
406 reference; all functional references are released by the \fIENGINE_finish()\fR
409 The second way to get a functional reference is by asking OpenSSL for a
410 default implementation for a given task, eg. by \fIENGINE_get_default_RSA()\fR,
411 \&\fIENGINE_get_default_cipher_engine()\fR, etc. These are discussed in the next
412 section, though they are not usually required by application programmers as
413 they are used automatically when creating and using the relevant
414 algorithm-specific types in OpenSSL, such as \s-1RSA\s0, \s-1DSA\s0, \s-1EVP_CIPHER_CTX\s0, etc.
415 .Sh "Default implementations"
416 .IX Subsection "Default implementations"
417 For each supported abstraction, the \s-1ENGINE\s0 code maintains an internal table
418 of state to control which implementations are available for a given
419 abstraction and which should be used by default. These implementations are
420 registered in the tables separated-out by an 'nid' index, because
421 abstractions like \s-1EVP_CIPHER\s0 and \s-1EVP_DIGEST\s0 support many distinct
422 algorithms and modes \- ENGINEs will support different numbers and
423 combinations of these. In the case of other abstractions like \s-1RSA\s0, \s-1DSA\s0,
424 etc, there is only one \*(L"algorithm\*(R" so all implementations implicitly
425 register using the same 'nid' index. ENGINEs can be \fBregistered\fR into
426 these tables to make themselves available for use automatically by the
427 various abstractions, eg. \s-1RSA\s0. For illustrative purposes, we continue with
428 the \s-1RSA\s0 example, though all comments apply similarly to the other
429 abstractions (they each get their own table and linkage to the
430 corresponding section of openssl code).
432 When a new \s-1RSA\s0 key is being created, ie. in \fIRSA_new_method()\fR, a
433 \&\*(L"get_default\*(R" call will be made to the \s-1ENGINE\s0 subsystem to process the \s-1RSA\s0
434 state table and return a functional reference to an initialised \s-1ENGINE\s0
435 whose \s-1RSA_METHOD\s0 should be used. If no \s-1ENGINE\s0 should (or can) be used, it
436 will return \s-1NULL\s0 and the \s-1RSA\s0 key will operate with a \s-1NULL\s0 \s-1ENGINE\s0 handle by
437 using the conventional \s-1RSA\s0 implementation in OpenSSL (and will from then on
438 behave the way it used to before the \s-1ENGINE\s0 \s-1API\s0 existed \- for details see
441 Each state table has a flag to note whether it has processed this
442 \&\*(L"get_default\*(R" query since the table was last modified, because to process
443 this question it must iterate across all the registered ENGINEs in the
444 table trying to initialise each of them in turn, in case one of them is
445 operational. If it returns a functional reference to an \s-1ENGINE\s0, it will
446 also cache another reference to speed up processing future queries (without
447 needing to iterate across the table). Likewise, it will cache a \s-1NULL\s0
448 response if no \s-1ENGINE\s0 was available so that future queries won't repeat the
449 same iteration unless the state table changes. This behaviour can also be
450 changed; if the \s-1ENGINE_TABLE_FLAG_NOINIT\s0 flag is set (using
451 \&\fIENGINE_set_table_flags()\fR), no attempted initialisations will take place,
452 instead the only way for the state table to return a non-NULL \s-1ENGINE\s0 to the
453 \&\*(L"get_default\*(R" query will be if one is expressly set in the table. Eg.
454 \&\fIENGINE_set_default_RSA()\fR does the same job as \fIENGINE_register_RSA()\fR except
455 that it also sets the state table's cached response for the \*(L"get_default\*(R"
458 In the case of abstractions like \s-1EVP_CIPHER\s0, where implementations are
459 indexed by 'nid', these flags and cached-responses are distinct for each
462 It is worth illustrating the difference between \*(L"registration\*(R" of ENGINEs
463 into these per-algorithm state tables and using the alternative
464 \&\*(L"set_default\*(R" functions. The latter handles both \*(L"registration\*(R" and also
465 setting the cached \*(L"default\*(R" \s-1ENGINE\s0 in each relevant state table \- so
466 registered ENGINEs will only have a chance to be initialised for use as a
467 default if a default \s-1ENGINE\s0 wasn't already set for the same state table.
468 Eg. if \s-1ENGINE\s0 X supports cipher nids {A,B} and \s-1RSA\s0, \s-1ENGINE\s0 Y supports
469 ciphers {A} and \s-1DSA\s0, and the following code is executed;
472 \& ENGINE_register_complete(X);
473 \& ENGINE_set_default(Y, ENGINE_METHOD_ALL);
474 \& e1 = ENGINE_get_default_RSA();
475 \& e2 = ENGINE_get_cipher_engine(A);
476 \& e3 = ENGINE_get_cipher_engine(B);
477 \& e4 = ENGINE_get_default_DSA();
478 \& e5 = ENGINE_get_cipher_engine(C);
480 The results would be as follows;
487 \& assert(e5 == NULL);
489 .Sh "Application requirements"
490 .IX Subsection "Application requirements"
491 This section will explain the basic things an application programmer should
492 support to make the most useful elements of the \s-1ENGINE\s0 functionality
493 available to the user. The first thing to consider is whether the
494 programmer wishes to make alternative \s-1ENGINE\s0 modules available to the
495 application and user. OpenSSL maintains an internal linked list of
496 \&\*(L"visible\*(R" ENGINEs from which it has to operate \- at start-up, this list is
497 empty and in fact if an application does not call any \s-1ENGINE\s0 \s-1API\s0 calls and
498 it uses static linking against openssl, then the resulting application
499 binary will not contain any alternative \s-1ENGINE\s0 code at all. So the first
500 consideration is whether any/all available \s-1ENGINE\s0 implementations should be
501 made visible to OpenSSL \- this is controlled by calling the various \*(L"load\*(R"
505 \& /* Make the "dynamic" ENGINE available */
506 \& void ENGINE_load_dynamic(void);
507 \& /* Make the CryptoSwift hardware acceleration support available */
508 \& void ENGINE_load_cswift(void);
509 \& /* Make support for nCipher's "CHIL" hardware available */
510 \& void ENGINE_load_chil(void);
512 \& /* Make ALL ENGINE implementations bundled with OpenSSL available */
513 \& void ENGINE_load_builtin_engines(void);
515 Having called any of these functions, \s-1ENGINE\s0 objects would have been
516 dynamically allocated and populated with these implementations and linked
517 into OpenSSL's internal linked list. At this point it is important to
518 mention an important \s-1API\s0 function;
521 \& void ENGINE_cleanup(void);
523 If no \s-1ENGINE\s0 \s-1API\s0 functions are called at all in an application, then there
524 are no inherent memory leaks to worry about from the \s-1ENGINE\s0 functionality,
525 however if any ENGINEs are \*(L"load\*(R"ed, even if they are never registered or
526 used, it is necessary to use the \fIENGINE_cleanup()\fR function to
527 correspondingly cleanup before program exit, if the caller wishes to avoid
528 memory leaks. This mechanism uses an internal callback registration table
529 so that any \s-1ENGINE\s0 \s-1API\s0 functionality that knows it requires cleanup can
530 register its cleanup details to be called during \fIENGINE_cleanup()\fR. This
531 approach allows \fIENGINE_cleanup()\fR to clean up after any \s-1ENGINE\s0 functionality
532 at all that your program uses, yet doesn't automatically create linker
533 dependencies to all possible \s-1ENGINE\s0 functionality \- only the cleanup
534 callbacks required by the functionality you do use will be required by the
537 The fact that ENGINEs are made visible to OpenSSL (and thus are linked into
538 the program and loaded into memory at run-time) does not mean they are
539 \&\*(L"registered\*(R" or called into use by OpenSSL automatically \- that behaviour
540 is something for the application to have control over. Some applications
541 will want to allow the user to specify exactly which \s-1ENGINE\s0 they want used
542 if any is to be used at all. Others may prefer to load all support and have
543 OpenSSL automatically use at run-time any \s-1ENGINE\s0 that is able to
544 successfully initialise \- ie. to assume that this corresponds to
545 acceleration hardware attached to the machine or some such thing. There are
546 probably numerous other ways in which applications may prefer to handle
547 things, so we will simply illustrate the consequences as they apply to a
548 couple of simple cases and leave developers to consider these and the
549 source code to openssl's builtin utilities as guides.
551 \&\fIUsing a specific \s-1ENGINE\s0 implementation\fR
553 Here we'll assume an application has been configured by its user or admin
554 to want to use the \*(L"\s-1ACME\s0\*(R" \s-1ENGINE\s0 if it is available in the version of
555 OpenSSL the application was compiled with. If it is available, it should be
556 used by default for all \s-1RSA\s0, \s-1DSA\s0, and symmetric cipher operation, otherwise
557 OpenSSL should use its builtin software as per usual. The following code
558 illustrates how to approach this;
562 \& const char *engine_id = "ACME";
563 \& ENGINE_load_builtin_engines();
564 \& e = ENGINE_by_id(engine_id);
566 \& /* the engine isn't available */
568 \& if(!ENGINE_init(e)) {
569 \& /* the engine couldn't initialise, release 'e' */
573 \& if(!ENGINE_set_default_RSA(e))
574 \& /* This should only happen when 'e' can't initialise, but the previous
575 \& * statement suggests it did. */
577 \& ENGINE_set_default_DSA(e);
578 \& ENGINE_set_default_ciphers(e);
579 \& /* Release the functional reference from ENGINE_init() */
581 \& /* Release the structural reference from ENGINE_by_id() */
584 \&\fIAutomatically using builtin \s-1ENGINE\s0 implementations\fR
586 Here we'll assume we want to load and register all \s-1ENGINE\s0 implementations
587 bundled with OpenSSL, such that for any cryptographic algorithm required by
588 OpenSSL \- if there is an \s-1ENGINE\s0 that implements it and can be initialise,
589 it should be used. The following code illustrates how this can work;
592 \& /* Load all bundled ENGINEs into memory and make them visible */
593 \& ENGINE_load_builtin_engines();
594 \& /* Register all of them for every algorithm they collectively implement */
595 \& ENGINE_register_all_complete();
597 That's all that's required. Eg. the next time OpenSSL tries to set up an
598 \&\s-1RSA\s0 key, any bundled ENGINEs that implement \s-1RSA_METHOD\s0 will be passed to
599 \&\fIENGINE_init()\fR and if any of those succeed, that \s-1ENGINE\s0 will be set as the
600 default for use with \s-1RSA\s0 from then on.
601 .Sh "Advanced configuration support"
602 .IX Subsection "Advanced configuration support"
603 There is a mechanism supported by the \s-1ENGINE\s0 framework that allows each
604 \&\s-1ENGINE\s0 implementation to define an arbitrary set of configuration
605 \&\*(L"commands\*(R" and expose them to OpenSSL and any applications based on
606 OpenSSL. This mechanism is entirely based on the use of name-value pairs
607 and and assumes \s-1ASCII\s0 input (no unicode or \s-1UTF\s0 for now!), so it is ideal if
608 applications want to provide a transparent way for users to provide
609 arbitrary configuration \*(L"directives\*(R" directly to such ENGINEs. It is also
610 possible for the application to dynamically interrogate the loaded \s-1ENGINE\s0
611 implementations for the names, descriptions, and input flags of their
612 available \*(L"control commands\*(R", providing a more flexible configuration
613 scheme. However, if the user is expected to know which \s-1ENGINE\s0 device he/she
614 is using (in the case of specialised hardware, this goes without saying)
615 then applications may not need to concern themselves with discovering the
616 supported control commands and simply prefer to allow settings to passed
617 into ENGINEs exactly as they are provided by the user.
619 Before illustrating how control commands work, it is worth mentioning what
620 they are typically used for. Broadly speaking there are two uses for
621 control commands; the first is to provide the necessary details to the
622 implementation (which may know nothing at all specific to the host system)
623 so that it can be initialised for use. This could include the path to any
624 driver or config files it needs to load, required network addresses,
625 smart-card identifiers, passwords to initialise password-protected devices,
626 logging information, etc etc. This class of commands typically needs to be
627 passed to an \s-1ENGINE\s0 \fBbefore\fR attempting to initialise it, ie. before
628 calling \fIENGINE_init()\fR. The other class of commands consist of settings or
629 operations that tweak certain behaviour or cause certain operations to take
630 place, and these commands may work either before or after \fIENGINE_init()\fR, or
631 in same cases both. \s-1ENGINE\s0 implementations should provide indications of
632 this in the descriptions attached to builtin control commands and/or in
633 external product documentation.
635 \&\fIIssuing control commands to an \s-1ENGINE\s0\fR
637 Let's illustrate by example; a function for which the caller supplies the
638 name of the \s-1ENGINE\s0 it wishes to use, a table of string-pairs for use before
639 initialisation, and another table for use after initialisation. Note that
640 the string-pairs used for control commands consist of a command \*(L"name\*(R"
641 followed by the command \*(L"parameter\*(R" \- the parameter could be \s-1NULL\s0 in some
642 cases but the name can not. This function should initialise the \s-1ENGINE\s0
643 (issuing the \*(L"pre\*(R" commands beforehand and the \*(L"post\*(R" commands afterwards)
644 and set it as the default for everything except \s-1RAND\s0 and then return a
645 boolean success or failure.
648 \& int generic_load_engine_fn(const char *engine_id,
649 \& const char **pre_cmds, int pre_num,
650 \& const char **post_cmds, int post_num)
652 \& ENGINE *e = ENGINE_by_id(engine_id);
654 \& while(pre_num--) {
655 \& if(!ENGINE_ctrl_cmd_string(e, pre_cmds[0], pre_cmds[1], 0)) {
656 \& fprintf(stderr, "Failed command (%s - %s:%s)\en", engine_id,
657 \& pre_cmds[0], pre_cmds[1] ? pre_cmds[1] : "(NULL)");
663 \& if(!ENGINE_init(e)) {
664 \& fprintf(stderr, "Failed initialisation\en");
668 \& /* ENGINE_init() returned a functional reference, so free the structural
669 \& * reference from ENGINE_by_id(). */
671 \& while(post_num--) {
672 \& if(!ENGINE_ctrl_cmd_string(e, post_cmds[0], post_cmds[1], 0)) {
673 \& fprintf(stderr, "Failed command (%s - %s:%s)\en", engine_id,
674 \& post_cmds[0], post_cmds[1] ? post_cmds[1] : "(NULL)");
680 \& ENGINE_set_default(e, ENGINE_METHOD_ALL & ~ENGINE_METHOD_RAND);
685 Note that \fIENGINE_ctrl_cmd_string()\fR accepts a boolean argument that can
686 relax the semantics of the function \- if set non-zero it will only return
687 failure if the \s-1ENGINE\s0 supported the given command name but failed while
688 executing it, if the \s-1ENGINE\s0 doesn't support the command name it will simply
689 return success without doing anything. In this case we assume the user is
690 only supplying commands specific to the given \s-1ENGINE\s0 so we set this to
693 \&\fIDiscovering supported control commands\fR
695 It is possible to discover at run-time the names, numerical-ids, descriptions
696 and input parameters of the control commands supported from a structural
697 reference to any \s-1ENGINE\s0. It is first important to note that some control
698 commands are defined by OpenSSL itself and it will intercept and handle these
699 control commands on behalf of the \s-1ENGINE\s0, ie. the \s-1ENGINE\s0's \fIctrl()\fR handler is not
700 used for the control command. openssl/engine.h defines a symbol,
701 \&\s-1ENGINE_CMD_BASE\s0, that all control commands implemented by ENGINEs from. Any
702 command value lower than this symbol is considered a \*(L"generic\*(R" command is
703 handled directly by the OpenSSL core routines.
705 It is using these \*(L"core\*(R" control commands that one can discover the the control
706 commands implemented by a given \s-1ENGINE\s0, specifically the commands;
709 \& #define ENGINE_HAS_CTRL_FUNCTION 10
710 \& #define ENGINE_CTRL_GET_FIRST_CMD_TYPE 11
711 \& #define ENGINE_CTRL_GET_NEXT_CMD_TYPE 12
712 \& #define ENGINE_CTRL_GET_CMD_FROM_NAME 13
713 \& #define ENGINE_CTRL_GET_NAME_LEN_FROM_CMD 14
714 \& #define ENGINE_CTRL_GET_NAME_FROM_CMD 15
715 \& #define ENGINE_CTRL_GET_DESC_LEN_FROM_CMD 16
716 \& #define ENGINE_CTRL_GET_DESC_FROM_CMD 17
717 \& #define ENGINE_CTRL_GET_CMD_FLAGS 18
719 Whilst these commands are automatically processed by the OpenSSL framework code,
720 they use various properties exposed by each \s-1ENGINE\s0 by which to process these
721 queries. An \s-1ENGINE\s0 has 3 properties it exposes that can affect this behaviour;
722 it can supply a \fIctrl()\fR handler, it can specify \s-1ENGINE_FLAGS_MANUAL_CMD_CTRL\s0 in
723 the \s-1ENGINE\s0's flags, and it can expose an array of control command descriptions.
724 If an \s-1ENGINE\s0 specifies the \s-1ENGINE_FLAGS_MANUAL_CMD_CTRL\s0 flag, then it will
725 simply pass all these \*(L"core\*(R" control commands directly to the \s-1ENGINE\s0's \fIctrl()\fR
726 handler (and thus, it must have supplied one), so it is up to the \s-1ENGINE\s0 to
727 reply to these \*(L"discovery\*(R" commands itself. If that flag is not set, then the
728 OpenSSL framework code will work with the following rules;
731 \& if no ctrl() handler supplied;
732 \& ENGINE_HAS_CTRL_FUNCTION returns FALSE (zero),
733 \& all other commands fail.
734 \& if a ctrl() handler was supplied but no array of control commands;
735 \& ENGINE_HAS_CTRL_FUNCTION returns TRUE,
736 \& all other commands fail.
737 \& if a ctrl() handler and array of control commands was supplied;
738 \& ENGINE_HAS_CTRL_FUNCTION returns TRUE,
739 \& all other commands proceed processing ...
741 If the \s-1ENGINE\s0's array of control commands is empty then all other commands will
742 fail, otherwise; \s-1ENGINE_CTRL_GET_FIRST_CMD_TYPE\s0 returns the identifier of
743 the first command supported by the \s-1ENGINE\s0, \s-1ENGINE_GET_NEXT_CMD_TYPE\s0 takes the
744 identifier of a command supported by the \s-1ENGINE\s0 and returns the next command
745 identifier or fails if there are no more, \s-1ENGINE_CMD_FROM_NAME\s0 takes a string
746 name for a command and returns the corresponding identifier or fails if no such
747 command name exists, and the remaining commands take a command identifier and
748 return properties of the corresponding commands. All except
749 \&\s-1ENGINE_CTRL_GET_FLAGS\s0 return the string length of a command name or description,
750 or populate a supplied character buffer with a copy of the command name or
751 description. \s-1ENGINE_CTRL_GET_FLAGS\s0 returns a bitwise-OR'd mask of the following
755 \& #define ENGINE_CMD_FLAG_NUMERIC (unsigned int)0x0001
756 \& #define ENGINE_CMD_FLAG_STRING (unsigned int)0x0002
757 \& #define ENGINE_CMD_FLAG_NO_INPUT (unsigned int)0x0004
758 \& #define ENGINE_CMD_FLAG_INTERNAL (unsigned int)0x0008
760 If the \s-1ENGINE_CMD_FLAG_INTERNAL\s0 flag is set, then any other flags are purely
761 informational to the caller \- this flag will prevent the command being usable
762 for any higher-level \s-1ENGINE\s0 functions such as \fIENGINE_ctrl_cmd_string()\fR.
763 \&\*(L"\s-1INTERNAL\s0\*(R" commands are not intended to be exposed to text-based configuration
764 by applications, administrations, users, etc. These can support arbitrary
765 operations via \fIENGINE_ctrl()\fR, including passing to and/or from the control
766 commands data of any arbitrary type. These commands are supported in the
767 discovery mechanisms simply to allow applications determinie if an \s-1ENGINE\s0
768 supports certain specific commands it might want to use (eg. application \*(L"foo\*(R"
769 might query various ENGINEs to see if they implement \*(L"\s-1FOO_GET_VENDOR_LOGO_GIF\s0\*(R" \-
770 and \s-1ENGINE\s0 could therefore decide whether or not to support this \*(L"foo\*(R"\-specific
772 .Sh "Future developments"
773 .IX Subsection "Future developments"
774 The \s-1ENGINE\s0 \s-1API\s0 and internal architecture is currently being reviewed. Slated for
775 possible release in 0.9.8 is support for transparent loading of \*(L"dynamic\*(R"
776 ENGINEs (built as self-contained shared-libraries). This would allow \s-1ENGINE\s0
777 implementations to be provided independantly of OpenSSL libraries and/or
778 OpenSSL-based applications, and would also remove any requirement for
779 applications to explicitly use the \*(L"dynamic\*(R" \s-1ENGINE\s0 to bind to shared-library
782 .IX Header "SEE ALSO"
783 rsa(3), dsa(3), dh(3), rand(3),
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