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1=pod
2
3=head1 NAME
4
5ENGINE_add, ENGINE_by_id, ENGINE_finish, ENGINE_get_first,
6ENGINE_get_last, ENGINE_get_next, ENGINE_get_prev,
7ENGINE_init, ENGINE_load_builtin_engines, ENGINE_remove
8- ENGINE cryptographic module support
9
10=head1 SYNOPSIS
11
12 #include <openssl/engine.h>
13
14 ENGINE *ENGINE_get_first(void);
15 ENGINE *ENGINE_get_last(void);
16 ENGINE *ENGINE_get_next(ENGINE *e);
17 ENGINE *ENGINE_get_prev(ENGINE *e);
18
19 int ENGINE_add(ENGINE *e);
20 int ENGINE_remove(ENGINE *e);
21
22 ENGINE *ENGINE_by_id(const char *id);
23
24 int ENGINE_init(ENGINE *e);
25 int ENGINE_finish(ENGINE *e);
26
27 void ENGINE_load_openssl(void);
28 void ENGINE_load_dynamic(void);
29 void ENGINE_load_cryptodev(void);
30 void ENGINE_load_builtin_engines(void);
31
32 void ENGINE_cleanup(void);
33
34 ENGINE *ENGINE_get_default_RSA(void);
35 ENGINE *ENGINE_get_default_DSA(void);
36 ENGINE *ENGINE_get_default_ECDH(void);
37 ENGINE *ENGINE_get_default_ECDSA(void);
38 ENGINE *ENGINE_get_default_DH(void);
39 ENGINE *ENGINE_get_default_RAND(void);
40 ENGINE *ENGINE_get_cipher_engine(int nid);
41 ENGINE *ENGINE_get_digest_engine(int nid);
42
43 int ENGINE_set_default_RSA(ENGINE *e);
44 int ENGINE_set_default_DSA(ENGINE *e);
45 int ENGINE_set_default_ECDH(ENGINE *e);
46 int ENGINE_set_default_ECDSA(ENGINE *e);
47 int ENGINE_set_default_DH(ENGINE *e);
48 int ENGINE_set_default_RAND(ENGINE *e);
49 int ENGINE_set_default_ciphers(ENGINE *e);
50 int ENGINE_set_default_digests(ENGINE *e);
51 int ENGINE_set_default_string(ENGINE *e, const char *list);
52
53 int ENGINE_set_default(ENGINE *e, unsigned int flags);
54
55 unsigned int ENGINE_get_table_flags(void);
56 void ENGINE_set_table_flags(unsigned int flags);
57
58 int ENGINE_register_RSA(ENGINE *e);
59 void ENGINE_unregister_RSA(ENGINE *e);
60 void ENGINE_register_all_RSA(void);
61 int ENGINE_register_DSA(ENGINE *e);
62 void ENGINE_unregister_DSA(ENGINE *e);
63 void ENGINE_register_all_DSA(void);
64 int ENGINE_register_ECDH(ENGINE *e);
65 void ENGINE_unregister_ECDH(ENGINE *e);
66 void ENGINE_register_all_ECDH(void);
67 int ENGINE_register_ECDSA(ENGINE *e);
68 void ENGINE_unregister_ECDSA(ENGINE *e);
69 void ENGINE_register_all_ECDSA(void);
70 int ENGINE_register_DH(ENGINE *e);
71 void ENGINE_unregister_DH(ENGINE *e);
72 void ENGINE_register_all_DH(void);
73 int ENGINE_register_RAND(ENGINE *e);
74 void ENGINE_unregister_RAND(ENGINE *e);
75 void ENGINE_register_all_RAND(void);
76 int ENGINE_register_STORE(ENGINE *e);
77 void ENGINE_unregister_STORE(ENGINE *e);
78 void ENGINE_register_all_STORE(void);
79 int ENGINE_register_ciphers(ENGINE *e);
80 void ENGINE_unregister_ciphers(ENGINE *e);
81 void ENGINE_register_all_ciphers(void);
82 int ENGINE_register_digests(ENGINE *e);
83 void ENGINE_unregister_digests(ENGINE *e);
84 void ENGINE_register_all_digests(void);
85 int ENGINE_register_complete(ENGINE *e);
86 int ENGINE_register_all_complete(void);
87
88 int ENGINE_ctrl(ENGINE *e, int cmd, long i, void *p, void (*f)(void));
89 int ENGINE_cmd_is_executable(ENGINE *e, int cmd);
90 int ENGINE_ctrl_cmd(ENGINE *e, const char *cmd_name,
91 long i, void *p, void (*f)(void), int cmd_optional);
92 int ENGINE_ctrl_cmd_string(ENGINE *e, const char *cmd_name, const char *arg,
93 int cmd_optional);
94
95 int ENGINE_set_ex_data(ENGINE *e, int idx, void *arg);
96 void *ENGINE_get_ex_data(const ENGINE *e, int idx);
97
98 int ENGINE_get_ex_new_index(long argl, void *argp, CRYPTO_EX_new *new_func,
99 CRYPTO_EX_dup *dup_func, CRYPTO_EX_free *free_func);
100
101 ENGINE *ENGINE_new(void);
102 int ENGINE_free(ENGINE *e);
103 int ENGINE_up_ref(ENGINE *e);
104
105 int ENGINE_set_id(ENGINE *e, const char *id);
106 int ENGINE_set_name(ENGINE *e, const char *name);
107 int ENGINE_set_RSA(ENGINE *e, const RSA_METHOD *rsa_meth);
108 int ENGINE_set_DSA(ENGINE *e, const DSA_METHOD *dsa_meth);
109 int ENGINE_set_ECDH(ENGINE *e, const ECDH_METHOD *dh_meth);
110 int ENGINE_set_ECDSA(ENGINE *e, const ECDSA_METHOD *dh_meth);
111 int ENGINE_set_DH(ENGINE *e, const DH_METHOD *dh_meth);
112 int ENGINE_set_RAND(ENGINE *e, const RAND_METHOD *rand_meth);
113 int ENGINE_set_STORE(ENGINE *e, const STORE_METHOD *rand_meth);
114 int ENGINE_set_destroy_function(ENGINE *e, ENGINE_GEN_INT_FUNC_PTR destroy_f);
115 int ENGINE_set_init_function(ENGINE *e, ENGINE_GEN_INT_FUNC_PTR init_f);
116 int ENGINE_set_finish_function(ENGINE *e, ENGINE_GEN_INT_FUNC_PTR finish_f);
117 int ENGINE_set_ctrl_function(ENGINE *e, ENGINE_CTRL_FUNC_PTR ctrl_f);
118 int ENGINE_set_load_privkey_function(ENGINE *e, ENGINE_LOAD_KEY_PTR loadpriv_f);
119 int ENGINE_set_load_pubkey_function(ENGINE *e, ENGINE_LOAD_KEY_PTR loadpub_f);
120 int ENGINE_set_ciphers(ENGINE *e, ENGINE_CIPHERS_PTR f);
121 int ENGINE_set_digests(ENGINE *e, ENGINE_DIGESTS_PTR f);
122 int ENGINE_set_flags(ENGINE *e, int flags);
123 int ENGINE_set_cmd_defns(ENGINE *e, const ENGINE_CMD_DEFN *defns);
124
125 const char *ENGINE_get_id(const ENGINE *e);
126 const char *ENGINE_get_name(const ENGINE *e);
127 const RSA_METHOD *ENGINE_get_RSA(const ENGINE *e);
128 const DSA_METHOD *ENGINE_get_DSA(const ENGINE *e);
129 const ECDH_METHOD *ENGINE_get_ECDH(const ENGINE *e);
130 const ECDSA_METHOD *ENGINE_get_ECDSA(const ENGINE *e);
131 const DH_METHOD *ENGINE_get_DH(const ENGINE *e);
132 const RAND_METHOD *ENGINE_get_RAND(const ENGINE *e);
133 const STORE_METHOD *ENGINE_get_STORE(const ENGINE *e);
134 ENGINE_GEN_INT_FUNC_PTR ENGINE_get_destroy_function(const ENGINE *e);
135 ENGINE_GEN_INT_FUNC_PTR ENGINE_get_init_function(const ENGINE *e);
136 ENGINE_GEN_INT_FUNC_PTR ENGINE_get_finish_function(const ENGINE *e);
137 ENGINE_CTRL_FUNC_PTR ENGINE_get_ctrl_function(const ENGINE *e);
138 ENGINE_LOAD_KEY_PTR ENGINE_get_load_privkey_function(const ENGINE *e);
139 ENGINE_LOAD_KEY_PTR ENGINE_get_load_pubkey_function(const ENGINE *e);
140 ENGINE_CIPHERS_PTR ENGINE_get_ciphers(const ENGINE *e);
141 ENGINE_DIGESTS_PTR ENGINE_get_digests(const ENGINE *e);
142 const EVP_CIPHER *ENGINE_get_cipher(ENGINE *e, int nid);
143 const EVP_MD *ENGINE_get_digest(ENGINE *e, int nid);
144 int ENGINE_get_flags(const ENGINE *e);
145 const ENGINE_CMD_DEFN *ENGINE_get_cmd_defns(const ENGINE *e);
146
147 EVP_PKEY *ENGINE_load_private_key(ENGINE *e, const char *key_id,
148 UI_METHOD *ui_method, void *callback_data);
149 EVP_PKEY *ENGINE_load_public_key(ENGINE *e, const char *key_id,
150 UI_METHOD *ui_method, void *callback_data);
151
152 void ENGINE_add_conf_module(void);
153
154=head1 DESCRIPTION
155
156These functions create, manipulate, and use cryptographic modules in the
157form of B<ENGINE> objects. These objects act as containers for
158implementations of cryptographic algorithms, and support a
159reference-counted mechanism to allow them to be dynamically loaded in and
160out of the running application.
161
162The cryptographic functionality that can be provided by an B<ENGINE>
163implementation includes the following abstractions;
164
165 RSA_METHOD - for providing alternative RSA implementations
166 DSA_METHOD, DH_METHOD, RAND_METHOD, ECDH_METHOD, ECDSA_METHOD,
167 STORE_METHOD - similarly for other OpenSSL APIs
168 EVP_CIPHER - potentially multiple cipher algorithms (indexed by 'nid')
169 EVP_DIGEST - potentially multiple hash algorithms (indexed by 'nid')
170 key-loading - loading public and/or private EVP_PKEY keys
171
172=head2 Reference counting and handles
173
174Due to the modular nature of the ENGINE API, pointers to ENGINEs need to be
175treated as handles - ie. not only as pointers, but also as references to
176the underlying ENGINE object. Ie. one should obtain a new reference when
177making copies of an ENGINE pointer if the copies will be used (and
178released) independently.
179
180ENGINE objects have two levels of reference-counting to match the way in
181which the objects are used. At the most basic level, each ENGINE pointer is
182inherently a B<structural> reference - a structural reference is required
183to use the pointer value at all, as this kind of reference is a guarantee
184that the structure can not be deallocated until the reference is released.
185
186However, a structural reference provides no guarantee that the ENGINE is
187initialised and able to use any of its cryptographic
188implementations. Indeed it's quite possible that most ENGINEs will not
189initialise at all in typical environments, as ENGINEs are typically used to
190support specialised hardware. To use an ENGINE's functionality, you need a
191B<functional> reference. This kind of reference can be considered a
192specialised form of structural reference, because each functional reference
193implicitly contains a structural reference as well - however to avoid
194difficult-to-find programming bugs, it is recommended to treat the two
195kinds of reference independently. If you have a functional reference to an
196ENGINE, you have a guarantee that the ENGINE has been initialised ready to
197perform cryptographic operations and will remain uninitialised
198until after you have released your reference.
199
200I<Structural references>
201
202This basic type of reference is used for instantiating new ENGINEs,
203iterating across OpenSSL's internal linked-list of loaded
204ENGINEs, reading information about an ENGINE, etc. Essentially a structural
205reference is sufficient if you only need to query or manipulate the data of
206an ENGINE implementation rather than use its functionality.
207
208The ENGINE_new() function returns a structural reference to a new (empty)
209ENGINE object. There are other ENGINE API functions that return structural
210references such as; ENGINE_by_id(), ENGINE_get_first(), ENGINE_get_last(),
211ENGINE_get_next(), ENGINE_get_prev(). All structural references should be
212released by a corresponding to call to the ENGINE_free() function - the
213ENGINE object itself will only actually be cleaned up and deallocated when
214the last structural reference is released.
215
216It should also be noted that many ENGINE API function calls that accept a
217structural reference will internally obtain another reference - typically
218this happens whenever the supplied ENGINE will be needed by OpenSSL after
219the function has returned. Eg. the function to add a new ENGINE to
220OpenSSL's internal list is ENGINE_add() - if this function returns success,
221then OpenSSL will have stored a new structural reference internally so the
222caller is still responsible for freeing their own reference with
223ENGINE_free() when they are finished with it. In a similar way, some
224functions will automatically release the structural reference passed to it
225if part of the function's job is to do so. Eg. the ENGINE_get_next() and
226ENGINE_get_prev() functions are used for iterating across the internal
227ENGINE list - they will return a new structural reference to the next (or
228previous) ENGINE in the list or NULL if at the end (or beginning) of the
229list, but in either case the structural reference passed to the function is
230released on behalf of the caller.
231
232To clarify a particular function's handling of references, one should
233always consult that function's documentation "man" page, or failing that
234the openssl/engine.h header file includes some hints.
235
236I<Functional references>
237
238As mentioned, functional references exist when the cryptographic
239functionality of an ENGINE is required to be available. A functional
240reference can be obtained in one of two ways; from an existing structural
241reference to the required ENGINE, or by asking OpenSSL for the default
242operational ENGINE for a given cryptographic purpose.
243
244To obtain a functional reference from an existing structural reference,
245call the ENGINE_init() function. This returns zero if the ENGINE was not
246already operational and couldn't be successfully initialised (eg. lack of
247system drivers, no special hardware attached, etc), otherwise it will
248return non-zero to indicate that the ENGINE is now operational and will
249have allocated a new B<functional> reference to the ENGINE. All functional
250references are released by calling ENGINE_finish() (which removes the
251implicit structural reference as well).
252
253The second way to get a functional reference is by asking OpenSSL for a
254default implementation for a given task, eg. by ENGINE_get_default_RSA(),
255ENGINE_get_default_cipher_engine(), etc. These are discussed in the next
256section, though they are not usually required by application programmers as
257they are used automatically when creating and using the relevant
258algorithm-specific types in OpenSSL, such as RSA, DSA, EVP_CIPHER_CTX, etc.
259
260=head2 Default implementations
261
262For each supported abstraction, the ENGINE code maintains an internal table
263of state to control which implementations are available for a given
264abstraction and which should be used by default. These implementations are
265registered in the tables and indexed by an 'nid' value, because
266abstractions like EVP_CIPHER and EVP_DIGEST support many distinct
267algorithms and modes, and ENGINEs can support arbitrarily many of them.
268In the case of other abstractions like RSA, DSA, etc, there is only one
269"algorithm" so all implementations implicitly register using the same 'nid'
270index.
271
272When a default ENGINE is requested for a given abstraction/algorithm/mode, (eg.
273when calling RSA_new_method(NULL)), a "get_default" call will be made to the
274ENGINE subsystem to process the corresponding state table and return a
275functional reference to an initialised ENGINE whose implementation should be
276used. If no ENGINE should (or can) be used, it will return NULL and the caller
277will operate with a NULL ENGINE handle - this usually equates to using the
278conventional software implementation. In the latter case, OpenSSL will from
279then on behave the way it used to before the ENGINE API existed.
280
281Each state table has a flag to note whether it has processed this
282"get_default" query since the table was last modified, because to process
283this question it must iterate across all the registered ENGINEs in the
284table trying to initialise each of them in turn, in case one of them is
285operational. If it returns a functional reference to an ENGINE, it will
286also cache another reference to speed up processing future queries (without
287needing to iterate across the table). Likewise, it will cache a NULL
288response if no ENGINE was available so that future queries won't repeat the
289same iteration unless the state table changes. This behaviour can also be
290changed; if the ENGINE_TABLE_FLAG_NOINIT flag is set (using
291ENGINE_set_table_flags()), no attempted initialisations will take place,
292instead the only way for the state table to return a non-NULL ENGINE to the
293"get_default" query will be if one is expressly set in the table. Eg.
294ENGINE_set_default_RSA() does the same job as ENGINE_register_RSA() except
295that it also sets the state table's cached response for the "get_default"
296query. In the case of abstractions like EVP_CIPHER, where implementations are
297indexed by 'nid', these flags and cached-responses are distinct for each 'nid'
298value.
299
300=head2 Application requirements
301
302This section will explain the basic things an application programmer should
303support to make the most useful elements of the ENGINE functionality
304available to the user. The first thing to consider is whether the
305programmer wishes to make alternative ENGINE modules available to the
306application and user. OpenSSL maintains an internal linked list of
307"visible" ENGINEs from which it has to operate - at start-up, this list is
308empty and in fact if an application does not call any ENGINE API calls and
309it uses static linking against openssl, then the resulting application
310binary will not contain any alternative ENGINE code at all. So the first
311consideration is whether any/all available ENGINE implementations should be
312made visible to OpenSSL - this is controlled by calling the various "load"
313functions, eg.
314
315 /* Make ALL ENGINE implementations bundled with OpenSSL available */
316 ENGINE_load_builtin_engines();
317
318Note that ENGINE_load_dynamic(void) is a placeholder and does not enable
319dynamic engine loading support.
320
321Having called any of these functions, ENGINE objects would have been
322dynamically allocated and populated with these implementations and linked
323into OpenSSL's internal linked list. At this point it is important to
324mention an important API function;
325
326 void ENGINE_cleanup(void);
327
328If no ENGINE API functions are called at all in an application, then there
329are no inherent memory leaks to worry about from the ENGINE functionality,
330however if any ENGINEs are loaded, even if they are never registered or
331used, it is necessary to use the ENGINE_cleanup() function to
332correspondingly cleanup before program exit, if the caller wishes to avoid
333memory leaks. This mechanism uses an internal callback registration table
334so that any ENGINE API functionality that knows it requires cleanup can
335register its cleanup details to be called during ENGINE_cleanup(). This
336approach allows ENGINE_cleanup() to clean up after any ENGINE functionality
337at all that your program uses, yet doesn't automatically create linker
338dependencies to all possible ENGINE functionality - only the cleanup
339callbacks required by the functionality you do use will be required by the
340linker.
341
342The fact that ENGINEs are made visible to OpenSSL (and thus are linked into
343the program and loaded into memory at run-time) does not mean they are
344"registered" or called into use by OpenSSL automatically - that behaviour
345is something for the application to control. Some applications
346will want to allow the user to specify exactly which ENGINE they want used
347if any is to be used at all. Others may prefer to load all support and have
348OpenSSL automatically use at run-time any ENGINE that is able to
349successfully initialise - ie. to assume that this corresponds to
350acceleration hardware attached to the machine or some such thing. There are
351probably numerous other ways in which applications may prefer to handle
352things, so we will simply illustrate the consequences as they apply to a
353couple of simple cases and leave developers to consider these and the
354source code to openssl's builtin utilities as guides.
355
356I<Using a specific ENGINE implementation>
357
358Here we'll assume an application has been configured by its user or admin
359to want to use the "ACME" ENGINE if it is available in the version of
360OpenSSL the application was compiled with. If it is available, it should be
361used by default for all RSA, DSA, and symmetric cipher operation, otherwise
362OpenSSL should use its builtin software as per usual. The following code
363illustrates how to approach this;
364
365 ENGINE *e;
366 const char *engine_id = "ACME";
367 ENGINE_load_builtin_engines();
368 e = ENGINE_by_id(engine_id);
369 if (!e)
370 /* the engine isn't available */
371 return;
372 if (!ENGINE_init(e)) {
373 /* the engine couldn't initialise, release 'e' */
374 ENGINE_free(e);
375 return;
376 }
377 if (!ENGINE_set_default_RSA(e))
378 /* This should only happen when 'e' can't initialise, but the previous
379 * statement suggests it did. */
380 abort();
381 ENGINE_set_default_DSA(e);
382 ENGINE_set_default_ciphers(e);
383 /* Release the functional reference from ENGINE_init() */
384 ENGINE_finish(e);
385 /* Release the structural reference from ENGINE_by_id() */
386 ENGINE_free(e);
387
388I<Automatically using builtin ENGINE implementations>
389
390Here we'll assume we want to load and register all ENGINE implementations
391bundled with OpenSSL, such that for any cryptographic algorithm required by
392OpenSSL - if there is an ENGINE that implements it and can be initialise,
393it should be used. The following code illustrates how this can work;
394
395 /* Load all bundled ENGINEs into memory and make them visible */
396 ENGINE_load_builtin_engines();
397 /* Register all of them for every algorithm they collectively implement */
398 ENGINE_register_all_complete();
399
400That's all that's required. Eg. the next time OpenSSL tries to set up an
401RSA key, any bundled ENGINEs that implement RSA_METHOD will be passed to
402ENGINE_init() and if any of those succeed, that ENGINE will be set as the
403default for RSA use from then on.
404
405=head2 Advanced configuration support
406
407There is a mechanism supported by the ENGINE framework that allows each
408ENGINE implementation to define an arbitrary set of configuration
409"commands" and expose them to OpenSSL and any applications based on
410OpenSSL. This mechanism is entirely based on the use of name-value pairs
411and assumes ASCII input (no unicode or UTF for now!), so it is ideal if
412applications want to provide a transparent way for users to provide
413arbitrary configuration "directives" directly to such ENGINEs. It is also
414possible for the application to dynamically interrogate the loaded ENGINE
415implementations for the names, descriptions, and input flags of their
416available "control commands", providing a more flexible configuration
417scheme. However, if the user is expected to know which ENGINE device he/she
418is using (in the case of specialised hardware, this goes without saying)
419then applications may not need to concern themselves with discovering the
420supported control commands and simply prefer to pass settings into ENGINEs
421exactly as they are provided by the user.
422
423Before illustrating how control commands work, it is worth mentioning what
424they are typically used for. Broadly speaking there are two uses for
425control commands; the first is to provide the necessary details to the
426implementation (which may know nothing at all specific to the host system)
427so that it can be initialised for use. This could include the path to any
428driver or config files it needs to load, required network addresses,
429smart-card identifiers, passwords to initialise protected devices,
430logging information, etc etc. This class of commands typically needs to be
431passed to an ENGINE B<before> attempting to initialise it, ie. before
432calling ENGINE_init(). The other class of commands consist of settings or
433operations that tweak certain behaviour or cause certain operations to take
434place, and these commands may work either before or after ENGINE_init(), or
435in some cases both. ENGINE implementations should provide indications of
436this in the descriptions attached to builtin control commands and/or in
437external product documentation.
438
439I<Issuing control commands to an ENGINE>
440
441Let's illustrate by example; a function for which the caller supplies the
442name of the ENGINE it wishes to use, a table of string-pairs for use before
443initialisation, and another table for use after initialisation. Note that
444the string-pairs used for control commands consist of a command "name"
445followed by the command "parameter" - the parameter could be NULL in some
446cases but the name can not. This function should initialise the ENGINE
447(issuing the "pre" commands beforehand and the "post" commands afterwards)
448and set it as the default for everything except RAND and then return a
449boolean success or failure.
450
451 int
452 generic_load_engine_fn(const char *engine_id,
453 const char **pre_cmds, int pre_num,
454 const char **post_cmds, int post_num)
455 {
456 ENGINE *e = ENGINE_by_id(engine_id);
457
458 if (!e)
459 return 0;
460 while (pre_num--) {
461 if (!ENGINE_ctrl_cmd_string(e,
462 pre_cmds[0], pre_cmds[1], 0)) {
463 fprintf(stderr,
464 "Failed command (%s - %s:%s)\n",
465 engine_id, pre_cmds[0],
466 pre_cmds[1] ? pre_cmds[1] : "(NULL)");
467 ENGINE_free(e);
468 return 0;
469 }
470 pre_cmds += 2;
471 }
472 if (!ENGINE_init(e)) {
473 fprintf(stderr, "Failed initialisation\n");
474 ENGINE_free(e);
475 return 0;
476 }
477 /*
478 * ENGINE_init() returned a functional reference,
479 * so free the structural reference from
480 * ENGINE_by_id().
481 */
482 ENGINE_free(e);
483 while (post_num--) {
484 if (!ENGINE_ctrl_cmd_string(e,
485 post_cmds[0], post_cmds[1], 0)) {
486 fprintf(stderr,
487 "Failed command (%s - %s:%s)\n",
488 engine_id, post_cmds[0],
489 post_cmds[1] ? post_cmds[1] : "(NULL)");
490 ENGINE_finish(e);
491 return 0;
492 }
493 post_cmds += 2;
494 }
495 ENGINE_set_default(e, ENGINE_METHOD_ALL & ~ENGINE_METHOD_RAND);
496 /* Success */
497 return 1;
498}
499
500Note that ENGINE_ctrl_cmd_string() accepts a boolean argument that can
501relax the semantics of the function - if set non-zero it will only return
502failure if the ENGINE supported the given command name but failed while
503executing it, if the ENGINE doesn't support the command name it will simply
504return success without doing anything. In this case we assume the user is
505only supplying commands specific to the given ENGINE so we set this to
506FALSE.
507
508I<Discovering supported control commands>
509
510It is possible to discover at run-time the names, numerical-ids, descriptions
511and input parameters of the control commands supported by an ENGINE using a
512structural reference. Note that some control commands are defined by OpenSSL
513itself and it will intercept and handle these control commands on behalf of the
514ENGINE, ie. the ENGINE's ctrl() handler is not used for the control command.
515openssl/engine.h defines an index, ENGINE_CMD_BASE, that all control commands
516implemented by ENGINEs should be numbered from. Any command value lower than
517this symbol is considered a "generic" command is handled directly by the
518OpenSSL core routines.
519
520It is using these "core" control commands that one can discover the control
521commands implemented by a given ENGINE, specifically the commands;
522
523 #define ENGINE_HAS_CTRL_FUNCTION 10
524 #define ENGINE_CTRL_GET_FIRST_CMD_TYPE 11
525 #define ENGINE_CTRL_GET_NEXT_CMD_TYPE 12
526 #define ENGINE_CTRL_GET_CMD_FROM_NAME 13
527 #define ENGINE_CTRL_GET_NAME_LEN_FROM_CMD 14
528 #define ENGINE_CTRL_GET_NAME_FROM_CMD 15
529 #define ENGINE_CTRL_GET_DESC_LEN_FROM_CMD 16
530 #define ENGINE_CTRL_GET_DESC_FROM_CMD 17
531 #define ENGINE_CTRL_GET_CMD_FLAGS 18
532
533Whilst these commands are automatically processed by the OpenSSL framework code,
534they use various properties exposed by each ENGINE to process these
535queries. An ENGINE has 3 properties it exposes that can affect how this behaves;
536it can supply a ctrl() handler, it can specify ENGINE_FLAGS_MANUAL_CMD_CTRL in
537the ENGINE's flags, and it can expose an array of control command descriptions.
538If an ENGINE specifies the ENGINE_FLAGS_MANUAL_CMD_CTRL flag, then it will
539simply pass all these "core" control commands directly to the ENGINE's ctrl()
540handler (and thus, it must have supplied one), so it is up to the ENGINE to
541reply to these "discovery" commands itself. If that flag is not set, then the
542OpenSSL framework code will work with the following rules;
543
544 if no ctrl() handler supplied;
545 ENGINE_HAS_CTRL_FUNCTION returns FALSE (zero),
546 all other commands fail.
547 if a ctrl() handler was supplied but no array of control commands;
548 ENGINE_HAS_CTRL_FUNCTION returns TRUE,
549 all other commands fail.
550 if a ctrl() handler and array of control commands was supplied;
551 ENGINE_HAS_CTRL_FUNCTION returns TRUE,
552 all other commands proceed processing ...
553
554If the ENGINE's array of control commands is empty then all other commands will
555fail, otherwise; ENGINE_CTRL_GET_FIRST_CMD_TYPE returns the identifier of
556the first command supported by the ENGINE, ENGINE_GET_NEXT_CMD_TYPE takes the
557identifier of a command supported by the ENGINE and returns the next command
558identifier or fails if there are no more, ENGINE_CMD_FROM_NAME takes a string
559name for a command and returns the corresponding identifier or fails if no such
560command name exists, and the remaining commands take a command identifier and
561return properties of the corresponding commands. All except
562ENGINE_CTRL_GET_FLAGS return the string length of a command name or description,
563or populate a supplied character buffer with a copy of the command name or
564description. ENGINE_CTRL_GET_FLAGS returns a bitwise-OR'd mask of the following
565possible values;
566
567 #define ENGINE_CMD_FLAG_NUMERIC (unsigned int)0x0001
568 #define ENGINE_CMD_FLAG_STRING (unsigned int)0x0002
569 #define ENGINE_CMD_FLAG_NO_INPUT (unsigned int)0x0004
570 #define ENGINE_CMD_FLAG_INTERNAL (unsigned int)0x0008
571
572If the ENGINE_CMD_FLAG_INTERNAL flag is set, then any other flags are purely
573informational to the caller - this flag will prevent the command being usable
574for any higher-level ENGINE functions such as ENGINE_ctrl_cmd_string().
575"INTERNAL" commands are not intended to be exposed to text-based configuration
576by applications, administrations, users, etc. These can support arbitrary
577operations via ENGINE_ctrl(), including passing to and/or from the control
578commands data of any arbitrary type. These commands are supported in the
579discovery mechanisms simply allow applications to determine if an ENGINE
580supports certain specific commands it might want to use (eg. application "foo"
581might query various ENGINEs to see if they implement "FOO_GET_VENDOR_LOGO_GIF" -
582and ENGINE could therefore decide whether or not to support this "foo"-specific
583extension).
584
585=head2 Future developments
586
587The ENGINE API and internal architecture is currently being reviewed. Slated for
588possible release in 0.9.8 is support for transparent loading of "dynamic"
589ENGINEs (built as self-contained shared-libraries). This would allow ENGINE
590implementations to be provided independently of OpenSSL libraries and/or
591OpenSSL-based applications, and would also remove any requirement for
592applications to explicitly use the "dynamic" ENGINE to bind to shared-library
593implementations.
594
595=head1 SEE ALSO
596
597L<rsa(3)|rsa(3)>, L<dsa(3)|dsa(3)>, L<dh(3)|dh(3)>, L<rand(3)|rand(3)>
598
599=cut
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