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-=pod
-=head1 NAME
-engine - ENGINE cryptographic module support
-=head1 SYNOPSIS
- #include <openssl/engine.h>
- ENGINE *ENGINE_get_first(void);
- ENGINE *ENGINE_get_last(void);
- ENGINE *ENGINE_get_next(ENGINE *e);
- ENGINE *ENGINE_get_prev(ENGINE *e);
- int ENGINE_add(ENGINE *e);
- int ENGINE_remove(ENGINE *e);
- ENGINE *ENGINE_by_id(const char *id);
- int ENGINE_init(ENGINE *e);
- int ENGINE_finish(ENGINE *e);
- void ENGINE_load_openssl(void);
- void ENGINE_load_dynamic(void);
- #ifndef OPENSSL_NO_STATIC_ENGINE
- void ENGINE_load_4758cca(void);
- void ENGINE_load_aep(void);
- void ENGINE_load_atalla(void);
- void ENGINE_load_chil(void);
- void ENGINE_load_cswift(void);
- void ENGINE_load_gmp(void);
- void ENGINE_load_nuron(void);
- void ENGINE_load_sureware(void);
- void ENGINE_load_ubsec(void);
- #endif
- void ENGINE_load_cryptodev(void);
- void ENGINE_load_builtin_engines(void);
- void ENGINE_cleanup(void);
- ENGINE *ENGINE_get_default_RSA(void);
- ENGINE *ENGINE_get_default_DSA(void);
- ENGINE *ENGINE_get_default_ECDH(void);
- ENGINE *ENGINE_get_default_ECDSA(void);
- ENGINE *ENGINE_get_default_DH(void);
- ENGINE *ENGINE_get_default_RAND(void);
- ENGINE *ENGINE_get_cipher_engine(int nid);
- ENGINE *ENGINE_get_digest_engine(int nid);
- int ENGINE_set_default_RSA(ENGINE *e);
- int ENGINE_set_default_DSA(ENGINE *e);
- int ENGINE_set_default_ECDH(ENGINE *e);
- int ENGINE_set_default_ECDSA(ENGINE *e);
- int ENGINE_set_default_DH(ENGINE *e);
- int ENGINE_set_default_RAND(ENGINE *e);
- int ENGINE_set_default_ciphers(ENGINE *e);
- int ENGINE_set_default_digests(ENGINE *e);
- int ENGINE_set_default_string(ENGINE *e, const char *list);
- int ENGINE_set_default(ENGINE *e, unsigned int flags);
- unsigned int ENGINE_get_table_flags(void);
- void ENGINE_set_table_flags(unsigned int flags);
- int ENGINE_register_RSA(ENGINE *e);
- void ENGINE_unregister_RSA(ENGINE *e);
- void ENGINE_register_all_RSA(void);
- int ENGINE_register_DSA(ENGINE *e);
- void ENGINE_unregister_DSA(ENGINE *e);
- void ENGINE_register_all_DSA(void);
- int ENGINE_register_ECDH(ENGINE *e);
- void ENGINE_unregister_ECDH(ENGINE *e);
- void ENGINE_register_all_ECDH(void);
- int ENGINE_register_ECDSA(ENGINE *e);
- void ENGINE_unregister_ECDSA(ENGINE *e);
- void ENGINE_register_all_ECDSA(void);
- int ENGINE_register_DH(ENGINE *e);
- void ENGINE_unregister_DH(ENGINE *e);
- void ENGINE_register_all_DH(void);
- int ENGINE_register_RAND(ENGINE *e);
- void ENGINE_unregister_RAND(ENGINE *e);
- void ENGINE_register_all_RAND(void);
- int ENGINE_register_STORE(ENGINE *e);
- void ENGINE_unregister_STORE(ENGINE *e);
- void ENGINE_register_all_STORE(void);
- int ENGINE_register_ciphers(ENGINE *e);
- void ENGINE_unregister_ciphers(ENGINE *e);
- void ENGINE_register_all_ciphers(void);
- int ENGINE_register_digests(ENGINE *e);
- void ENGINE_unregister_digests(ENGINE *e);
- void ENGINE_register_all_digests(void);
- int ENGINE_register_complete(ENGINE *e);
- int ENGINE_register_all_complete(void);
- int ENGINE_ctrl(ENGINE *e, int cmd, long i, void *p, void (*f)(void));
- int ENGINE_cmd_is_executable(ENGINE *e, int cmd);
- int ENGINE_ctrl_cmd(ENGINE *e, const char *cmd_name,
-         long i, void *p, void (*f)(void), int cmd_optional);
- int ENGINE_ctrl_cmd_string(ENGINE *e, const char *cmd_name, const char *arg,
-         int cmd_optional);
- int ENGINE_set_ex_data(ENGINE *e, int idx, void *arg);
- void *ENGINE_get_ex_data(const ENGINE *e, int idx);
- int ENGINE_get_ex_new_index(long argl, void *argp, CRYPTO_EX_new *new_func,
-         CRYPTO_EX_dup *dup_func, CRYPTO_EX_free *free_func);
- ENGINE *ENGINE_new(void);
- int ENGINE_free(ENGINE *e);
- int ENGINE_up_ref(ENGINE *e);
- int ENGINE_set_id(ENGINE *e, const char *id);
- int ENGINE_set_name(ENGINE *e, const char *name);
- int ENGINE_set_RSA(ENGINE *e, const RSA_METHOD *rsa_meth);
- int ENGINE_set_DSA(ENGINE *e, const DSA_METHOD *dsa_meth);
- int ENGINE_set_ECDH(ENGINE *e, const ECDH_METHOD *dh_meth);
- int ENGINE_set_ECDSA(ENGINE *e, const ECDSA_METHOD *dh_meth);
- int ENGINE_set_DH(ENGINE *e, const DH_METHOD *dh_meth);
- int ENGINE_set_RAND(ENGINE *e, const RAND_METHOD *rand_meth);
- int ENGINE_set_STORE(ENGINE *e, const STORE_METHOD *rand_meth);
- int ENGINE_set_destroy_function(ENGINE *e, ENGINE_GEN_INT_FUNC_PTR destroy_f);
- int ENGINE_set_init_function(ENGINE *e, ENGINE_GEN_INT_FUNC_PTR init_f);
- int ENGINE_set_finish_function(ENGINE *e, ENGINE_GEN_INT_FUNC_PTR finish_f);
- int ENGINE_set_ctrl_function(ENGINE *e, ENGINE_CTRL_FUNC_PTR ctrl_f);
- int ENGINE_set_load_privkey_function(ENGINE *e, ENGINE_LOAD_KEY_PTR loadpriv_f);
- int ENGINE_set_load_pubkey_function(ENGINE *e, ENGINE_LOAD_KEY_PTR loadpub_f);
- int ENGINE_set_ciphers(ENGINE *e, ENGINE_CIPHERS_PTR f);
- int ENGINE_set_digests(ENGINE *e, ENGINE_DIGESTS_PTR f);
- int ENGINE_set_flags(ENGINE *e, int flags);
- int ENGINE_set_cmd_defns(ENGINE *e, const ENGINE_CMD_DEFN *defns);
- const char *ENGINE_get_id(const ENGINE *e);
- const char *ENGINE_get_name(const ENGINE *e);
- const RSA_METHOD *ENGINE_get_RSA(const ENGINE *e);
- const DSA_METHOD *ENGINE_get_DSA(const ENGINE *e);
- const ECDH_METHOD *ENGINE_get_ECDH(const ENGINE *e);
- const ECDSA_METHOD *ENGINE_get_ECDSA(const ENGINE *e);
- const DH_METHOD *ENGINE_get_DH(const ENGINE *e);
- const RAND_METHOD *ENGINE_get_RAND(const ENGINE *e);
- const STORE_METHOD *ENGINE_get_STORE(const ENGINE *e);
- ENGINE_GEN_INT_FUNC_PTR ENGINE_get_destroy_function(const ENGINE *e);
- ENGINE_GEN_INT_FUNC_PTR ENGINE_get_init_function(const ENGINE *e);
- ENGINE_GEN_INT_FUNC_PTR ENGINE_get_finish_function(const ENGINE *e);
- ENGINE_CTRL_FUNC_PTR ENGINE_get_ctrl_function(const ENGINE *e);
- ENGINE_LOAD_KEY_PTR ENGINE_get_load_privkey_function(const ENGINE *e);
- ENGINE_LOAD_KEY_PTR ENGINE_get_load_pubkey_function(const ENGINE *e);
- ENGINE_CIPHERS_PTR ENGINE_get_ciphers(const ENGINE *e);
- ENGINE_DIGESTS_PTR ENGINE_get_digests(const ENGINE *e);
- const EVP_CIPHER *ENGINE_get_cipher(ENGINE *e, int nid);
- const EVP_MD *ENGINE_get_digest(ENGINE *e, int nid);
- int ENGINE_get_flags(const ENGINE *e);
- const ENGINE_CMD_DEFN *ENGINE_get_cmd_defns(const ENGINE *e);
- EVP_PKEY *ENGINE_load_private_key(ENGINE *e, const char *key_id,
-     UI_METHOD *ui_method, void *callback_data);
- EVP_PKEY *ENGINE_load_public_key(ENGINE *e, const char *key_id,
-     UI_METHOD *ui_method, void *callback_data);
- void ENGINE_add_conf_module(void);
-=head1 DESCRIPTION
-These functions create, manipulate, and use cryptographic modules in the
-form of B<ENGINE> objects. These objects act as containers for
-implementations of cryptographic algorithms, and support a
-reference-counted mechanism to allow them to be dynamically loaded in and
-out of the running application.
-The cryptographic functionality that can be provided by an B<ENGINE>
-implementation includes the following abstractions;
- RSA_METHOD - for providing alternative RSA implementations
- DSA_METHOD, DH_METHOD, RAND_METHOD, ECDH_METHOD, ECDSA_METHOD,
-       STORE_METHOD - similarly for other OpenSSL APIs
- EVP_CIPHER - potentially multiple cipher algorithms (indexed by 'nid')
- EVP_DIGEST - potentially multiple hash algorithms (indexed by 'nid')
- key-loading - loading public and/or private EVP_PKEY keys
-=head2 Reference counting and handles
-Due to the modular nature of the ENGINE API, pointers to ENGINEs need to be
-treated as handles - ie. not only as pointers, but also as references to
-the underlying ENGINE object. Ie. one should obtain a new reference when
-making copies of an ENGINE pointer if the copies will be used (and
-released) independently.
-ENGINE objects have two levels of reference-counting to match the way in
-which the objects are used. At the most basic level, each ENGINE pointer is
-inherently a B<structural> reference - a structural reference is required
-to use the pointer value at all, as this kind of reference is a guarantee
-that the structure can not be deallocated until the reference is released.
-However, a structural reference provides no guarantee that the ENGINE is
-initiliased and able to use any of its cryptographic
-implementations. Indeed it's quite possible that most ENGINEs will not
-initialise at all in typical environments, as ENGINEs are typically used to
-support specialised hardware. To use an ENGINE's functionality, you need a
-B<functional> reference. This kind of reference can be considered a
-specialised form of structural reference, because each functional reference
-implicitly contains a structural reference as well - however to avoid
-difficult-to-find programming bugs, it is recommended to treat the two
-kinds of reference independently. If you have a functional reference to an
-ENGINE, you have a guarantee that the ENGINE has been initialised ready to
-perform cryptographic operations and will remain uninitialised
-until after you have released your reference.
-I<Structural references>
-This basic type of reference is used for instantiating new ENGINEs,
-iterating across OpenSSL's internal linked-list of loaded
-ENGINEs, reading information about an ENGINE, etc. Essentially a structural
-reference is sufficient if you only need to query or manipulate the data of
-an ENGINE implementation rather than use its functionality.
-The ENGINE_new() function returns a structural reference to a new (empty)
-ENGINE object. There are other ENGINE API functions that return structural
-references such as; ENGINE_by_id(), ENGINE_get_first(), ENGINE_get_last(),
-ENGINE_get_next(), ENGINE_get_prev(). All structural references should be
-released by a corresponding to call to the ENGINE_free() function - the
-ENGINE object itself will only actually be cleaned up and deallocated when
-the last structural reference is released.
-It should also be noted that many ENGINE API function calls that accept a
-structural reference will internally obtain another reference - typically
-this happens whenever the supplied ENGINE will be needed by OpenSSL after
-the function has returned. Eg. the function to add a new ENGINE to
-OpenSSL's internal list is ENGINE_add() - if this function returns success,
-then OpenSSL will have stored a new structural reference internally so the
-caller is still responsible for freeing their own reference with
-ENGINE_free() when they are finished with it. In a similar way, some
-functions will automatically release the structural reference passed to it
-if part of the function's job is to do so. Eg. the ENGINE_get_next() and
-ENGINE_get_prev() functions are used for iterating across the internal
-ENGINE list - they will return a new structural reference to the next (or
-previous) ENGINE in the list or NULL if at the end (or beginning) of the
-list, but in either case the structural reference passed to the function is
-released on behalf of the caller.
-To clarify a particular function's handling of references, one should
-always consult that function's documentation "man" page, or failing that
-the openssl/engine.h header file includes some hints.
-I<Functional references>
-As mentioned, functional references exist when the cryptographic
-functionality of an ENGINE is required to be available. A functional
-reference can be obtained in one of two ways; from an existing structural
-reference to the required ENGINE, or by asking OpenSSL for the default
-operational ENGINE for a given cryptographic purpose.
-To obtain a functional reference from an existing structural reference,
-call the ENGINE_init() function. This returns zero if the ENGINE was not
-already operational and couldn't be successfully initialised (eg. lack of
-system drivers, no special hardware attached, etc), otherwise it will
-return non-zero to indicate that the ENGINE is now operational and will
-have allocated a new B<functional> reference to the ENGINE. All functional
-references are released by calling ENGINE_finish() (which removes the
-implicit structural reference as well).
-The second way to get a functional reference is by asking OpenSSL for a
-default implementation for a given task, eg. by ENGINE_get_default_RSA(),
-ENGINE_get_default_cipher_engine(), etc. These are discussed in the next
-section, though they are not usually required by application programmers as
-they are used automatically when creating and using the relevant
-algorithm-specific types in OpenSSL, such as RSA, DSA, EVP_CIPHER_CTX, etc.
-=head2 Default implementations
-For each supported abstraction, the ENGINE code maintains an internal table
-of state to control which implementations are available for a given
-abstraction and which should be used by default. These implementations are
-registered in the tables and indexed by an 'nid' value, because
-abstractions like EVP_CIPHER and EVP_DIGEST support many distinct
-algorithms and modes, and ENGINEs can support arbitrarily many of them.
-In the case of other abstractions like RSA, DSA, etc, there is only one
-"algorithm" so all implementations implicitly register using the same 'nid'
-index.
-When a default ENGINE is requested for a given abstraction/algorithm/mode, (eg.
-when calling RSA_new_method(NULL)), a "get_default" call will be made to the
-ENGINE subsystem to process the corresponding state table and return a
-functional reference to an initialised ENGINE whose implementation should be
-used. If no ENGINE should (or can) be used, it will return NULL and the caller
-will operate with a NULL ENGINE handle - this usually equates to using the
-conventional software implementation. In the latter case, OpenSSL will from
-then on behave the way it used to before the ENGINE API existed.
-Each state table has a flag to note whether it has processed this
-"get_default" query since the table was last modified, because to process
-this question it must iterate across all the registered ENGINEs in the
-table trying to initialise each of them in turn, in case one of them is
-operational. If it returns a functional reference to an ENGINE, it will
-also cache another reference to speed up processing future queries (without
-needing to iterate across the table). Likewise, it will cache a NULL
-response if no ENGINE was available so that future queries won't repeat the
-same iteration unless the state table changes. This behaviour can also be
-changed; if the ENGINE_TABLE_FLAG_NOINIT flag is set (using
-ENGINE_set_table_flags()), no attempted initialisations will take place,
-instead the only way for the state table to return a non-NULL ENGINE to the
-"get_default" query will be if one is expressly set in the table. Eg.
-ENGINE_set_default_RSA() does the same job as ENGINE_register_RSA() except
-that it also sets the state table's cached response for the "get_default"
-query. In the case of abstractions like EVP_CIPHER, where implementations are
-indexed by 'nid', these flags and cached-responses are distinct for each 'nid'
-value.
-=head2 Application requirements
-This section will explain the basic things an application programmer should
-support to make the most useful elements of the ENGINE functionality
-available to the user. The first thing to consider is whether the
-programmer wishes to make alternative ENGINE modules available to the
-application and user. OpenSSL maintains an internal linked list of
-"visible" ENGINEs from which it has to operate - at start-up, this list is
-empty and in fact if an application does not call any ENGINE API calls and
-it uses static linking against openssl, then the resulting application
-binary will not contain any alternative ENGINE code at all. So the first
-consideration is whether any/all available ENGINE implementations should be
-made visible to OpenSSL - this is controlled by calling the various "load"
-functions, eg.
- /* Make the "dynamic" ENGINE available */
- void ENGINE_load_dynamic(void);
- /* Make the CryptoSwift hardware acceleration support available */
- void ENGINE_load_cswift(void);
- /* Make support for nCipher's "CHIL" hardware available */
- void ENGINE_load_chil(void);
- ...
- /* Make ALL ENGINE implementations bundled with OpenSSL available */
- void ENGINE_load_builtin_engines(void);
-Having called any of these functions, ENGINE objects would have been
-dynamically allocated and populated with these implementations and linked
-into OpenSSL's internal linked list. At this point it is important to
-mention an important API function;
- void ENGINE_cleanup(void);
-If no ENGINE API functions are called at all in an application, then there
-are no inherent memory leaks to worry about from the ENGINE functionality,
-however if any ENGINEs are loaded, even if they are never registered or
-used, it is necessary to use the ENGINE_cleanup() function to
-correspondingly cleanup before program exit, if the caller wishes to avoid
-memory leaks. This mechanism uses an internal callback registration table
-so that any ENGINE API functionality that knows it requires cleanup can
-register its cleanup details to be called during ENGINE_cleanup(). This
-approach allows ENGINE_cleanup() to clean up after any ENGINE functionality
-at all that your program uses, yet doesn't automatically create linker
-dependencies to all possible ENGINE functionality - only the cleanup
-callbacks required by the functionality you do use will be required by the
-linker.
-The fact that ENGINEs are made visible to OpenSSL (and thus are linked into
-the program and loaded into memory at run-time) does not mean they are
-"registered" or called into use by OpenSSL automatically - that behaviour
-is something for the application to control. Some applications
-will want to allow the user to specify exactly which ENGINE they want used
-if any is to be used at all. Others may prefer to load all support and have
-OpenSSL automatically use at run-time any ENGINE that is able to
-successfully initialise - ie. to assume that this corresponds to
-acceleration hardware attached to the machine or some such thing. There are
-probably numerous other ways in which applications may prefer to handle
-things, so we will simply illustrate the consequences as they apply to a
-couple of simple cases and leave developers to consider these and the
-source code to openssl's builtin utilities as guides.
-I<Using a specific ENGINE implementation>
-Here we'll assume an application has been configured by its user or admin
-to want to use the "ACME" ENGINE if it is available in the version of
-OpenSSL the application was compiled with. If it is available, it should be
-used by default for all RSA, DSA, and symmetric cipher operation, otherwise
-OpenSSL should use its builtin software as per usual. The following code
-illustrates how to approach this;
- ENGINE *e;
- const char *engine_id = "ACME";
- ENGINE_load_builtin_engines();
- e = ENGINE_by_id(engine_id);
- if(!e)
-     /* the engine isn't available */
-     return;
- if(!ENGINE_init(e)) {
-     /* the engine couldn't initialise, release 'e' */
-     ENGINE_free(e);
-     return;
- }
- if(!ENGINE_set_default_RSA(e))
-     /* This should only happen when 'e' can't initialise, but the previous
-      * statement suggests it did. */
-     abort();
- ENGINE_set_default_DSA(e);
- ENGINE_set_default_ciphers(e);
- /* Release the functional reference from ENGINE_init() */
- ENGINE_finish(e);
- /* Release the structural reference from ENGINE_by_id() */
- ENGINE_free(e);
-I<Automatically using builtin ENGINE implementations>
-Here we'll assume we want to load and register all ENGINE implementations
-bundled with OpenSSL, such that for any cryptographic algorithm required by
-OpenSSL - if there is an ENGINE that implements it and can be initialise,
-it should be used. The following code illustrates how this can work;
- /* Load all bundled ENGINEs into memory and make them visible */
- ENGINE_load_builtin_engines();
- /* Register all of them for every algorithm they collectively implement */
- ENGINE_register_all_complete();
-That's all that's required. Eg. the next time OpenSSL tries to set up an
-RSA key, any bundled ENGINEs that implement RSA_METHOD will be passed to
-ENGINE_init() and if any of those succeed, that ENGINE will be set as the
-default for RSA use from then on.
-=head2 Advanced configuration support
-There is a mechanism supported by the ENGINE framework that allows each
-ENGINE implementation to define an arbitrary set of configuration
-"commands" and expose them to OpenSSL and any applications based on
-OpenSSL. This mechanism is entirely based on the use of name-value pairs
-and assumes ASCII input (no unicode or UTF for now!), so it is ideal if
-applications want to provide a transparent way for users to provide
-arbitrary configuration "directives" directly to such ENGINEs. It is also
-possible for the application to dynamically interrogate the loaded ENGINE
-implementations for the names, descriptions, and input flags of their
-available "control commands", providing a more flexible configuration
-scheme. However, if the user is expected to know which ENGINE device he/she
-is using (in the case of specialised hardware, this goes without saying)
-then applications may not need to concern themselves with discovering the
-supported control commands and simply prefer to pass settings into ENGINEs
-exactly as they are provided by the user.
-Before illustrating how control commands work, it is worth mentioning what
-they are typically used for. Broadly speaking there are two uses for
-control commands; the first is to provide the necessary details to the
-implementation (which may know nothing at all specific to the host system)
-so that it can be initialised for use. This could include the path to any
-driver or config files it needs to load, required network addresses,
-smart-card identifiers, passwords to initialise protected devices,
-logging information, etc etc. This class of commands typically needs to be
-passed to an ENGINE B<before> attempting to initialise it, ie. before
-calling ENGINE_init(). The other class of commands consist of settings or
-operations that tweak certain behaviour or cause certain operations to take
-place, and these commands may work either before or after ENGINE_init(), or
-in some cases both. ENGINE implementations should provide indications of
-this in the descriptions attached to builtin control commands and/or in
-external product documentation.
-I<Issuing control commands to an ENGINE>
-Let's illustrate by example; a function for which the caller supplies the
-name of the ENGINE it wishes to use, a table of string-pairs for use before
-initialisation, and another table for use after initialisation. Note that
-the string-pairs used for control commands consist of a command "name"
-followed by the command "parameter" - the parameter could be NULL in some
-cases but the name can not. This function should initialise the ENGINE
-(issuing the "pre" commands beforehand and the "post" commands afterwards)
-and set it as the default for everything except RAND and then return a
-boolean success or failure.
- int generic_load_engine_fn(const char *engine_id,
-                            const char **pre_cmds, int pre_num,
-                            const char **post_cmds, int post_num)
- {
-     ENGINE *e = ENGINE_by_id(engine_id);
-     if(!e) return 0;
-     while(pre_num--) {
-         if(!ENGINE_ctrl_cmd_string(e, pre_cmds[0], pre_cmds[1], 0)) {
-             fprintf(stderr, "Failed command (%s - %s:%s)\n", engine_id,
-                 pre_cmds[0], pre_cmds[1] ? pre_cmds[1] : "(NULL)");
-             ENGINE_free(e);
-             return 0;
-         }
-         pre_cmds += 2;
-     }
-     if(!ENGINE_init(e)) {
-         fprintf(stderr, "Failed initialisation\n");
-         ENGINE_free(e);
-         return 0;
-     }
-     /* ENGINE_init() returned a functional reference, so free the structural
-      * reference from ENGINE_by_id(). */
-     ENGINE_free(e);
-     while(post_num--) {
-         if(!ENGINE_ctrl_cmd_string(e, post_cmds[0], post_cmds[1], 0)) {
-             fprintf(stderr, "Failed command (%s - %s:%s)\n", engine_id,
-                 post_cmds[0], post_cmds[1] ? post_cmds[1] : "(NULL)");
-             ENGINE_finish(e);
-             return 0;
-         }
-         post_cmds += 2;
-     }
-     ENGINE_set_default(e, ENGINE_METHOD_ALL & ~ENGINE_METHOD_RAND);
-     /* Success */
-     return 1;
- }
-Note that ENGINE_ctrl_cmd_string() accepts a boolean argument that can
-relax the semantics of the function - if set non-zero it will only return
-failure if the ENGINE supported the given command name but failed while
-executing it, if the ENGINE doesn't support the command name it will simply
-return success without doing anything. In this case we assume the user is
-only supplying commands specific to the given ENGINE so we set this to
-FALSE.
-I<Discovering supported control commands>
-It is possible to discover at run-time the names, numerical-ids, descriptions
-and input parameters of the control commands supported by an ENGINE using a
-structural reference. Note that some control commands are defined by OpenSSL
-itself and it will intercept and handle these control commands on behalf of the
-ENGINE, ie. the ENGINE's ctrl() handler is not used for the control command.
-openssl/engine.h defines an index, ENGINE_CMD_BASE, that all control commands
-implemented by ENGINEs should be numbered from. Any command value lower than
-this symbol is considered a "generic" command is handled directly by the
-OpenSSL core routines.
-It is using these "core" control commands that one can discover the the control
-commands implemented by a given ENGINE, specifically the commands;
- #define ENGINE_HAS_CTRL_FUNCTION               10
- #define ENGINE_CTRL_GET_FIRST_CMD_TYPE         11
- #define ENGINE_CTRL_GET_NEXT_CMD_TYPE          12
- #define ENGINE_CTRL_GET_CMD_FROM_NAME          13
- #define ENGINE_CTRL_GET_NAME_LEN_FROM_CMD      14
- #define ENGINE_CTRL_GET_NAME_FROM_CMD          15
- #define ENGINE_CTRL_GET_DESC_LEN_FROM_CMD      16
- #define ENGINE_CTRL_GET_DESC_FROM_CMD          17
- #define ENGINE_CTRL_GET_CMD_FLAGS              18
-Whilst these commands are automatically processed by the OpenSSL framework code,
-they use various properties exposed by each ENGINE to process these
-queries. An ENGINE has 3 properties it exposes that can affect how this behaves;
-it can supply a ctrl() handler, it can specify ENGINE_FLAGS_MANUAL_CMD_CTRL in
-the ENGINE's flags, and it can expose an array of control command descriptions.
-If an ENGINE specifies the ENGINE_FLAGS_MANUAL_CMD_CTRL flag, then it will
-simply pass all these "core" control commands directly to the ENGINE's ctrl()
-handler (and thus, it must have supplied one), so it is up to the ENGINE to
-reply to these "discovery" commands itself. If that flag is not set, then the
-OpenSSL framework code will work with the following rules;
- if no ctrl() handler supplied;
-     ENGINE_HAS_CTRL_FUNCTION returns FALSE (zero),
-     all other commands fail.
- if a ctrl() handler was supplied but no array of control commands;
-     ENGINE_HAS_CTRL_FUNCTION returns TRUE,
-     all other commands fail.
- if a ctrl() handler and array of control commands was supplied;
-     ENGINE_HAS_CTRL_FUNCTION returns TRUE,
-     all other commands proceed processing ...
-If the ENGINE's array of control commands is empty then all other commands will
-fail, otherwise; ENGINE_CTRL_GET_FIRST_CMD_TYPE returns the identifier of
-the first command supported by the ENGINE, ENGINE_GET_NEXT_CMD_TYPE takes the
-identifier of a command supported by the ENGINE and returns the next command
-identifier or fails if there are no more, ENGINE_CMD_FROM_NAME takes a string
-name for a command and returns the corresponding identifier or fails if no such
-command name exists, and the remaining commands take a command identifier and
-return properties of the corresponding commands. All except
-ENGINE_CTRL_GET_FLAGS return the string length of a command name or description,
-or populate a supplied character buffer with a copy of the command name or
-description. ENGINE_CTRL_GET_FLAGS returns a bitwise-OR'd mask of the following
-possible values;
- #define ENGINE_CMD_FLAG_NUMERIC                (unsigned int)0x0001
- #define ENGINE_CMD_FLAG_STRING                 (unsigned int)0x0002
- #define ENGINE_CMD_FLAG_NO_INPUT               (unsigned int)0x0004
- #define ENGINE_CMD_FLAG_INTERNAL               (unsigned int)0x0008
-If the ENGINE_CMD_FLAG_INTERNAL flag is set, then any other flags are purely
-informational to the caller - this flag will prevent the command being usable
-for any higher-level ENGINE functions such as ENGINE_ctrl_cmd_string().
-"INTERNAL" commands are not intended to be exposed to text-based configuration
-by applications, administrations, users, etc. These can support arbitrary
-operations via ENGINE_ctrl(), including passing to and/or from the control
-commands data of any arbitrary type. These commands are supported in the
-discovery mechanisms simply to allow applications determinie if an ENGINE
-supports certain specific commands it might want to use (eg. application "foo"
-might query various ENGINEs to see if they implement "FOO_GET_VENDOR_LOGO_GIF" -
-and ENGINE could therefore decide whether or not to support this "foo"-specific
-extension).
-=head2 Future developments
-The ENGINE API and internal architecture is currently being reviewed. Slated for
-possible release in 0.9.8 is support for transparent loading of "dynamic"
-ENGINEs (built as self-contained shared-libraries). This would allow ENGINE
-implementations to be provided independently of OpenSSL libraries and/or
-OpenSSL-based applications, and would also remove any requirement for
-applications to explicitly use the "dynamic" ENGINE to bind to shared-library
-implementations.
-=head1 SEE ALSO
-L<rsa(3)|rsa(3)>, L<dsa(3)|dsa(3)>, L<dh(3)|dh(3)>, L<rand(3)|rand(3)>
-=cut