engine - ENGINE cryptographic module support


        #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);
        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);
        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);


       These functions create, manipulate, and use cryptographic modules in
       the form of ENGINE objects. These objects act as containers for imple-
       mentations of cryptographic algorithms, and support a reference-counted
       mechanism to allow them to be dynamically loaded in and out of the run-
       ning application.

       The cryptographic functionality that can be provided by an ENGINE
       implementation includes the following abstractions;

        RSA_METHOD - for providing alternative RSA implementations
              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

       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 refer-
       ences to the underlying ENGINE object. Ie. one should obtain a new ref-
       erence 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 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 ref-
       erence is released.

       However, a structural reference provides no guarantee that the ENGINE
       is initialised and able to use any of its cryptographic implementa-
       tions. Indeed it's quite possible that most ENGINEs will not initialise
       at all in typical environments, as ENGINEs are typically used to sup-
       port specialised hardware. To use an ENGINE's functionality, you need a
       functional reference. This kind of reference can be considered a spe-
       cialised form of structural reference, because each functional refer-
       ence 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 ref-
       erence to an ENGINE, you have a guarantee that the ENGINE has been ini-
       tialised and is ready to perform cryptographic operations, and will
       remain initialised until after you have released your reference.

       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 ref-
       erence 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

       It should also be noted that many ENGINE API function calls that accept
       a structural reference will internally obtain another reference - typi-
       cally 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 refer-
       ence 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.

       Functional references

       As mentioned, functional references exist when the cryptographic func-
       tionality of an ENGINE is required to be available. A functional refer-
       ence 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 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.

       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 implemen-
       tations are registered in the tables and indexed by an 'nid' value,
       because abstractions like EVP_CIPHER and EVP_DIGEST support many dis-
       tinct 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/algo-
       rithm/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 initialisa-
       tions 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 abstrac-
       tions like EVP_CIPHER, where implementations are indexed by 'nid',
       these flags and cached-responses are distinct for each 'nid' value.

       Application requirements

       This section will explain the basic things an application programmer
       should support to make the most useful elements of the ENGINE function-
       ality 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 imple-
       mentations 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 impor-
       tant 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 func-
       tionality, however if any ENGINEs are loaded, even if they are never
       registered or used, it is necessary to use the ENGINE_cleanup() func-
       tion 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 functional-
       ity 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 be-
       haviour 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.

       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 ver-
       sion of OpenSSL the application was compiled with. If it is available,
       it should be used by default for all RSA, DSA, and symmetric cipher
       operations, 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";
        e = ENGINE_by_id(engine_id);
            /* the engine isn't available */
        if(!ENGINE_init(e)) {
            /* the engine couldn't initialise, release 'e' */
            /* This should only happen when 'e' can't initialise, but the previous
             * statement suggests it did. */
        /* Release the functional reference from ENGINE_init() */
        /* Release the structural reference from ENGINE_by_id() */

       Automatically using builtin ENGINE implementations

       Here we'll assume we want to load and register all ENGINE implementa-
       tions bundled with OpenSSL, such that for any cryptographic algorithm
       required by OpenSSL - if there is an ENGINE that implements it and can
       be initialised, it should be used. The following code illustrates how
       this can work;

        /* Load all bundled ENGINEs into memory and make them visible */
        /* Register all of them for every algorithm they collectively implement */

       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.

       Advanced configuration support

       There is a mechanism supported by the ENGINE framework that allows each
       ENGINE implementation to define an arbitrary set of configuration "com-
       mands" 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 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.

       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 initialisa-
       tion. 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)");
                    return 0;
                pre_cmds += 2;
            if(!ENGINE_init(e)) {
                fprintf(stderr, "Failed initialisation\n");
                return 0;
            /* ENGINE_init() returned a functional reference, so free the structural
             * reference from ENGINE_by_id(). */
            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)");
                    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.

       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() han-
       dler 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 sym-
       bol 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 com-

        #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 frame-
       work 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 com-
       mands 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 bit-
       wise-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 arbi-
       trary 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_VEN-
       DOR_LOGO_GIF" - and ENGINE could therefore decide whether or not to
       support this "foo"-specific extension).

       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.


       rsa(3), dsa(3), dh(3), rand(3)

1.0.2t                            2019-09-10                         engine(3)

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