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            #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);
            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);
            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);
            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
           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 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
           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 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.
           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
           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.
           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
           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'
       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 -
           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.
           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";
            e = ENGINE_by_id(engine_id);
                /* the engine isn't available */
            if(!ENGINE_init(e)) {
                /* the engine couldn't initialise, release 'e' */
           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 */
            /* 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
           "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 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.
                                       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()
           handler is not used for the control command.  openssl/engine.h defines
            #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(),
           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)

    1.0.1e 2013-02-11 engine(3)


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