engine(3)                    OpenSSL                    engine(3)


     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);

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engine(3)                    OpenSSL                    engine(3)

      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);

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engine(3)                    OpenSSL                    engine(3)

      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);

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engine(3)                    OpenSSL                    engine(3)

      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.

     However, a structural reference provides no guarantee that
     the ENGINE is initialised 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 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

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engine(3)                    OpenSSL                    engine(3)

     kinds of reference independently. If you have a functional
     reference to an ENGINE, you have a guarantee that the ENGINE
     has been initialised 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 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.

     Functional references

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engine(3)                    OpenSSL                    engine(3)

     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 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 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'

     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.

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engine(3)                    OpenSSL                    engine(3)

     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.

     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

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engine(3)                    OpenSSL                    engine(3)

     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.

     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 operations,
     otherwise OpenSSL should use its builtin software as per
     usual. The following code illustrates how to approach this;

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engine(3)                    OpenSSL                    engine(3)

      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
     implementations 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 "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

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engine(3)                    OpenSSL                    engine(3)

     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 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.

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engine(3)                    OpenSSL                    engine(3)

      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;
          /* 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

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engine(3)                    OpenSSL                    engine(3)

     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

     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

      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;
          all other commands fail.
      if a ctrl() handler and array of control commands was supplied;
          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,

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engine(3)                    OpenSSL                    engine(3)

     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

     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


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

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