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engine(3) OpenSSL engine(3)
NAME
engine - ENGINE cryptographic module support
SYNOPSIS
#include
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_cswift(void);
void ENGINE_load_chil(void);
void ENGINE_load_atalla(void);
void ENGINE_load_nuron(void);
void ENGINE_load_ubsec(void);
void ENGINE_load_aep(void);
void ENGINE_load_sureware(void);
void ENGINE_load_4758cca(void);
void ENGINE_load_openbsd_dev_crypto(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_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_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_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_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)());
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)(), 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_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_DH(ENGINE *e, const DH_METHOD *dh_meth);
int ENGINE_set_RAND(ENGINE *e, const RAND_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 DH_METHOD *ENGINE_get_DH(const ENGINE *e);
const RAND_METHOD *ENGINE_get_RAND(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);
DESCRIPTION
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
DSA_METHOD, DH_METHOD, RAND_METHOD - alternative DSA, DH, and RAND
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. you should obtain a new ref-
erence when making copies of an ENGINE pointer if the copies will be
used (and released) independantly.
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 - you need a structural
reference simply to refer to the pointer value at all, as this kind of
reference is your guarantee that the structure can not be deallocated
until you release your reference.
However, a structural reference provides no guarantee that the ENGINE
has been initiliased to be usable to perform any of its cryptographic
implementations - and indeed it's quite possible that most ENGINEs will
not initialised at all on standard setups, 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 consid-
ered a specialised form of structural reference, because each func-
tional reference implicitly contains a structural reference as well -
however to avoid difficult-to-find programming bugs, it is recommended
to treat the two kinds of reference independantly. If you have a func-
tional reference to an ENGINE, you have a guarantee that the ENGINE has
been initialised ready to perform cryptographic operations and will not
be uninitialised or cleaned up until after you have released your ref-
erence.
We will discuss the two kinds of reference separately, including how to
tell which one you are dealing with at any given point in time (after
all they are both simply (ENGINE *) pointers, the difference is in the
way they are used).
Structural references
This basic type of reference is typically used for creating new ENGINEs
dynamically, iterating across OpenSSL's internal linked-list of loaded
ENGINEs, reading information about an ENGINE, etc. Essentially a struc-
tural 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. Other than that, structural references come from
return values to various ENGINE API functions 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 - 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. In
this case, the supplied ENGINE pointer is, from the point of the view
of the caller, both a structural reference and a functional reference -
so if the caller intends to use it as a functional reference it should
free the structural reference with ENGINE_free() first. If the caller
wishes to use it only as a structural reference (eg. if the
ENGINE_init() call was simply to test if the ENGINE seems avail-
able/online), then it should free the functional reference; all func-
tional references are released by the ENGINE_finish() function.
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 separated-out by an 'nid' index,
because abstractions like EVP_CIPHER and EVP_DIGEST support many dis-
tinct algorithms and modes - ENGINEs will support different numbers and
combinations of these. 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. ENGINEs can be registered into
these tables to make themselves available for use automatically by the
various abstractions, eg. RSA. For illustrative purposes, we continue
with the RSA example, though all comments apply similarly to the other
abstractions (they each get their own table and linkage to the corre-
sponding section of openssl code).
When a new RSA key is being created, ie. in RSA_new_method(), a
"get_default" call will be made to the ENGINE subsystem to process the
RSA state table and return a functional reference to an initialised
ENGINE whose RSA_METHOD should be used. If no ENGINE should (or can) be
used, it will return NULL and the RSA key will operate with a NULL
ENGINE handle by using the conventional RSA implementation in OpenSSL
(and will from then on behave the way it used to before the ENGINE API
existed - for details see RSA_new_method(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 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 abstractions like EVP_CIPHER, where implementations are
indexed by 'nid', these flags and cached-responses are distinct for
each 'nid' value.
It is worth illustrating the difference between "registration" of
ENGINEs into these per-algorithm state tables and using the alternative
"set_default" functions. The latter handles both "registration" and
also setting the cached "default" ENGINE in each relevant state table -
so registered ENGINEs will only have a chance to be initialised for use
as a default if a default ENGINE wasn't already set for the same state
table. Eg. if ENGINE X supports cipher nids {A,B} and RSA, ENGINE Y
supports ciphers {A} and DSA, and the following code is executed;
ENGINE_register_complete(X);
ENGINE_set_default(Y, ENGINE_METHOD_ALL);
e1 = ENGINE_get_default_RSA();
e2 = ENGINE_get_cipher_engine(A);
e3 = ENGINE_get_cipher_engine(B);
e4 = ENGINE_get_default_DSA();
e5 = ENGINE_get_cipher_engine(C);
The results would be as follows;
assert(e1 == X);
assert(e2 == Y);
assert(e3 == X);
assert(e4 == Y);
assert(e5 == NULL);
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 "load"ed, 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 have control over. 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 appli-
cations may prefer to handle things, so we will simply illustrate the
consequences as they apply to a couple of simple cases and leave devel-
opers to consider these and the source code to openssl's builtin utili-
ties 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
operation, otherwise OpenSSL should use its builtin software as per
usual. The following code illustrates how to approach this;
ENGINE *e;
const char *engine_id = "ACME";
ENGINE_load_builtin_engines();
e = ENGINE_by_id(engine_id);
if(!e)
/* the engine isn't available */
return;
if(!ENGINE_init(e)) {
/* the engine couldn't initialise, release 'e' */
ENGINE_free(e);
return;
}
if(!ENGINE_set_default_RSA(e))
/* This should only happen when 'e' can't initialise, but the previous
* statement suggests it did. */
abort();
ENGINE_set_default_DSA(e);
ENGINE_set_default_ciphers(e);
/* Release the functional reference from ENGINE_init() */
ENGINE_finish(e);
/* Release the structural reference from ENGINE_by_id() */
ENGINE_free(e);
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 initialise, it should be used. The following code illustrates how
this can work;
/* Load all bundled ENGINEs into memory and make them visible */
ENGINE_load_builtin_engines();
/* Register all of them for every algorithm they collectively implement */
ENGINE_register_all_complete();
That's all that's required. Eg. the next time OpenSSL tries to set up
an RSA key, any bundled ENGINEs that implement RSA_METHOD will be
passed to ENGINE_init() and if any of those succeed, that ENGINE will
be set as the default for use with RSA 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 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 allow settings to passed 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 password-
protected devices, logging information, etc etc. This class of commands
typically needs to be passed to an ENGINE before attempting to ini-
tialise it, ie. before calling ENGINE_init(). The other class of com-
mands 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 same cases both. ENGINE
implementations should provide indications of this in the descriptions
attached to builtin control commands and/or in external product docu-
mentation.
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)");
ENGINE_free(e);
return 0;
}
pre_cmds += 2;
}
if(!ENGINE_init(e)) {
fprintf(stderr, "Failed initialisation\n");
ENGINE_free(e);
return 0;
}
/* ENGINE_init() returned a functional reference, so free the structural
* reference from ENGINE_by_id(). */
ENGINE_free(e);
while(post_num--) {
if(!ENGINE_ctrl_cmd_string(e, post_cmds[0], post_cmds[1], 0)) {
fprintf(stderr, "Failed command (%s - %s:%s)\n", engine_id,
post_cmds[0], post_cmds[1] ? post_cmds[1] : "(NULL)");
ENGINE_finish(e);
return 0;
}
post_cmds += 2;
}
ENGINE_set_default(e, ENGINE_METHOD_ALL & ~ENGINE_METHOD_RAND);
/* Success */
return 1;
}
Note that ENGINE_ctrl_cmd_string() accepts a boolean argument that can
relax the semantics of the function - if set non-zero it will only
return failure if the ENGINE supported the given command name but
failed while executing it, if the ENGINE doesn't support the command
name it will simply return success without doing anything. In this case
we assume the user is only supplying commands specific to the given
ENGINE so we set this to FALSE.
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
from a structural reference to any ENGINE. It is first important to
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 a symbol, ENGINE_CMD_BASE, that all
control commands implemented by ENGINEs from. Any command value lower
than this symbol is considered a "generic" command is handled directly
by the OpenSSL core routines.
It is using these "core" control commands that one can discover the the
control commands implemented by a given ENGINE, specifically the com-
mands;
#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 by which
to process these queries. An ENGINE has 3 properties it exposes that
can affect this behaviour; it can supply a ctrl() handler, it can spec-
ify ENGINE_FLAGS_MANUAL_CMD_CTRL in the ENGINE's flags, and it can
expose an array of control command descriptions. If an ENGINE speci-
fies the ENGINE_FLAGS_MANUAL_CMD_CTRL flag, then it will simply pass
all these "core" control commands directly to the ENGINE's ctrl() han-
dler (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 independantly 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.
SEE ALSO
rsa(3), dsa(3), dh(3), rand(3), RSA_new_method(3)
0.9.7d 2005-02-24 engine(3)
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