ENGINE(3) OpenSSL ENGINE(3)NAMEengine - ENGINE cryptographic module support
SYNOPSIS
#include <openssl/engine.h>
ENGINE *ENGINE_get_first(void);
ENGINE *ENGINE_get_last(void);
ENGINE *ENGINE_get_next(ENGINE *e);
ENGINE *ENGINE_get_prev(ENGINE *e);
int ENGINE_add(ENGINE *e);
int ENGINE_remove(ENGINE *e);
ENGINE *ENGINE_by_id(const char *id);
int ENGINE_init(ENGINE *e);
int ENGINE_finish(ENGINE *e);
void ENGINE_load_openssl(void);
void ENGINE_load_dynamic(void);
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);
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ENGINE(3) OpenSSL ENGINE(3)
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);
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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 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;
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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 references to the underlying ENGINE
object. Ie. you should obtain a new reference 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
considered a specialised form of structural reference,
because each functional reference implicitly contains a
structural reference as well - however to avoid difficult-
to-find programming bugs, it is recommended to treat the two
kinds of reference independantly. If you have a functional
reference to an ENGINE, you have a guarantee that the ENGINE
has been initialised ready to perform cryptographic opera-
tions and will not be uninitialised or cleaned up until
after you have released your reference.
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 structural reference is suffi-
cient if you only need to query or manipulate the data of an
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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 - typically this happens whenever the sup-
plied 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 struc-
tural reference internally so the caller is still responsi-
ble 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 crypto-
graphic functionality of an ENGINE is required to be avail-
able. 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
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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 available/online), then it should free the functional
reference; all functional 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 implementations are registered in the
tables separated-out by an 'nid' index, because abstractions
like EVP_CIPHER and EVP_DIGEST support many distinct algo-
rithms 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 con-
tinue with the RSA example, though all comments apply simi-
larly to the other abstractions (they each get their own
table and linkage to the corresponding 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 implemen-
tation 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)).
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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). Like-
wise, it will cache a NULL response if no ENGINE was avail-
able 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 implemen-
tations are indexed by 'nid', these flags and cached-
responses are distinct for each 'nid' value.
It is worth illustrating the difference between "registra-
tion" 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);
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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 con-
tain any alternative ENGINE code at all. So the first con-
sideration is whether any/all available ENGINE implementa-
tions should be made visible to OpenSSL - this is controlled
by calling the various "load" functions, eg.
/* Make the "dynamic" ENGINE available */
void ENGINE_load_dynamic(void);
/* Make the CryptoSwift hardware acceleration support available */
void ENGINE_load_cswift(void);
/* Make support for nCipher's "CHIL" hardware available */
void ENGINE_load_chil(void);
...
/* Make ALL ENGINE implementations bundled with OpenSSL available */
void ENGINE_load_builtin_engines(void);
Having called any of these functions, ENGINE objects would
have been dynamically allocated and populated with these
implementations and linked into OpenSSL's internal linked
list. At this point it is important to mention an important
API function;
void ENGINE_cleanup(void);
If no ENGINE API functions are called at all in an applica-
tion, then there are no inherent memory leaks to worry about
from the ENGINE functionality, however if any ENGINEs are
"load"ed, 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 inter-
nal callback registration table so that any ENGINE API func-
tionality 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 pos-
sible ENGINE functionality - only the cleanup callbacks
required by the functionality you do use will be required by
the linker.
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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 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 ini-
tialise - ie. to assume that this corresponds to accelera-
tion 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 com-
piled 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
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Here we'll assume we want to load and register all ENGINE
implementations bundled with OpenSSL, such that for any
cryptographic algorithm required by OpenSSL - if there is an
ENGINE that implements it and can be initialise, it should
be used. The following code illustrates how this can work;
/* Load all bundled ENGINEs into memory and make them visible */
ENGINE_load_builtin_engines();
/* Register all of them for every algorithm they collectively implement */
ENGINE_register_all_complete();
That's all that's required. Eg. the next time OpenSSL tries
to set up an RSA key, any bundled ENGINEs that implement
RSA_METHOD will be passed to ENGINE_init() and if any of
those succeed, that ENGINE will be set as the default for
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 "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 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 appli-
cation to dynamically interrogate the loaded ENGINE imple-
mentations for the names, descriptions, and input flags of
their available "control commands", providing a more flexi-
ble 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 appli-
cations 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 speak-
ing 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 ini-
tialise password-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
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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 indica-
tions of this in the descriptions attached to builtin con-
trol 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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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 argu-
ment 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 con-
trol commands supported from a structural reference to any
ENGINE. It is first important to note that some control
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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 "gen-
eric" command is handled directly by the OpenSSL core rou-
tines.
It is using these "core" control commands that one can dis-
cover 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 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 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,
MirOS BSD #10-current 2005-02-05 13
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 iden-
tifier 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 charac-
ter 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(). "INTER-
NAL" commands are not intended to be exposed to text-based
configuration by applications, administrations, users, etc.
These can support arbitrary operations via ENGINE_ctrl(),
including passing to and/or from the control commands data
of any arbitrary type. These commands are supported in the
discovery mechanisms simply to allow applications determinie
if an ENGINE supports certain specific commands it might
want to use (eg. application "foo" might query various
ENGINEs to see if they implement "FOO_GET_VENDOR_LOGO_GIF" -
and ENGINE could therefore decide whether or not to support
this "foo"-specific extension).
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 implementa-
tions.
SEE ALSOrsa(3), dsa(3), dh(3), rand(3), RSA_new_method(3)MirOS BSD #10-current 2005-02-05 14
