All of the functions described on this page are deprecated. Applications should instead use the provider
APIs.
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
DSA_METHOD, DH_METHOD, RAND_METHOD, ECDH_METHOD, ECDSA_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
Referencecountingandhandles
Due to the modular nature of the ENGINE API, pointers to ENGINEs need to be treated as handles - i.e. 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 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.
Structuralreferences
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. If the argument to
ENGINE_free() is NULL, nothing is done.
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.
Functionalreferences
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
(e.g. lack of system drivers, no special hardware attached, etc), otherwise it will return nonzero 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, e.g. 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.
Defaultimplementations
For each supported abstraction, the ENGINE code maintains an internal table of state to control which
implementations are available for a given abstraction and which should be used by default. These
implementations are registered in the tables and indexed by an 'nid' value, because abstractions like
EVP_CIPHER and EVP_DIGEST support many distinct algorithms and modes, and ENGINEs can support arbitrarily
many of them. In the case of other abstractions like RSA, DSA, etc, there is only one "algorithm" so all
implementations implicitly register using the same 'nid' index.
When a default ENGINE is requested for a given abstraction/algorithm/mode, (e.g. when calling
RSA_new_method(NULL)), a "get_default" call will be made to the ENGINE subsystem to process the
corresponding state table and return a functional reference to an initialised ENGINE whose implementation
should be used. If no ENGINE should (or can) be used, it will return NULL and the caller will operate
with a NULL ENGINE handle - this usually equates to using the conventional software implementation. In
the latter case, OpenSSL will from then on behave the way it used to before the ENGINE API existed.
Each state table has a flag to note whether it has processed this "get_default" query since the table was
last modified, because to process this question it must iterate across all the registered ENGINEs in the
table trying to initialise each of them in turn, in case one of them is operational. If it returns a
functional reference to an ENGINE, it will also cache another reference to speed up processing future
queries (without needing to iterate across the table). Likewise, it will cache a NULL response if no
ENGINE was available so that future queries won't repeat the same iteration unless the state table
changes. This behaviour can also be changed; if the ENGINE_TABLE_FLAG_NOINIT flag is set (using
ENGINE_set_table_flags()), no attempted initialisations will take place, instead the only way for the
state table to return a non-NULL ENGINE to the "get_default" query will be if one is expressly set in the
table. Eg. ENGINE_set_default_RSA() does the same job as ENGINE_register_RSA() except that it also sets
the state table's cached response for the "get_default" query. In the case of abstractions like
EVP_CIPHER, where implementations are indexed by 'nid', these flags and cached-responses are distinct for
each 'nid' value.
Applicationrequirements
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.
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
- i.e. 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 built-in utilities as guides.
If no ENGINE API functions are called within an application, then OpenSSL will not allocate any internal
resources. Prior to OpenSSL 1.1.0, however, if any ENGINEs are loaded, even if not registered or used,
it was necessary to call ENGINE_cleanup() before the program exits.
UsingaspecificENGINEimplementation
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 built-in 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);
Automaticallyusingbuilt-inENGINEimplementations
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 */
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 RSA use from then on.
Advancedconfigurationsupport
There is a mechanism supported by the ENGINE framework that allows each ENGINE implementation to define
an arbitrary set of configuration "commands" and expose them to OpenSSL and any applications based on
OpenSSL. This mechanism is entirely based on the use of name-value pairs and assumes ASCII input (no
unicode or UTF for now!), so it is ideal if applications want to provide a transparent way for users to
provide arbitrary configuration "directives" directly to such ENGINEs. It is also possible for the
application to dynamically interrogate the loaded ENGINE implementations for the names, descriptions, and
input flags of their available "control commands", providing a more flexible configuration scheme.
However, if the user is expected to know which ENGINE device he/she is using (in the case of specialised
hardware, this goes without saying) then applications may not need to concern themselves with discovering
the supported control commands and simply prefer to pass settings into ENGINEs exactly as they are
provided by the user.
Before illustrating how control commands work, it is worth mentioning what they are typically used for.
Broadly speaking there are two uses for control commands; the first is to provide the necessary details
to the implementation (which may know nothing at all specific to the host system) so that it can be
initialised for use. This could include the path to any driver or config files it needs to load, required
network addresses, smart-card identifiers, passwords to initialise protected devices, logging
information, etc etc. This class of commands typically needs to be passed to an ENGINE before attempting
to initialise it, i.e. 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 built-in control commands and/or in external product
documentation.
IssuingcontrolcommandstoanENGINE
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.
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 nonzero 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.
Discoveringsupportedcontrolcommands
It is possible to discover at run-time the names, numerical-ids, descriptions and input parameters of the
control commands supported by an ENGINE using a structural reference. Note that some control commands are
defined by OpenSSL itself and it will intercept and handle these control commands on behalf of the
ENGINE, i.e. 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 routines.
It is using these "core" control commands that one can discover the control commands implemented by a
given ENGINE, specifically the commands:
ENGINE_HAS_CTRL_FUNCTION
ENGINE_CTRL_GET_FIRST_CMD_TYPE
ENGINE_CTRL_GET_NEXT_CMD_TYPE
ENGINE_CTRL_GET_CMD_FROM_NAME
ENGINE_CTRL_GET_NAME_LEN_FROM_CMD
ENGINE_CTRL_GET_NAME_FROM_CMD
ENGINE_CTRL_GET_DESC_LEN_FROM_CMD
ENGINE_CTRL_GET_DESC_FROM_CMD
ENGINE_CTRL_GET_CMD_FLAGS
Whilst these commands are automatically processed by the OpenSSL framework code, they use various
properties exposed by each ENGINE to process these queries. An ENGINE has 3 properties it exposes that
can affect how this behaves; it can supply a ctrl() handler, it can specify ENGINE_FLAGS_MANUAL_CMD_CTRL
in the ENGINE's flags, and it can expose an array of control command descriptions. If an ENGINE
specifies the ENGINE_FLAGS_MANUAL_CMD_CTRL flag, then it will simply pass all these "core" control
commands directly to the ENGINE's ctrl() handler (and thus, it must have supplied one), so it is up to
the ENGINE to reply to these "discovery" commands itself. If that flag is not set, then the OpenSSL
framework code will work with the following rules:
if no ctrl() handler supplied;
ENGINE_HAS_CTRL_FUNCTION returns FALSE (zero),
all other commands fail.
if a ctrl() handler was supplied but no array of control commands;
ENGINE_HAS_CTRL_FUNCTION returns TRUE,
all other commands fail.
if a ctrl() handler and array of control commands was supplied;
ENGINE_HAS_CTRL_FUNCTION returns TRUE,
all other commands proceed processing ...
If the ENGINE's array of control commands is empty then all other commands will fail, otherwise;
ENGINE_CTRL_GET_FIRST_CMD_TYPE returns the identifier of the first command supported by the ENGINE,
ENGINE_GET_NEXT_CMD_TYPE takes the identifier of a command supported by the ENGINE and returns the next
command identifier or fails if there are no more, ENGINE_CMD_FROM_NAME takes a string name for a command
and returns the corresponding identifier or fails if no such command name exists, and the remaining
commands take a command identifier and return properties of the corresponding commands. All except
ENGINE_CTRL_GET_FLAGS return the string length of a command name or description, or populate a supplied
character buffer with a copy of the command name or description. ENGINE_CTRL_GET_FLAGS returns a bitwise-
OR'd mask of the following possible values:
ENGINE_CMD_FLAG_NUMERIC
ENGINE_CMD_FLAG_STRING
ENGINE_CMD_FLAG_NO_INPUT
ENGINE_CMD_FLAG_INTERNAL
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 to determine if an ENGINE supports
certain specific commands it might want to use (e.g. 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).
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