path: root/src/lib/libcrypto/man/engine.3

.\" $OpenBSD: engine.3,v 1.16 2018/04/15 17:02:03 schwarze Exp $
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.\" This file was written by Geoff Thorpe <geoff@openssl.org>
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.Dd $Mdocdate: April 15 2018 $
.Dt ENGINE 3
.Os
.Sh NAME
.Nm engine
.Nd ENGINE cryptographic module support
.Sh DESCRIPTION
These functions create, manipulate, and use cryptographic modules
in the form of
.Vt 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.
.Pp
The cryptographic functionality that can be provided by an
.Vt ENGINE
implementation includes the following abstractions:
.Pp
.Bl -bullet -compact
.It
.Vt RSA_METHOD :
for providing alternative RSA implementations
.It
.Vt DSA_METHOD , DH_METHOD , RAND_METHOD , ECDH_METHOD ,
.Vt ECDSA_METHOD , STORE_METHOD :
similarly for other OpenSSL APIs
.It
.Vt EVP_CIPHER :
potentially multiple cipher algorithms (indexed by 'nid')
.It
.Vt EVP_DIGEST :
potentially multiple hash algorithms (indexed by 'nid')
.It
key-loading: loading public and/or private EVP_PKEY keys
.El
.Ss Reference counting and handles
Due to the modular nature of the
.Nm engine
API, pointers to
.Vt ENGINE Ns s
need to be treated as handles - i.e. not only as pointers, but also
as references to the underlying
.Vt ENGINE
object.
One should obtain a new reference when making copies of an
.Vt ENGINE
pointer if the copies will be used (and released) independently.
.Pp
.Vt ENGINE
objects have two levels of reference-counting to match the way in
which the objects are used.
At the most basic level, each
.Vt ENGINE
pointer is inherently a
.Sy 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 cannot
be deallocated until the reference is released.
.Pp
However, a structural reference provides no guarantee that the
.Vt ENGINE
is initialised and able to use any of its cryptographic implementations.
Indeed it's quite possible that most
.Vt ENGINE Ns s
will not initialise at all in typical environments, as
.Vt ENGINE Ns s
are typically used to support specialised hardware.
To use an
.Vt ENGINE Ap s
functionality, you need a
.Sy 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
.Vt ENGINE ,
you have a guarantee that the
.Vt ENGINE
has been initialised and is ready to perform cryptographic operations and
will remain uninitialised until after you have released your
reference.
.Pp
.Em Structural references
.Pp
This basic type of reference is used for instantiating new
.Vt ENGINE Ns s ,
iterating across OpenSSL's internal linked-list of loaded
.Vt ENGINE Ns s ,
reading information about an
.Vt ENGINE ,
etc.
Essentially a structural reference is sufficient if you only need to
query or manipulate the data of an
.Vt ENGINE
implementation rather than use its functionality.
.Ss Application requirements
This section will explain the basic things an application programmer
should support to make the most useful elements of the
.Nm engine
functionality available to the user.
The first thing to consider is whether the programmer wishes to make
alternative
.Vt ENGINE
modules available to the application and user.
OpenSSL maintains an internal linked list of "visible"
.Vt ENGINE Ns s
from which it has to operate.
At start-up, this list is empty, and in fact if an application does
not call any
.Nm engine
API calls and it uses static
linking against openssl, then the resulting application binary will
not contain any alternative
.Nm engine
code at all.
So the first consideration is whether any/all available
.Vt ENGINE
implementations should be made visible to OpenSSL.
.Pp
Having called any of these functions,
.Vt ENGINE
objects would have been dynamically allocated and populated with
these implementations and linked into OpenSSL's internal linked
list.
.Pp
The fact that
.Vt ENGINE Ns s
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
.Vt 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
.Vt ENGINE
that is able to successfully initialised - 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 builtin utilities as guides.
.Pp
.Em Using a specific ENGINE implementation
.Pp
Here we'll assume an application has been configured by its user or
admin to want to use the "ACME"
.Vt 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 usual.
The following code illustrates how to approach this:
.Bd -literal
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);
.Ed
.Pp
.Em Automatically using builtin ENGINE implementations
.Pp
Here we'll assume we want to load and register all
.Vt ENGINE
implementations bundled with OpenSSL, such that for any cryptographic
algorithm required by OpenSSL - if there is an
.Vt ENGINE
that implements it and can be initialised, it should be used.
The following code illustrates how this can work;
.Bd -literal
/* 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();
.Ed
.Pp
That's all that's required.
For example, the next time OpenSSL tries to set up an RSA key, any bundled
.Vt ENGINE Ns s
that implement
.Vt RSA_METHOD
will be passed to
.Xr ENGINE_init 3
and if any of those succeed, that
.Vt ENGINE
will be set as the default for RSA use from then on.
.Ss Advanced configuration support
There is a mechanism supported by the
.Nm engine
framework that allows each
.Vt 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
.Vt ENGINE Ns s .
It is also possible for the application to dynamically interrogate the
loaded
.Vt 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
.Vt 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
.Vt ENGINE s
exactly as they are provided by the user.
.Pp
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.
This class of commands typically needs to be passed to an
.Vt ENGINE
.Sy before
attempting to initialise it, i.e. before calling
.Xr ENGINE_init 3 .
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
.Xr ENGINE_init 3 ,
or in some cases both.
.Vt ENGINE
implementations should provide indications of this in the descriptions
attached to builtin control commands and/or in external product
documentation.
.Pp
.Em Issuing control commands to an ENGINE
.Pp
Let's illustrate by example; a function for which the caller supplies
the name of the
.Vt 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
.Dv NULL
in some cases but the name cannot.
This function should initialise the
.Vt 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.
.Bd -literal
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)\en",
			    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\en");
		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)\en",
			    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;
}
.Ed
.Pp
Note that
.Fn ENGINE_ctrl_cmd_string
accepts a boolean argument that can relax the semantics of the function.
If set to non-zero it will only return failure if the
.Vt ENGINE
supported the given command name but failed while executing it, if the
.Vt 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
.Vt ENGINE
so we set this to FALSE.
.Pp
.Em Discovering supported control commands
.Pp
It is possible to discover at run-time the names, numerical-ids,
descriptions and input parameters of the control commands supported by an
.Vt 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
.Vt ENGINE ,
i.e. the
.Vt ENGINE Ap s
ctrl() handler is not used for the control command.
.In openssl/engine.h
defines an index,
.Dv ENGINE_CMD_BASE ,
that all control commands implemented by
.Vt ENGINE Ns s
should be numbered from.
Any command value lower than this symbol is considered a "generic"
command is handled directly by the OpenSSL core routines.
.Pp
It is using these "core" control commands that one can discover the
control commands implemented by a given
.Vt ENGINE ,
specifically the commands:
.Bd -literal
#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
.Ed
.Pp
Whilst these commands are automatically processed by the OpenSSL
framework code, they use various properties exposed by each
.Vt ENGINE
to process these queries.
An
.Vt ENGINE
has 3 properties it exposes that can affect how this behaves;
it can supply a ctrl() handler, it can specify
.Dv ENGINE_FLAGS_MANUAL_CMD_CTRL
in the
.Vt ENGINE Ap s
flags, and it can expose an array of control command descriptions.
If an
.Vt ENGINE
specifies the
.Dv ENGINE_FLAGS_MANUAL_CMD_CTRL
flag, then it will simply pass all these "core" control commands
directly to the
.Vt ENGINE Ap s
ctrl() handler (and thus, it must have supplied one), so it is up
to the
.Vt 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;
.Bl -tag -width Ds
.It If no ctrl() handler is supplied:
.Dv ENGINE_HAS_CTRL_FUNCTION
returns FALSE (zero), all other commands fail.
.It If a ctrl() handler was supplied but no array of control commands:
.Dv ENGINE_HAS_CTRL_FUNCTION
returns TRUE, all other commands fail.
.It If a ctrl() handler and array of control commands was supplied:
.Dv ENGINE_HAS_CTRL_FUNCTION
returns TRUE, all other commands proceed processing...
.El
.Pp
If the
.Vt ENGINE Ns s
array of control commands is empty, then all other commands will fail.
Otherwise
.Dv ENGINE_CTRL_GET_FIRST_CMD_TYPE
returns the identifier of the first command supported by the
.Vt ENGINE ,
.Dv ENGINE_GET_NEXT_CMD_TYPE
takes the identifier of a command supported by the
.Vt ENGINE
and returns the next command identifier or fails if there are no more,
.Dv 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
.Dv 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.
.Dv ENGINE_CTRL_GET_FLAGS
returns a bitwise-OR'd mask of the following possible values:
.Bd -literal
#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
.Ed
.Pp
If the
.Dv 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
.Vt ENGINE
functions such as
.Fn 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
.Fn 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 allow
applications to determine if an
.Vt ENGINE
supports certain specific commands it might want to use (e.g.
application "foo" might query various
.Vt ENGINE Ns s
to see if they implement "FOO_GET_VENDOR_LOGO_GIF" - and
.Vt ENGINE
could therefore decide whether or not to support this "foo"-specific
extension).
.Sh SEE ALSO
.Xr DH_new 3 ,
.Xr DSA_new 3 ,
.Xr ENGINE_add_conf_module 3 ,
.Xr ENGINE_set_ex_data 3 ,
.Xr RSA_new 3
.Sh HISTORY
The engine API first appeared in OpenSSL 0.9.7
and has been available since
.Ox 3.2 .