xref: /openssl/README-ENGINES.md (revision af33b200)
1Engines
2=======
3
4Deprecation Note
5----------------
6
7The ENGINE API was introduced in OpenSSL version 0.9.6 as a low level
8interface for adding alternative implementations of cryptographic
9primitives, most notably for integrating hardware crypto devices.
10
11The ENGINE interface has its limitations and it has been superseded
12by the [PROVIDER API](README-PROVIDERS.md), it is deprecated in OpenSSL
13version 3.0. The following documentation is retained as an aid for
14users who need to maintain or support existing ENGINE implementations.
15Support for new hardware devices or new algorithms should be added
16via providers, and existing engines should be converted to providers
17as soon as possible.
18
19Built-in ENGINE implementations
20-------------------------------
21
22There are currently built-in ENGINE implementations for the following
23crypto devices:
24
25- Microsoft CryptoAPI
26- VIA Padlock
27- nCipher CHIL
28
29In addition, dynamic binding to external ENGINE implementations is now
30provided by a special ENGINE called "dynamic". See the "DYNAMIC ENGINE"
31section below for details.
32
33At this stage, a number of things are still needed and are being worked on:
34
351. Integration of EVP support.
362. Configuration support.
373. Documentation!
38
39Integration of EVP support
40--------------------------
41
42With respect to EVP, this relates to support for ciphers and digests in
43the ENGINE model so that alternative implementations of existing
44algorithms/modes (or previously unimplemented ones) can be provided by
45ENGINE implementations.
46
47Configuration support
48---------------------
49
50Configuration support currently exists in the ENGINE API itself, in the
51form of "control commands". These allow an application to expose to the
52user/admin the set of commands and parameter types a given ENGINE
53implementation supports, and for an application to directly feed string
54based input to those ENGINEs, in the form of name-value pairs. This is an
55extensible way for ENGINEs to define their own "configuration" mechanisms
56that are specific to a given ENGINE (eg. for a particular hardware
57device) but that should be consistent across *all* OpenSSL-based
58applications when they use that ENGINE. Work is in progress (or at least
59in planning) for supporting these control commands from the CONF (or
60NCONF) code so that applications using OpenSSL's existing configuration
61file format can have ENGINE settings specified in much the same way.
62Presently however, applications must use the ENGINE API itself to provide
63such functionality. To see first hand the types of commands available
64with the various compiled-in ENGINEs (see further down for dynamic
65ENGINEs), use the "engine" openssl utility with full verbosity, i.e.:
66
67    openssl engine -vvvv
68
69Documentation
70-------------
71
72Documentation? Volunteers welcome! The source code is reasonably well
73self-documenting, but some summaries and usage instructions are needed -
74moreover, they are needed in the same POD format the existing OpenSSL
75documentation is provided in. Any complete or incomplete contributions
76would help make this happen.
77
78STABILITY & BUG-REPORTS
79=======================
80
81What already exists is fairly stable as far as it has been tested, but
82the test base has been a bit small most of the time. For the most part,
83the vendors of the devices these ENGINEs support have contributed to the
84development and/or testing of the implementations, and *usually* (with no
85guarantees) have experience in using the ENGINE support to drive their
86devices from common OpenSSL-based applications. Bugs and/or inexplicable
87behaviour in using a specific ENGINE implementation should be sent to the
88author of that implementation (if it is mentioned in the corresponding C
89file), and in the case of implementations for commercial hardware
90devices, also through whatever vendor support channels are available. If
91none of this is possible, or the problem seems to be something about the
92ENGINE API itself (ie. not necessarily specific to a particular ENGINE
93implementation) then you should mail complete details to the relevant
94OpenSSL mailing list. For a definition of "complete details", refer to
95the OpenSSL "README" file. As for which list to send it to:
96
97- openssl-users: if you are *using* the ENGINE abstraction, either in an
98  pre-compiled application or in your own application code.
99
100- openssl-dev: if you are discussing problems with OpenSSL source code.
101
102USAGE
103=====
104
105The default "openssl" ENGINE is always chosen when performing crypto
106operations unless you specify otherwise. You must actively tell the
107openssl utility commands to use anything else through a new command line
108switch called "-engine". Also, if you want to use the ENGINE support in
109your own code to do something similar, you must likewise explicitly
110select the ENGINE implementation you want.
111
112Depending on the type of hardware, system, and configuration, "settings"
113may need to be applied to an ENGINE for it to function as expected/hoped.
114The recommended way of doing this is for the application to support
115ENGINE "control commands" so that each ENGINE implementation can provide
116whatever configuration primitives it might require and the application
117can allow the user/admin (and thus the hardware vendor's support desk
118also) to provide any such input directly to the ENGINE implementation.
119This way, applications do not need to know anything specific to any
120device, they only need to provide the means to carry such user/admin
121input through to the ENGINE in question. Ie. this connects *you* (and
122your helpdesk) to the specific ENGINE implementation (and device), and
123allows application authors to not get buried in hassle supporting
124arbitrary devices they know (and care) nothing about.
125
126A new "openssl" utility, "openssl engine", has been added in that allows
127for testing and examination of ENGINE implementations. Basic usage
128instructions are available by specifying the "-?" command line switch.
129
130DYNAMIC ENGINES
131===============
132
133The new "dynamic" ENGINE provides a low-overhead way to support ENGINE
134implementations that aren't pre-compiled and linked into OpenSSL-based
135applications. This could be because existing compiled-in implementations
136have known problems and you wish to use a newer version with an existing
137application. It could equally be because the application (or OpenSSL
138library) you are using simply doesn't have support for the ENGINE you
139wish to use, and the ENGINE provider (eg. hardware vendor) is providing
140you with a self-contained implementation in the form of a shared-library.
141The other use-case for "dynamic" is with applications that wish to
142maintain the smallest foot-print possible and so do not link in various
143ENGINE implementations from OpenSSL, but instead leaves you to provide
144them, if you want them, in the form of "dynamic"-loadable
145shared-libraries. It should be possible for hardware vendors to provide
146their own shared-libraries to support arbitrary hardware to work with
147applications based on OpenSSL 0.9.7 or later. If you're using an
148application based on 0.9.7 (or later) and the support you desire is only
149announced for versions later than the one you need, ask the vendor to
150backport their ENGINE to the version you need.
151
152How does "dynamic" work?
153------------------------
154
155The dynamic ENGINE has a special flag in its implementation such that
156every time application code asks for the 'dynamic' ENGINE, it in fact
157gets its own copy of it. As such, multi-threaded code (or code that
158multiplexes multiple uses of 'dynamic' in a single application in any
159way at all) does not get confused by 'dynamic' being used to do many
160independent things. Other ENGINEs typically don't do this so there is
161only ever 1 ENGINE structure of its type (and reference counts are used
162to keep order). The dynamic ENGINE itself provides absolutely no
163cryptographic functionality, and any attempt to "initialise" the ENGINE
164automatically fails. All it does provide are a few "control commands"
165that can be used to control how it will load an external ENGINE
166implementation from a shared-library. To see these control commands,
167use the command-line;
168
169    openssl engine -vvvv dynamic
170
171The "SO_PATH" control command should be used to identify the
172shared-library that contains the ENGINE implementation, and "NO_VCHECK"
173might possibly be useful if there is a minor version conflict and you
174(or a vendor helpdesk) is convinced you can safely ignore it.
175"ID" is probably only needed if a shared-library implements
176multiple ENGINEs, but if you know the engine id you expect to be using,
177it doesn't hurt to specify it (and this provides a sanity check if
178nothing else). "LIST_ADD" is only required if you actually wish the
179loaded ENGINE to be discoverable by application code later on using the
180ENGINE's "id". For most applications, this isn't necessary - but some
181application authors may have nifty reasons for using it. The "LOAD"
182command is the only one that takes no parameters and is the command
183that uses the settings from any previous commands to actually *load*
184the shared-library ENGINE implementation. If this command succeeds, the
185(copy of the) 'dynamic' ENGINE will magically morph into the ENGINE
186that has been loaded from the shared-library. As such, any control
187commands supported by the loaded ENGINE could then be executed as per
188normal. For instance, if ENGINE "foo" is implemented in the shared-library
189"libfoo.so" and it supports some special control command "CMD_FOO", the
190following code would load and use it (NB: obviously this code has no
191error checking);
192
193    ENGINE *e = ENGINE_by_id("dynamic");
194    ENGINE_ctrl_cmd_string(e, "SO_PATH", "/lib/libfoo.so", 0);
195    ENGINE_ctrl_cmd_string(e, "ID", "foo", 0);
196    ENGINE_ctrl_cmd_string(e, "LOAD", NULL, 0);
197    ENGINE_ctrl_cmd_string(e, "CMD_FOO", "some input data", 0);
198
199For testing, the "openssl engine" utility can be useful for this sort
200of thing. For example the above code excerpt would achieve much the
201same result as;
202
203    openssl engine dynamic \
204              -pre SO_PATH:/lib/libfoo.so \
205              -pre ID:foo \
206              -pre LOAD \
207              -pre "CMD_FOO:some input data"
208
209Or to simply see the list of commands supported by the "foo" ENGINE;
210
211    openssl engine -vvvv dynamic \
212              -pre SO_PATH:/lib/libfoo.so \
213              -pre ID:foo \
214              -pre LOAD
215
216Applications that support the ENGINE API and more specifically, the
217"control commands" mechanism, will provide some way for you to pass
218such commands through to ENGINEs. As such, you would select "dynamic"
219as the ENGINE to use, and the parameters/commands you pass would
220control the *actual* ENGINE used. Each command is actually a name-value
221pair and the value can sometimes be omitted (eg. the "LOAD" command).
222Whilst the syntax demonstrated in "openssl engine" uses a colon to
223separate the command name from the value, applications may provide
224their own syntax for making that separation (eg. a win32 registry
225key-value pair may be used by some applications). The reason for the
226"-pre" syntax in the "openssl engine" utility is that some commands
227might be issued to an ENGINE *after* it has been initialised for use.
228Eg. if an ENGINE implementation requires a smart-card to be inserted
229during initialisation (or a PIN to be typed, or whatever), there may be
230a control command you can issue afterwards to "forget" the smart-card
231so that additional initialisation is no longer possible. In
232applications such as web-servers, where potentially volatile code may
233run on the same host system, this may provide some arguable security
234value. In such a case, the command would be passed to the ENGINE after
235it has been initialised for use, and so the "-post" switch would be
236used instead. Applications may provide a different syntax for
237supporting this distinction, and some may simply not provide it at all
238("-pre" is almost always what you're after, in reality).
239
240How do I build a "dynamic" ENGINE?
241----------------------------------
242
243This question is trickier - currently OpenSSL bundles various ENGINE
244implementations that are statically built in, and any application that
245calls the "ENGINE_load_builtin_engines()" function will automatically
246have all such ENGINEs available (and occupying memory). Applications
247that don't call that function have no ENGINEs available like that and
248would have to use "dynamic" to load any such ENGINE - but on the other
249hand such applications would only have the memory footprint of any
250ENGINEs explicitly loaded using user/admin provided control commands.
251The main advantage of not statically linking ENGINEs and only using
252"dynamic" for hardware support is that any installation using no
253"external" ENGINE suffers no unnecessary memory footprint from unused
254ENGINEs. Likewise, installations that do require an ENGINE incur the
255overheads from only *that* ENGINE once it has been loaded.
256
257Sounds good? Maybe, but currently building an ENGINE implementation as
258a shared-library that can be loaded by "dynamic" isn't automated in
259OpenSSL's build process. It can be done manually quite easily however.
260Such a shared-library can either be built with any OpenSSL code it
261needs statically linked in, or it can link dynamically against OpenSSL
262if OpenSSL itself is built as a shared library. The instructions are
263the same in each case, but in the former (statically linked any
264dependencies on OpenSSL) you must ensure OpenSSL is built with
265position-independent code ("PIC"). The default OpenSSL compilation may
266already specify the relevant flags to do this, but you should consult
267with your compiler documentation if you are in any doubt.
268
269This example will show building the "atalla" ENGINE in the
270crypto/engine/ directory as a shared-library for use via the "dynamic"
271ENGINE.
272
2731.  "cd" to the crypto/engine/ directory of a pre-compiled OpenSSL
274    source tree.
275
2762.  Recompile at least one source file so you can see all the compiler
277    flags (and syntax) being used to build normally. Eg;
278
279        touch hw_atalla.c ; make
280
281    will rebuild "hw_atalla.o" using all such flags.
282
2833.  Manually enter the same compilation line to compile the
284    "hw_atalla.c" file but with the following two changes;
285
286    - add "-DENGINE_DYNAMIC_SUPPORT" to the command line switches,
287    - change the output file from "hw_atalla.o" to something new,
288      eg. "tmp_atalla.o"
289
2904.  Link "tmp_atalla.o" into a shared-library using the top-level
291    OpenSSL libraries to resolve any dependencies. The syntax for doing
292    this depends heavily on your system/compiler and is a nightmare
293    known well to anyone who has worked with shared-library portability
294    before. 'gcc' on Linux, for example, would use the following syntax;
295
296        gcc -shared -o dyn_atalla.so tmp_atalla.o -L../.. -lcrypto
297
2985.  Test your shared library using "openssl engine" as explained in the
299    previous section. Eg. from the top-level directory, you might try
300
301        apps/openssl engine -vvvv dynamic \
302              -pre SO_PATH:./crypto/engine/dyn_atalla.so -pre LOAD
303
304If the shared-library loads successfully, you will see both "-pre"
305commands marked as "SUCCESS" and the list of control commands
306displayed (because of "-vvvv") will be the control commands for the
307*atalla* ENGINE (ie. *not* the 'dynamic' ENGINE). You can also add
308the "-t" switch to the utility if you want it to try and initialise
309the atalla ENGINE for use to test any possible hardware/driver issues.
310
311PROBLEMS
312========
313
314It seems like the ENGINE part doesn't work too well with CryptoSwift on Win32.
315A quick test done right before the release showed that trying "openssl speed
316-engine cswift" generated errors. If the DSO gets enabled, an attempt is made
317to write at memory address 0x00000002.
318