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290 lines
16 KiB
Plaintext
ENGINE
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======
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With OpenSSL 0.9.6, a new component was added to support alternative
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cryptography implementations, most commonly for interfacing with external
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crypto devices (eg. accelerator cards). This component is called ENGINE,
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and its presence in OpenSSL 0.9.6 (and subsequent bug-fix releases)
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caused a little confusion as 0.9.6** releases were rolled in two
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versions, a "standard" and an "engine" version. In development for 0.9.7,
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the ENGINE code has been merged into the main branch and will be present
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in the standard releases from 0.9.7 forwards.
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There are currently built-in ENGINE implementations for the following
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crypto devices:
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o CryptoSwift
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o Compaq Atalla
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o nCipher CHIL
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o Nuron
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o Broadcom uBSec
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In addition, dynamic binding to external ENGINE implementations is now
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provided by a special ENGINE called "dynamic". See the "DYNAMIC ENGINE"
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section below for details.
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At this stage, a number of things are still needed and are being worked on:
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1 Integration of EVP support.
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2 Configuration support.
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3 Documentation!
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1 With respect to EVP, this relates to support for ciphers and digests in
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the ENGINE model so that alternative implementations of existing
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algorithms/modes (or previously unimplemented ones) can be provided by
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ENGINE implementations.
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2 Configuration support currently exists in the ENGINE API itself, in the
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form of "control commands". These allow an application to expose to the
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user/admin the set of commands and parameter types a given ENGINE
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implementation supports, and for an application to directly feed string
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based input to those ENGINEs, in the form of name-value pairs. This is an
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extensible way for ENGINEs to define their own "configuration" mechanisms
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that are specific to a given ENGINE (eg. for a particular hardware
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device) but that should be consistent across *all* OpenSSL-based
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applications when they use that ENGINE. Work is in progress (or at least
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in planning) for supporting these control commands from the CONF (or
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NCONF) code so that applications using OpenSSL's existing configuration
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file format can have ENGINE settings specified in much the same way.
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Presently however, applications must use the ENGINE API itself to provide
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such functionality. To see first hand the types of commands available
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with the various compiled-in ENGINEs (see further down for dynamic
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ENGINEs), use the "engine" openssl utility with full verbosity, ie;
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openssl engine -vvvv
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3 Documentation? Volunteers welcome! The source code is reasonably well
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self-documenting, but some summaries and usage instructions are needed -
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moreover, they are needed in the same POD format the existing OpenSSL
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documentation is provided in. Any complete or incomplete contributions
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would help make this happen.
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STABILITY & BUG-REPORTS
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=======================
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What already exists is fairly stable as far as it has been tested, but
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the test base has been a bit small most of the time. For the most part,
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the vendors of the devices these ENGINEs support have contributed to the
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development and/or testing of the implementations, and *usually* (with no
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guarantees) have experience in using the ENGINE support to drive their
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devices from common OpenSSL-based applications. Bugs and/or inexplicable
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behaviour in using a specific ENGINE implementation should be sent to the
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author of that implementation (if it is mentioned in the corresponding C
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file), and in the case of implementations for commercial hardware
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devices, also through whatever vendor support channels are available. If
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none of this is possible, or the problem seems to be something about the
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ENGINE API itself (ie. not necessarily specific to a particular ENGINE
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implementation) then you should mail complete details to the relevant
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OpenSSL mailing list. For a definition of "complete details", refer to
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the OpenSSL "README" file. As for which list to send it to;
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openssl-users: if you are *using* the ENGINE abstraction, either in an
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pre-compiled application or in your own application code.
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openssl-dev: if you are discussing problems with OpenSSL source code.
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USAGE
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=====
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The default "openssl" ENGINE is always chosen when performing crypto
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operations unless you specify otherwise. You must actively tell the
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openssl utility commands to use anything else through a new command line
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switch called "-engine". Also, if you want to use the ENGINE support in
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your own code to do something similar, you must likewise explicitly
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select the ENGINE implementation you want.
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Depending on the type of hardware, system, and configuration, "settings"
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may need to be applied to an ENGINE for it to function as expected/hoped.
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The recommended way of doing this is for the application to support
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ENGINE "control commands" so that each ENGINE implementation can provide
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whatever configuration primitives it might require and the application
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can allow the user/admin (and thus the hardware vendor's support desk
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also) to provide any such input directly to the ENGINE implementation.
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This way, applications do not need to know anything specific to any
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device, they only need to provide the means to carry such user/admin
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input through to the ENGINE in question. Ie. this connects *you* (and
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your helpdesk) to the specific ENGINE implementation (and device), and
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allows application authors to not get buried in hassle supporting
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arbitrary devices they know (and care) nothing about.
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A new "openssl" utility, "openssl engine", has been added in that allows
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for testing and examination of ENGINE implementations. Basic usage
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instructions are available by specifying the "-?" command line switch.
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DYNAMIC ENGINES
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===============
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The new "dynamic" ENGINE provides a low-overhead way to support ENGINE
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implementations that aren't pre-compiled and linked into OpenSSL-based
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applications. This could be because existing compiled-in implementations
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have known problems and you wish to use a newer version with an existing
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application. It could equally be because the application (or OpenSSL
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library) you are using simply doesn't have support for the ENGINE you
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wish to use, and the ENGINE provider (eg. hardware vendor) is providing
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you with a self-contained implementation in the form of a shared-library.
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The other use-case for "dynamic" is with applications that wish to
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maintain the smallest foot-print possible and so do not link in various
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ENGINE implementations from OpenSSL, but instead leaves you to provide
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them, if you want them, in the form of "dynamic"-loadable
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shared-libraries. It should be possible for hardware vendors to provide
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their own shared-libraries to support arbitrary hardware to work with
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applications based on OpenSSL 0.9.7 or later. If you're using an
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application based on 0.9.7 (or later) and the support you desire is only
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announced for versions later than the one you need, ask the vendor to
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backport their ENGINE to the version you need.
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How does "dynamic" work?
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------------------------
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The dynamic ENGINE has a special flag in its implementation such that
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every time application code asks for the 'dynamic' ENGINE, it in fact
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gets its own copy of it. As such, multi-threaded code (or code that
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multiplexes multiple uses of 'dynamic' in a single application in any
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way at all) does not get confused by 'dynamic' being used to do many
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independent things. Other ENGINEs typically don't do this so there is
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only ever 1 ENGINE structure of its type (and reference counts are used
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to keep order). The dynamic ENGINE itself provides absolutely no
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cryptographic functionality, and any attempt to "initialise" the ENGINE
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automatically fails. All it does provide are a few "control commands"
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that can be used to control how it will load an external ENGINE
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implementation from a shared-library. To see these control commands,
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use the command-line;
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openssl engine -vvvv dynamic
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The "SO_PATH" control command should be used to identify the
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shared-library that contains the ENGINE implementation, and "NO_VCHECK"
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might possibly be useful if there is a minor version conflict and you
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(or a vendor helpdesk) is convinced you can safely ignore it.
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"ID" is probably only needed if a shared-library implements
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multiple ENGINEs, but if you know the engine id you expect to be using,
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it doesn't hurt to specify it (and this provides a sanity check if
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nothing else). "LIST_ADD" is only required if you actually wish the
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loaded ENGINE to be discoverable by application code later on using the
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ENGINE's "id". For most applications, this isn't necessary - but some
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application authors may have nifty reasons for using it. The "LOAD"
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command is the only one that takes no parameters and is the command
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that uses the settings from any previous commands to actually *load*
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the shared-library ENGINE implementation. If this command succeeds, the
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(copy of the) 'dynamic' ENGINE will magically morph into the ENGINE
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that has been loaded from the shared-library. As such, any control
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commands supported by the loaded ENGINE could then be executed as per
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normal. Eg. if ENGINE "foo" is implemented in the shared-library
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"libfoo.so" and it supports some special control command "CMD_FOO", the
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following code would load and use it (NB: obviously this code has no
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error checking);
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ENGINE *e = ENGINE_by_id("dynamic");
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ENGINE_ctrl_cmd_string(e, "SO_PATH", "/lib/libfoo.so", 0);
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ENGINE_ctrl_cmd_string(e, "ID", "foo", 0);
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ENGINE_ctrl_cmd_string(e, "LOAD", NULL, 0);
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ENGINE_ctrl_cmd_string(e, "CMD_FOO", "some input data", 0);
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For testing, the "openssl engine" utility can be useful for this sort
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of thing. For example the above code excerpt would achieve much the
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same result as;
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openssl engine dynamic \
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-pre SO_PATH:/lib/libfoo.so \
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-pre ID:foo \
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-pre LOAD \
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-pre "CMD_FOO:some input data"
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Or to simply see the list of commands supported by the "foo" ENGINE;
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openssl engine -vvvv dynamic \
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-pre SO_PATH:/lib/libfoo.so \
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-pre ID:foo \
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-pre LOAD
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Applications that support the ENGINE API and more specifically, the
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"control commands" mechanism, will provide some way for you to pass
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such commands through to ENGINEs. As such, you would select "dynamic"
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as the ENGINE to use, and the parameters/commands you pass would
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control the *actual* ENGINE used. Each command is actually a name-value
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pair and the value can sometimes be omitted (eg. the "LOAD" command).
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Whilst the syntax demonstrated in "openssl engine" uses a colon to
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separate the command name from the value, applications may provide
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their own syntax for making that separation (eg. a win32 registry
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key-value pair may be used by some applications). The reason for the
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"-pre" syntax in the "openssl engine" utility is that some commands
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might be issued to an ENGINE *after* it has been initialised for use.
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Eg. if an ENGINE implementation requires a smart-card to be inserted
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during initialisation (or a PIN to be typed, or whatever), there may be
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a control command you can issue afterwards to "forget" the smart-card
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so that additional initialisation is no longer possible. In
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applications such as web-servers, where potentially volatile code may
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run on the same host system, this may provide some arguable security
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value. In such a case, the command would be passed to the ENGINE after
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it has been initialised for use, and so the "-post" switch would be
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used instead. Applications may provide a different syntax for
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supporting this distinction, and some may simply not provide it at all
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("-pre" is almost always what you're after, in reality).
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How do I build a "dynamic" ENGINE?
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----------------------------------
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This question is trickier - currently OpenSSL bundles various ENGINE
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implementations that are statically built in, and any application that
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calls the "ENGINE_load_builtin_engines()" function will automatically
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have all such ENGINEs available (and occupying memory). Applications
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that don't call that function have no ENGINEs available like that and
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would have to use "dynamic" to load any such ENGINE - but on the other
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hand such applications would only have the memory footprint of any
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ENGINEs explicitly loaded using user/admin provided control commands.
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The main advantage of not statically linking ENGINEs and only using
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"dynamic" for hardware support is that any installation using no
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"external" ENGINE suffers no unnecessary memory footprint from unused
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ENGINEs. Likewise, installations that do require an ENGINE incur the
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overheads from only *that* ENGINE once it has been loaded.
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Sounds good? Maybe, but currently building an ENGINE implementation as
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a shared-library that can be loaded by "dynamic" isn't automated in
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OpenSSL's build process. It can be done manually quite easily however.
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Such a shared-library can either be built with any OpenSSL code it
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needs statically linked in, or it can link dynamically against OpenSSL
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if OpenSSL itself is built as a shared library. The instructions are
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the same in each case, but in the former (statically linked any
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dependencies on OpenSSL) you must ensure OpenSSL is built with
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position-independent code ("PIC"). The default OpenSSL compilation may
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already specify the relevant flags to do this, but you should consult
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with your compiler documentation if you are in any doubt.
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This example will show building the "atalla" ENGINE in the
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crypto/engine/ directory as a shared-library for use via the "dynamic"
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ENGINE.
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1) "cd" to the crypto/engine/ directory of a pre-compiled OpenSSL
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source tree.
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2) Recompile at least one source file so you can see all the compiler
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flags (and syntax) being used to build normally. Eg;
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touch hw_atalla.c ; make
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will rebuild "hw_atalla.o" using all such flags.
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3) Manually enter the same compilation line to compile the
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"hw_atalla.c" file but with the following two changes;
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(a) add "-DENGINE_DYNAMIC_SUPPORT" to the command line switches,
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(b) change the output file from "hw_atalla.o" to something new,
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eg. "tmp_atalla.o"
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4) Link "tmp_atalla.o" into a shared-library using the top-level
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OpenSSL libraries to resolve any dependencies. The syntax for doing
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this depends heavily on your system/compiler and is a nightmare
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known well to anyone who has worked with shared-library portability
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before. 'gcc' on Linux, for example, would use the following syntax;
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gcc -shared -o dyn_atalla.so tmp_atalla.o -L../.. -lcrypto
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5) Test your shared library using "openssl engine" as explained in the
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previous section. Eg. from the top-level directory, you might try;
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apps/openssl engine -vvvv dynamic \
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-pre SO_PATH:./crypto/engine/dyn_atalla.so -pre LOAD
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If the shared-library loads successfully, you will see both "-pre"
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commands marked as "SUCCESS" and the list of control commands
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displayed (because of "-vvvv") will be the control commands for the
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*atalla* ENGINE (ie. *not* the 'dynamic' ENGINE). You can also add
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the "-t" switch to the utility if you want it to try and initialise
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the atalla ENGINE for use to test any possible hardware/driver
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issues.
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PROBLEMS
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========
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It seems like the ENGINE part doesn't work too well with CryptoSwift on Win32.
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A quick test done right before the release showed that trying "openssl speed
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-engine cswift" generated errors. If the DSO gets enabled, an attempt is made
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to write at memory address 0x00000002.
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