  ENGINE
  ======

  With OpenSSL 0.9.6, a new component was added to support alternative
  cryptography implementations, most commonly for interfacing with external
  crypto devices (eg. accelerator cards). This component is called ENGINE,
  and its presence in OpenSSL 0.9.6 (and subsequent bug-fix releases)
  caused a little confusion as 0.9.6** releases were rolled in two
  versions, a "standard" and an "engine" version. In development for 0.9.7,
  the ENGINE code has been merged into the main branch and will be present
  in the standard releases from 0.9.7 forwards.

  There are currently built-in ENGINE implementations for the following
  crypto devices:

      o Microsoft CryptoAPI
      o VIA Padlock
      o nCipher CHIL

  In addition, dynamic binding to external ENGINE implementations is now
  provided by a special ENGINE called "dynamic". See the "DYNAMIC ENGINE"
  section below for details.

  At this stage, a number of things are still needed and are being worked on:

      1 Integration of EVP support.
      2 Configuration support.
      3 Documentation!

1 With respect to EVP, this relates to support for ciphers and digests in
  the ENGINE model so that alternative implementations of existing
  algorithms/modes (or previously unimplemented ones) can be provided by
  ENGINE implementations.

2 Configuration support currently exists in the ENGINE API itself, in the
  form of "control commands". These allow an application to expose to the
  user/admin the set of commands and parameter types a given ENGINE
  implementation supports, and for an application to directly feed string
  based input to those ENGINEs, in the form of name-value pairs. This is an
  extensible way for ENGINEs to define their own "configuration" mechanisms
  that are specific to a given ENGINE (eg. for a particular hardware
  device) but that should be consistent across *all* OpenSSL-based
  applications when they use that ENGINE. Work is in progress (or at least
  in planning) for supporting these control commands from the CONF (or
  NCONF) code so that applications using OpenSSL's existing configuration
  file format can have ENGINE settings specified in much the same way.
  Presently however, applications must use the ENGINE API itself to provide
  such functionality. To see first hand the types of commands available
  with the various compiled-in ENGINEs (see further down for dynamic
  ENGINEs), use the "engine" openssl utility with full verbosity, ie;
       openssl engine -vvvv

3 Documentation? Volunteers welcome! The source code is reasonably well
  self-documenting, but some summaries and usage instructions are needed -
  moreover, they are needed in the same POD format the existing OpenSSL
  documentation is provided in. Any complete or incomplete contributions
  would help make this happen.

  STABILITY & BUG-REPORTS
  =======================

  What already exists is fairly stable as far as it has been tested, but
  the test base has been a bit small most of the time. For the most part,
  the vendors of the devices these ENGINEs support have contributed to the
  development and/or testing of the implementations, and *usually* (with no
  guarantees) have experience in using the ENGINE support to drive their
  devices from common OpenSSL-based applications. Bugs and/or inexplicable
  behaviour in using a specific ENGINE implementation should be sent to the
  author of that implementation (if it is mentioned in the corresponding C
  file), and in the case of implementations for commercial hardware
  devices, also through whatever vendor support channels are available.  If
  none of this is possible, or the problem seems to be something about the
  ENGINE API itself (ie. not necessarily specific to a particular ENGINE
  implementation) then you should mail complete details to the relevant
  OpenSSL mailing list. For a definition of "complete details", refer to
  the OpenSSL "README" file. As for which list to send it to;

     openssl-users: if you are *using* the ENGINE abstraction, either in an
          pre-compiled application or in your own application code.

     openssl-dev: if you are discussing problems with OpenSSL source code.

  USAGE
  =====

  The default "openssl" ENGINE is always chosen when performing crypto
  operations unless you specify otherwise. You must actively tell the
  openssl utility commands to use anything else through a new command line
  switch called "-engine". Also, if you want to use the ENGINE support in
  your own code to do something similar, you must likewise explicitly
  select the ENGINE implementation you want.

  Depending on the type of hardware, system, and configuration, "settings"
  may need to be applied to an ENGINE for it to function as expected/hoped.
  The recommended way of doing this is for the application to support
  ENGINE "control commands" so that each ENGINE implementation can provide
  whatever configuration primitives it might require and the application
  can allow the user/admin (and thus the hardware vendor's support desk
  also) to provide any such input directly to the ENGINE implementation.
  This way, applications do not need to know anything specific to any
  device, they only need to provide the means to carry such user/admin
  input through to the ENGINE in question. Ie. this connects *you* (and
  your helpdesk) to the specific ENGINE implementation (and device), and
  allows application authors to not get buried in hassle supporting
  arbitrary devices they know (and care) nothing about.

  A new "openssl" utility, "openssl engine", has been added in that allows
  for testing and examination of ENGINE implementations. Basic usage
  instructions are available by specifying the "-?" command line switch.

  DYNAMIC ENGINES
  ===============

  The new "dynamic" ENGINE provides a low-overhead way to support ENGINE
  implementations that aren't pre-compiled and linked into OpenSSL-based
  applications. This could be because existing compiled-in implementations
  have known problems and you wish to use a newer version with an existing
  application. It could equally be because the application (or OpenSSL
  library) you are using simply doesn't have support for the ENGINE you
  wish to use, and the ENGINE provider (eg. hardware vendor) is providing
  you with a self-contained implementation in the form of a shared-library.
  The other use-case for "dynamic" is with applications that wish to
  maintain the smallest foot-print possible and so do not link in various
  ENGINE implementations from OpenSSL, but instead leaves you to provide
  them, if you want them, in the form of "dynamic"-loadable
  shared-libraries. It should be possible for hardware vendors to provide
  their own shared-libraries to support arbitrary hardware to work with
  applications based on OpenSSL 0.9.7 or later. If you're using an
  application based on 0.9.7 (or later) and the support you desire is only
  announced for versions later than the one you need, ask the vendor to
  backport their ENGINE to the version you need.

  How does "dynamic" work?
  ------------------------
    The dynamic ENGINE has a special flag in its implementation such that
    every time application code asks for the 'dynamic' ENGINE, it in fact
    gets its own copy of it. As such, multi-threaded code (or code that
    multiplexes multiple uses of 'dynamic' in a single application in any
    way at all) does not get confused by 'dynamic' being used to do many
    independent things. Other ENGINEs typically don't do this so there is
    only ever 1 ENGINE structure of its type (and reference counts are used
    to keep order). The dynamic ENGINE itself provides absolutely no
    cryptographic functionality, and any attempt to "initialise" the ENGINE
    automatically fails. All it does provide are a few "control commands"
    that can be used to control how it will load an external ENGINE
    implementation from a shared-library. To see these control commands,
    use the command-line;

       openssl engine -vvvv dynamic

    The "SO_PATH" control command should be used to identify the
    shared-library that contains the ENGINE implementation, and "NO_VCHECK"
    might possibly be useful if there is a minor version conflict and you
    (or a vendor helpdesk) is convinced you can safely ignore it.
    "ID" is probably only needed if a shared-library implements
    multiple ENGINEs, but if you know the engine id you expect to be using,
    it doesn't hurt to specify it (and this provides a sanity check if
    nothing else). "LIST_ADD" is only required if you actually wish the
    loaded ENGINE to be discoverable by application code later on using the
    ENGINE's "id". For most applications, this isn't necessary - but some
    application authors may have nifty reasons for using it. The "LOAD"
    command is the only one that takes no parameters and is the command
    that uses the settings from any previous commands to actually *load*
    the shared-library ENGINE implementation. If this command succeeds, the
    (copy of the) 'dynamic' ENGINE will magically morph into the ENGINE
    that has been loaded from the shared-library. As such, any control
    commands supported by the loaded ENGINE could then be executed as per
    normal. Eg. if ENGINE "foo" is implemented in the shared-library
    "libfoo.so" and it supports some special control command "CMD_FOO", the
    following code would load and use it (NB: obviously this code has no
    error checking);

       ENGINE *e = ENGINE_by_id("dynamic");
       ENGINE_ctrl_cmd_string(e, "SO_PATH", "/lib/libfoo.so", 0);
       ENGINE_ctrl_cmd_string(e, "ID", "foo", 0);
       ENGINE_ctrl_cmd_string(e, "LOAD", NULL, 0);
       ENGINE_ctrl_cmd_string(e, "CMD_FOO", "some input data", 0);

    For testing, the "openssl engine" utility can be useful for this sort
    of thing. For example the above code excerpt would achieve much the
    same result as;

       openssl engine dynamic \
                 -pre SO_PATH:/lib/libfoo.so \
                 -pre ID:foo \
                 -pre LOAD \
                 -pre "CMD_FOO:some input data"

    Or to simply see the list of commands supported by the "foo" ENGINE;

       openssl engine -vvvv dynamic \
                 -pre SO_PATH:/lib/libfoo.so \
                 -pre ID:foo \
                 -pre LOAD

    Applications that support the ENGINE API and more specifically, the
    "control commands" mechanism, will provide some way for you to pass
    such commands through to ENGINEs. As such, you would select "dynamic"
    as the ENGINE to use, and the parameters/commands you pass would
    control the *actual* ENGINE used. Each command is actually a name-value
    pair and the value can sometimes be omitted (eg. the "LOAD" command).
    Whilst the syntax demonstrated in "openssl engine" uses a colon to
    separate the command name from the value, applications may provide
    their own syntax for making that separation (eg. a win32 registry
    key-value pair may be used by some applications). The reason for the
    "-pre" syntax in the "openssl engine" utility is that some commands
    might be issued to an ENGINE *after* it has been initialised for use.
    Eg. if an ENGINE implementation requires a smart-card to be inserted
    during initialisation (or a PIN to be typed, or whatever), there may be
    a control command you can issue afterwards to "forget" the smart-card
    so that additional initialisation is no longer possible. In
    applications such as web-servers, where potentially volatile code may
    run on the same host system, this may provide some arguable security
    value. In such a case, the command would be passed to the ENGINE after
    it has been initialised for use, and so the "-post" switch would be
    used instead. Applications may provide a different syntax for
    supporting this distinction, and some may simply not provide it at all
    ("-pre" is almost always what you're after, in reality).

  How do I build a "dynamic" ENGINE?
  ----------------------------------
    This question is trickier - currently OpenSSL bundles various ENGINE
    implementations that are statically built in, and any application that
    calls the "ENGINE_load_builtin_engines()" function will automatically
    have all such ENGINEs available (and occupying memory). Applications
    that don't call that function have no ENGINEs available like that and
    would have to use "dynamic" to load any such ENGINE - but on the other
    hand such applications would only have the memory footprint of any
    ENGINEs explicitly loaded using user/admin provided control commands.
    The main advantage of not statically linking ENGINEs and only using
    "dynamic" for hardware support is that any installation using no
    "external" ENGINE suffers no unnecessary memory footprint from unused
    ENGINEs. Likewise, installations that do require an ENGINE incur the
    overheads from only *that* ENGINE once it has been loaded.

    Sounds good? Maybe, but currently building an ENGINE implementation as
    a shared-library that can be loaded by "dynamic" isn't automated in
    OpenSSL's build process. It can be done manually quite easily however.
    Such a shared-library can either be built with any OpenSSL code it
    needs statically linked in, or it can link dynamically against OpenSSL
    if OpenSSL itself is built as a shared library. The instructions are
    the same in each case, but in the former (statically linked any
    dependencies on OpenSSL) you must ensure OpenSSL is built with
    position-independent code ("PIC"). The default OpenSSL compilation may
    already specify the relevant flags to do this, but you should consult
    with your compiler documentation if you are in any doubt.

    This example will show building the "atalla" ENGINE in the
    crypto/engine/ directory as a shared-library for use via the "dynamic"
    ENGINE.
    1) "cd" to the crypto/engine/ directory of a pre-compiled OpenSSL
       source tree.
    2) Recompile at least one source file so you can see all the compiler
       flags (and syntax) being used to build normally. Eg;
           touch hw_atalla.c ; make
       will rebuild "hw_atalla.o" using all such flags.
    3) Manually enter the same compilation line to compile the
       "hw_atalla.c" file but with the following two changes;
         (a) add "-DENGINE_DYNAMIC_SUPPORT" to the command line switches,
	 (b) change the output file from "hw_atalla.o" to something new,
             eg. "tmp_atalla.o"
    4) Link "tmp_atalla.o" into a shared-library using the top-level
       OpenSSL libraries to resolve any dependencies. The syntax for doing
       this depends heavily on your system/compiler and is a nightmare
       known well to anyone who has worked with shared-library portability
       before. 'gcc' on Linux, for example, would use the following syntax;
          gcc -shared -o dyn_atalla.so tmp_atalla.o -L../.. -lcrypto
    5) Test your shared library using "openssl engine" as explained in the
       previous section. Eg. from the top-level directory, you might try;
          apps/openssl engine -vvvv dynamic \
              -pre SO_PATH:./crypto/engine/dyn_atalla.so -pre LOAD
       If the shared-library loads successfully, you will see both "-pre"
       commands marked as "SUCCESS" and the list of control commands
       displayed (because of "-vvvv") will be the control commands for the
       *atalla* ENGINE (ie. *not* the 'dynamic' ENGINE). You can also add
       the "-t" switch to the utility if you want it to try and initialise
       the atalla ENGINE for use to test any possible hardware/driver
       issues.

  PROBLEMS
  ========

  It seems like the ENGINE part doesn't work too well with CryptoSwift on Win32.
  A quick test done right before the release showed that trying "openssl speed
  -engine cswift" generated errors. If the DSO gets enabled, an attempt is made
  to write at memory address 0x00000002.