readme.engine

mediastreamer2是开源的网络传输媒体流的库

    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.