1 files changed, 208 insertions, 275 deletions

diff --git a/src/lib/libcrypto/engine/README b/src/lib/libcrypto/engine/README
index 96595e6f35..6b69b70f57 100644
--- a/src/lib/libcrypto/engine/README
+++ b/src/lib/libcrypto/engine/README
@@ -1,278 +1,211 @@
-NOTES, THOUGHTS, and EVERYTHING
--------------------------------
-
-(1) Concurrency and locking ... I made a change to the ENGINE_free code
-    because I spotted a potential hold-up in proceedings (doing too
-    much inside a lock including calling a callback), there may be
-    other bits like this. What do the speed/optimisation freaks think
-    of this aspect of the code and design? There's lots of locking for
-    manipulation functions and I need that to keep things nice and
-    solid, but this manipulation is mostly (de)initialisation, I would
-    think that most run-time locking is purely in the ENGINE_init and
-    ENGINE_finish calls that might be made when getting handles for
-    RSA (and friends') structures. These would be mostly reference
-    count operations as the functional references should always be 1
-    or greater at run-time to prevent init/deinit thrashing.
-
-(2) nCipher support, via the HWCryptoHook API, is now in the code.
-    Apparently this hasn't been tested too much yet, but it looks
-    good. :-) Atalla support has been added too, but shares a lot in
-    common with Ben's original hooks in bn_exp.c (although it has been
-    ENGINE-ified, and error handling wrapped around it) and it's also
-    had some low-volume testing, so it should be usable.
-
-(3) Of more concern, we need to work out (a) how to put together usable
-    RAND_METHODs for units that just have one "get n or less random
-    bytes" function, (b) we also need to determine how to hook the code
-    in crypto/rand/ to use the ENGINE defaults in a way similar to what
-    has been done in crypto/rsa/, crypto/dsa/, etc.
-
-(4) ENGINE should really grow to encompass more than 3 public key
-    algorithms and randomness gathering. The structure/data level of
-    the engine code is hidden from code outside the crypto/engine/
-    directory so change shouldn't be too viral. More important though
-    is how things should evolve ... this needs thought and discussion.
-
-
------------------------------------==*==-----------------------------------
-
-More notes 2000-08-01
----------------------
-
-Geoff Thorpe, who designed the engine part, wrote a pretty good description
-of the thoughts he had when he built it, good enough to include verbatim here
-(with his permission)                                   -- Richard Levitte
-
-
-Date: Tue, 1 Aug 2000 16:54:08 +0100 (BST)
-From: Geoff Thorpe
-Subject: Re: The thoughts to merge BRANCH_engine into the main trunk are
- emerging
-
-Hi there,
-
-I'm going to try and do some justice to this, but I'm a little short on
-time and the there is an endless amount that could be discussed on this
-subject. sigh ... please bear with me :-)
-
-> The changes in BRANCH_engine dig deep into the core of OpenSSL, for example
-> into the RSA and RAND routines, adding a level of indirection which is needed
-> to keep the abstraction, as far as I understand.  It would be a good thing if
-> those who do play with those things took a look at the changes that have been
-> done in the branch and say out loud how much (or hopefully little) we've made
-> fools of ourselves.
-
-The point here is that the code that has emerged in the BRANCH_engine
-branch was based on some initial requirements of mine that I went in and
-addressed, and Richard has picked up the ball and run with it too. It
-would be really useful to get some review of the approach we've taken, but
-first I think I need to describe as best I can the reasons behind what has
-been done so far, in particular what issues we have tried to address when
-doing this, and what issues we have intentionally (or necessarily) tried
-to avoid.
-
-methods, engines, and evps
---------------------------
-
-There has been some dicussion, particularly with Steve, about where this
-ENGINE stuff might fit into the conceptual picture as/when we start to
-abstract algorithms a little bit to make the library more extensible. In
-particular, it would desirable to have algorithms (symmetric, hash, pkc,
-etc) abstracted in some way that allows them to be just objects sitting in
-a list (or database) ... it'll just happen that the "DSA" object doesn't
-support encryption whereas the "RSA" object does. This requires a lot of
-consideration to begin to know how to tackle it; in particular how
-encapsulated should these things be? If the objects also understand their
-own ASN1 encodings and what-not, then it would for example be possible to
-add support for elliptic-curve DSA in as a new algorithm and automatically
-have ECC-DSA certificates supported in SSL applications. Possible, but not
-easy. :-)
-
-Whatever, it seems that the way to go (if I've grok'd Steve's comments on
-this in the past) is to amalgamate these things in EVP as is already done
-(I think) for ciphers or hashes (Steve, please correct/elaborate). I
-certainly think something should be done in this direction because right
-now we have different source directories, types, functions, and methods
-for each algorithm - even when conceptually they are very much different
-feathers of the same bird. (This is certainly all true for the public-key
-stuff, and may be partially true for the other parts.)
-
-ENGINE was *not* conceived as a way of solving this, far from it. Nor was
-it conceived as a way of replacing the various "***_METHOD"s. It was
-conceived as an abstraction of a sort of "virtual crypto device". If we
-lived in a world where "EVP_ALGO"s (or something like them) encapsulated
-particular algorithms like RSA,DSA,MD5,RC4,etc, and "***_METHOD"s
-encapsulated interfaces to algorithms (eg. some algo's might support a
-PKC_METHOD, a HASH_METHOD, or a CIPHER_METHOD, who knows?), then I would
-think that ENGINE would encapsulate an implementation of arbitrarily many
-of those algorithms - perhaps as alternatives to existing algorithms
-and/or perhaps as new previously unimplemented algorithms. An ENGINE could
-be used to contain an alternative software implementation, a wrapper for a
-hardware acceleration and/or key-management unit, a comms-wrapper for
-distributing cryptographic operations to remote machines, or any other
-"devices" your imagination can dream up.
-
-However, what has been done in the ENGINE branch so far is nothing more
-than starting to get our toes wet. I had a couple of self-imposed
-requirements when putting the initial abstraction together, and I may have
-already posed these in one form or another on the list, but briefly;
-
-   (i) only bother with public key algorithms for now, and maybe RAND too
-       (motivated by the need to get hardware support going and the fact
-       this was a comparitively easy subset to address to begin with).
-
-  (ii) don't change (if at all possible) the existing crypto code, ie. the
-       implementations, the way the ***_METHODs work, etc.
-
- (iii) ensure that if no function from the ENGINE code is ever called then
-       things work the way they always did, and there is no memory
-       allocation (otherwise the failure to cleanup would be a problem -
-       this is part of the reason no STACKs were used, the other part of
-       the reason being I found them inappropriate).
-
-  (iv) ensure that all the built-in crypto was encapsulated by one of
-       these "ENGINE"s and that this engine was automatically selected as
-       the default.
-
-   (v) provide the minimum hooking possible in the existing crypto code
-       so that global functions (eg. RSA_public_encrypt) do not need any
-       extra parameter, yet will use whatever the current default ENGINE
-       for that RSA key is, and that the default can be set "per-key"
-       and globally (new keys will assume the global default, and keys
-       without their own default will be operated on using the global
-       default). NB: Try and make (v) conflict as little as possible with
-       (ii). :-)
-
-  (vi) wrap the ENGINE code up in duct tape so you can't even see the
-       corners. Ie. expose no structures at all, just black-box pointers.
-
-   (v) maintain internally a list of ENGINEs on which a calling
-       application can iterate, interrogate, etc. Allow a calling
-       application to hook in new ENGINEs, remove ENGINEs from the list,
-       and enforce uniqueness within the global list of each ENGINE's
-       "unique id".
-
-  (vi) keep reference counts for everything - eg. this includes storing a
-       reference inside each RSA structure to the ENGINE that it uses.
-       This is freed when the RSA structure is destroyed, or has its
-       ENGINE explicitly changed. The net effect needs to be that at any
-       time, it is deterministic to know whether an ENGINE is in use or
-       can be safely removed (or unloaded in the case of the other type
-       of reference) without invalidating function pointers that may or
-       may not be used indavertently in the future. This was actually
-       one of the biggest problems to overcome in the existing OpenSSL
-       code - implementations had always been assumed to be ever-present,
-       so there was no trivial way to get round this.
-
- (vii) distinguish between structural references and functional
-       references.
-
-A *little* detail
+Notes: 2001-09-24
 -----------------
 
-While my mind is on it; I'll illustrate the bit in item (vii). This idea
-turned out to be very handy - the ENGINEs themselves need to be operated
-on and manipulated simply as objects without necessarily trying to
-"enable" them for use. Eg. most host machines will not have the necessary
-hardware or software to support all the engines one might compile into
-OpenSSL, yet it needs to be possible to iterate across the ENGINEs,
-querying their names, properties, etc - all happening in a thread-safe
-manner that uses reference counts (if you imagine two threads iterating
-through a list and one thread removing the ENGINE the other is currently
-looking at - you can see the gotcha waiting to happen). For all of this,
-*structural references* are used and operate much like the other reference
-counts in OpenSSL.
-
-The other kind of reference count is for *functional* references - these
-indicate a reference on which the caller can actually assume the
-particular ENGINE to be initialised and usable to perform the operations
-it implements. Any increment or decrement of the functional reference
-count automatically invokes a corresponding change in the structural
-reference count, as it is fairly obvious that a functional reference is a
-restricted case of a structural reference. So struct_ref >= funct_ref at
-all times. NB: functional references are usually obtained by a call to
-ENGINE_init(), but can also be created implicitly by calls that require a
-new functional reference to be created, eg. ENGINE_set_default(). Either
-way the only time the underlying ENGINE's "init" function is really called
-is when the (functional) reference count increases to 1, similarly the
-underlying "finish" handler is only called as the count goes down to 0.
-The effect of this, for example, is that if you set the default ENGINE for
-RSA operations to be "cswift", then its functional reference count will
-already be at least 1 so the CryptoSwift shared-library and the card will
-stay loaded and initialised until such time as all RSA keys using the
-cswift ENGINE are changed or destroyed and the default ENGINE for RSA
-operations has been changed. This prevents repeated thrashing of init and
-finish handling if the count keeps getting down as far as zero.
-
-Otherwise, the way the ENGINE code has been put together I think pretty
-much reflects the above points. The reason for the ENGINE structure having
-individual RSA_METHOD, DSA_METHOD, etc pointers is simply that it was the
-easiest way to go about things for now, to hook it all into the raw
-RSA,DSA,etc code, and I was trying to the keep the structure invisible
-anyway so that the way this is internally managed could be easily changed
-later on when we start to work out what's to be done about these other
-abstractions.
-
-Down the line, if some EVP-based technique emerges for adequately
-encapsulating algorithms and all their various bits and pieces, then I can
-imagine that "ENGINE" would turn into a reference-counting database of
-these EVP things, of which the default "openssl" ENGINE would be the
-library's own object database of pre-built software implemented algorithms
-(and such). It would also be cool to see the idea of "METHOD"s detached
-from the algorithms themselves ... so RSA, DSA, ElGamal, etc can all
-expose essentially the same METHOD (aka interface), which would include
-any querying/flagging stuff to identify what the algorithm can/can't do,
-its name, and other stuff like max/min block sizes, key sizes, etc. This
-would result in ENGINE similarly detaching its internal database of
-algorithm implementations from the function definitions that return
-interfaces to them. I think ...
-
-As for DSOs etc. Well the DSO code is pretty handy (but could be made much
-more so) for loading vendor's driver-libraries and talking to them in some
-generic way, but right now there's still big problems associated with
-actually putting OpenSSL code (ie. new ENGINEs, or anything else for that
-matter) in dynamically loadable libraries. These problems won't go away in
-a hurry so I don't think we should expect to have any kind of
-shared-library extensions any time soon - but solving the problems is a
-good thing to aim for, and would as a side-effect probably help make
-OpenSSL more usable as a shared-library itself (looking at the things
-needed to do this will show you why).
-
-One of the problems is that if you look at any of the ENGINE
-implementations, eg. hw_cswift.c or hw_ncipher.c, you'll see how it needs
-a variety of functionality and definitions from various areas of OpenSSL,
-including crypto/bn/, crypto/err/, crypto/ itself (locking for example),
-crypto/dso/, crypto/engine/, crypto/rsa, etc etc etc. So if similar code
-were to be suctioned off into shared libraries, the shared libraries would
-either have to duplicate all the definitions and code and avoid loader
-conflicts, or OpenSSL would have to somehow expose all that functionality
-to the shared-library. If this isn't a big enough problem, the issue of
-binary compatibility will be - anyone writing Apache modules can tell you
-that (Ralf? Ben? :-). However, I don't think OpenSSL would need to be
-quite so forgiving as Apache should be, so OpenSSL could simply tell its
-version to the DSO and leave the DSO with the problem of deciding whether
-to proceed or bail out for fear of binary incompatibilities.
-
-Certainly one thing that would go a long way to addressing this is to
-embark on a bit of an opaqueness mission. I've set the ENGINE code up with
-this in mind - it's so draconian that even to declare your own ENGINE, you
-have to get the engine code to create the underlying ENGINE structure, and
-then feed in the new ENGINE's function/method pointers through various
-"set" functions. The more of the code that takes on such a black-box
-approach, the more of the code that will be (a) easy to expose to shared
-libraries that need it, and (b) easy to expose to applications wanting to
-use OpenSSL itself as a shared-library. From my own explorations in
-OpenSSL, the biggest leviathan I've seen that is a problem in this respect
-is the BIGNUM code. Trying to "expose" the bignum code through any kind of
-organised "METHODs", let alone do all the necessary bignum operations
-solely through functions rather than direct access to the structures and
-macros, will be a massive pain in the "r"s.
-
-Anyway, I'm done for now - hope it was readable. Thoughts?
-
-Cheers,
-Geoff
-
-
------------------------------------==*==-----------------------------------
+This "description" (if one chooses to call it that) needed some major updating
+so here goes. This update addresses a change being made at the same time to
+OpenSSL, and it pretty much completely restructures the underlying mechanics of
+the "ENGINE" code. So it serves a double purpose of being a "ENGINE internals
+for masochists" document *and* a rather extensive commit log message. (I'd get
+lynched for sticking all this in CHANGES or the commit mails :-).
+
+ENGINE_TABLE underlies this restructuring, as described in the internal header
+"eng_int.h", implemented in eng_table.c, and used in each of the "class" files;
+tb_rsa.c, tb_dsa.c, etc.
+
+However, "EVP_CIPHER" underlies the motivation and design of ENGINE_TABLE so
+I'll mention a bit about that first. EVP_CIPHER (and most of this applies
+equally to EVP_MD for digests) is both a "method" and a algorithm/mode
+identifier that, in the current API, "lingers". These cipher description +
+implementation structures can be defined or obtained directly by applications,
+or can be loaded "en masse" into EVP storage so that they can be catalogued and
+searched in various ways, ie. two ways of encrypting with the "des_cbc"
+algorithm/mode pair are;
+
+(i) directly;
+     const EVP_CIPHER *cipher = EVP_des_cbc();
+     EVP_EncryptInit(&ctx, cipher, key, iv);
+     [ ... use EVP_EncryptUpdate() and EVP_EncryptFinal() ...]
+
+(ii) indirectly; 
+     OpenSSL_add_all_ciphers();
+     cipher = EVP_get_cipherbyname("des_cbc");
+     EVP_EncryptInit(&ctx, cipher, key, iv);
+     [ ... etc ... ]
+
+The latter is more generally used because it also allows ciphers/digests to be
+looked up based on other identifiers which can be useful for automatic cipher
+selection, eg. in SSL/TLS, or by user-controllable configuration.
+
+The important point about this is that EVP_CIPHER definitions and structures are
+passed around with impunity and there is no safe way, without requiring massive
+rewrites of many applications, to assume that EVP_CIPHERs can be reference
+counted. One an EVP_CIPHER is exposed to the caller, neither it nor anything it
+comes from can "safely" be destroyed. Unless of course the way of getting to
+such ciphers is via entirely distinct API calls that didn't exist before.
+However existing API usage cannot be made to understand when an EVP_CIPHER
+pointer, that has been passed to the caller, is no longer being used.
+
+The other problem with the existing API w.r.t. to hooking EVP_CIPHER support
+into ENGINE is storage - the OBJ_NAME-based storage used by EVP to register
+ciphers simultaneously registers cipher *types* and cipher *implementations* -
+they are effectively the same thing, an "EVP_CIPHER" pointer. The problem with
+hooking in ENGINEs is that multiple ENGINEs may implement the same ciphers. The
+solution is necessarily that ENGINE-provided ciphers simply are not registered,
+stored, or exposed to the caller in the same manner as existing ciphers. This is
+especially necessary considering the fact ENGINE uses reference counts to allow
+for cleanup, modularity, and DSO support - yet EVP_CIPHERs, as exposed to
+callers in the current API, support no such controls.
+
+Another sticking point for integrating cipher support into ENGINE is linkage.
+Already there is a problem with the way ENGINE supports RSA, DSA, etc whereby
+they are available *because* they're part of a giant ENGINE called "openssl".
+Ie. all implementations *have* to come from an ENGINE, but we get round that by
+having a giant ENGINE with all the software support encapsulated. This creates
+linker hassles if nothing else - linking a 1-line application that calls 2 basic
+RSA functions (eg. "RSA_free(RSA_new());") will result in large quantities of
+ENGINE code being linked in *and* because of that DSA, DH, and RAND also. If we
+continue with this approach for EVP_CIPHER support (even if it *was* possible)
+we would lose our ability to link selectively by selectively loading certain
+implementations of certain functionality. Touching any part of any kind of
+crypto would result in massive static linkage of everything else. So the
+solution is to change the way ENGINE feeds existing "classes", ie. how the
+hooking to ENGINE works from RSA, DSA, DH, RAND, as well as adding new hooking
+for EVP_CIPHER, and EVP_MD.
+
+The way this is now being done is by mostly reverting back to how things used to
+work prior to ENGINE :-). Ie. RSA now has a "RSA_METHOD" pointer again - this
+was previously replaced by an "ENGINE" pointer and all RSA code that required
+the RSA_METHOD would call ENGINE_get_RSA() each time on its ENGINE handle to
+temporarily get and use the ENGINE's RSA implementation. Apart from being more
+efficient, switching back to each RSA having an RSA_METHOD pointer also allows
+us to conceivably operate with *no* ENGINE. As we'll see, this removes any need
+for a fallback ENGINE that encapsulates default implementations - we can simply
+have our RSA structure pointing its RSA_METHOD pointer to the software
+implementation and have its ENGINE pointer set to NULL.
+
+A look at the EVP_CIPHER hooking is most explanatory, the RSA, DSA (etc) cases
+turn out to be degenerate forms of the same thing. The EVP storage of ciphers,
+and the existing EVP API functions that return "software" implementations and
+descriptions remain untouched. However, the storage takes more meaning in terms
+of "cipher description" and less meaning in terms of "implementation". When an
+EVP_CIPHER_CTX is actually initialised with an EVP_CIPHER method and is about to
+begin en/decryption, the hooking to ENGINE comes into play. What happens is that
+cipher-specific ENGINE code is asked for an ENGINE pointer (a functional
+reference) for any ENGINE that is registered to perform the algo/mode that the
+provided EVP_CIPHER structure represents. Under normal circumstances, that
+ENGINE code will return NULL because no ENGINEs will have had any cipher
+implementations *registered*. As such, a NULL ENGINE pointer is stored in the
+EVP_CIPHER_CTX context, and the EVP_CIPHER structure is left hooked into the
+context and so is used as the implementation. Pretty much how things work now
+except we'd have a redundant ENGINE pointer set to NULL and doing nothing.
+
+Conversely, if an ENGINE *has* been registered to perform the algorithm/mode
+combination represented by the provided EVP_CIPHER, then a functional reference
+to that ENGINE will be returned to the EVP_CIPHER_CTX during initialisation.
+That functional reference will be stored in the context (and released on
+cleanup) - and having that reference provides a *safe* way to use an EVP_CIPHER
+definition that is private to the ENGINE. Ie. the EVP_CIPHER provided by the
+application will actually be replaced by an EVP_CIPHER from the registered
+ENGINE - it will support the same algorithm/mode as the original but will be a
+completely different implementation. Because this EVP_CIPHER isn't stored in the
+EVP storage, nor is it returned to applications from traditional API functions,
+there is no associated problem with it not having reference counts. And of
+course, when one of these "private" cipher implementations is hooked into
+EVP_CIPHER_CTX, it is done whilst the EVP_CIPHER_CTX holds a functional
+reference to the ENGINE that owns it, thus the use of the ENGINE's EVP_CIPHER is
+safe.
+
+The "cipher-specific ENGINE code" I mentioned is implemented in tb_cipher.c but
+in essence it is simply an instantiation of "ENGINE_TABLE" code for use by
+EVP_CIPHER code. tb_digest.c is virtually identical but, of course, it is for
+use by EVP_MD code. Ditto for tb_rsa.c, tb_dsa.c, etc. These instantiations of
+ENGINE_TABLE essentially provide linker-separation of the classes so that even
+if ENGINEs implement *all* possible algorithms, an application using only
+EVP_CIPHER code will link at most code relating to EVP_CIPHER, tb_cipher.c, core
+ENGINE code that is independant of class, and of course the ENGINE
+implementation that the application loaded. It will *not* however link any
+class-specific ENGINE code for digests, RSA, etc nor will it bleed over into
+other APIs, such as the RSA/DSA/etc library code.
+
+ENGINE_TABLE is a little more complicated than may seem necessary but this is
+mostly to avoid a lot of "init()"-thrashing on ENGINEs (that may have to load
+DSOs, and other expensive setup that shouldn't be thrashed unnecessarily) *and*
+to duplicate "default" behaviour. Basically an ENGINE_TABLE instantiation, for
+example tb_cipher.c, implements a hash-table keyed by integer "nid" values.
+These nids provide the uniquenness of an algorithm/mode - and each nid will hash
+to a potentially NULL "ENGINE_PILE". An ENGINE_PILE is essentially a list of
+pointers to ENGINEs that implement that particular 'nid'. Each "pile" uses some
+caching tricks such that requests on that 'nid' will be cached and all future
+requests will return immediately (well, at least with minimal operation) unless
+a change is made to the pile, eg. perhaps an ENGINE was unloaded. The reason is
+that an application could have support for 10 ENGINEs statically linked
+in, and the machine in question may not have any of the hardware those 10
+ENGINEs support. If each of those ENGINEs has a "des_cbc" implementation, we
+want to avoid every EVP_CIPHER_CTX setup from trying (and failing) to initialise
+each of those 10 ENGINEs. Instead, the first such request will try to do that
+and will either return (and cache) a NULL ENGINE pointer or will return a
+functional reference to the first that successfully initialised. In the latter
+case it will also cache an extra functional reference to the ENGINE as a
+"default" for that 'nid'. The caching is acknowledged by a 'uptodate' variable
+that is unset only if un/registration takes place on that pile. Ie. if
+implementations of "des_cbc" are added or removed. This behaviour can be
+tweaked; the ENGINE_TABLE_FLAG_NOINIT value can be passed to
+ENGINE_set_table_flags(), in which case the only ENGINEs that tb_cipher.c will
+try to initialise from the "pile" will be those that are already initialised
+(ie. it's simply an increment of the functional reference count, and no real
+"initialisation" will take place).
+
+RSA, DSA, DH, and RAND all have their own ENGINE_TABLE code as well, and the
+difference is that they all use an implicit 'nid' of 1. Whereas EVP_CIPHERs are
+actually qualitatively different depending on 'nid' (the "des_cbc" EVP_CIPHER is
+not an interoperable implementation of "aes_256_cbc"), RSA_METHODs are
+necessarily interoperable and don't have different flavours, only different
+implementations. In other words, the ENGINE_TABLE for RSA will either be empty,
+or will have a single ENGING_PILE hashed to by the 'nid' 1 and that pile
+represents ENGINEs that implement the single "type" of RSA there is.
+
+Cleanup - the registration and unregistration may pose questions about how
+cleanup works with the ENGINE_PILE doing all this caching nonsense (ie. when the
+application or EVP_CIPHER code releases its last reference to an ENGINE, the
+ENGINE_PILE code may still have references and thus those ENGINEs will stay
+hooked in forever). The way this is handled is via "unregistration". With these
+new ENGINE changes, an abstract ENGINE can be loaded and initialised, but that
+is an algorithm-agnostic process. Even if initialised, it will not have
+registered any of its implementations (to do so would link all class "table"
+code despite the fact the application may use only ciphers, for example). This
+is deliberately a distinct step. Moreover, registration and unregistration has
+nothing to do with whether an ENGINE is *functional* or not (ie. you can even
+register an ENGINE and its implementations without it being operational, you may
+not even have the drivers to make it operate). What actually happens with
+respect to cleanup is managed inside eng_lib.c with the "engine_cleanup_***"
+functions. These functions are internal-only and each part of ENGINE code that
+could require cleanup will, upon performing its first allocation, register a
+callback with the "engine_cleanup" code. The other part of this that makes it
+tick is that the ENGINE_TABLE instantiations (tb_***.c) use NULL as their
+initialised state. So if RSA code asks for an ENGINE and no ENGINE has
+registered an implementation, the code will simply return NULL and the tb_rsa.c
+state will be unchanged. Thus, no cleanup is required unless registration takes
+place. ENGINE_cleanup() will simply iterate across a list of registered cleanup
+callbacks calling each in turn, and will then internally delete its own storage
+(a STACK). When a cleanup callback is next registered (eg. if the cleanup() is
+part of a gracefull restart and the application wants to cleanup all state then
+start again), the internal STACK storage will be freshly allocated. This is much
+the same as the situation in the ENGINE_TABLE instantiations ... NULL is the
+initialised state, so only modification operations (not queries) will cause that
+code to have to register a cleanup.
+
+What else? The bignum callbacks and associated ENGINE functions have been
+removed for two obvious reasons; (i) there was no way to generalise them to the
+mechanism now used by RSA/DSA/..., because there's no such thing as a BIGNUM
+method, and (ii) because of (i), there was no meaningful way for library or
+application code to automatically hook and use ENGINE supplied bignum functions
+anyway. Also, ENGINE_cpy() has been removed (although an internal-only version
+exists) - the idea of providing an ENGINE_cpy() function probably wasn't a good
+one and now certainly doesn't make sense in any generalised way. Some of the
+RSA, DSA, DH, and RAND functions that were fiddled during the original ENGINE
+changes have now, as a consequence, been reverted back. This is because the
+hooking of ENGINE is now automatic (and passive, it can interally use a NULL
+ENGINE pointer to simply ignore ENGINE from then on).
+
+Hell, that should be enough for now ... comments welcome: geoff@openssl.org