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+Notes: 2001-09-24
+-----------------
+
+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 independent 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
+