Delete bundled copy of OpenSSL and replace with README.

openssl/crypto/engine/README

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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 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
-