1 Notes: 2001-09-24 2 ----------------- 3 4 This "description" (if one chooses to call it that) needed some major updating 5 so here goes. This update addresses a change being made at the same time to 6 OpenSSL, and it pretty much completely restructures the underlying mechanics of 7 the "ENGINE" code. So it serves a double purpose of being a "ENGINE internals 8 for masochists" document *and* a rather extensive commit log message. (I'd get 9 lynched for sticking all this in CHANGES or the commit mails :-). 10 11 ENGINE_TABLE underlies this restructuring, as described in the internal header 12 "eng_int.h", implemented in eng_table.c, and used in each of the "class" files; 13 tb_rsa.c, tb_dsa.c, etc. 14 15 However, "EVP_CIPHER" underlies the motivation and design of ENGINE_TABLE so 16 I'll mention a bit about that first. EVP_CIPHER (and most of this applies 17 equally to EVP_MD for digests) is both a "method" and a algorithm/mode 18 identifier that, in the current API, "lingers". These cipher description + 19 implementation structures can be defined or obtained directly by applications, 20 or can be loaded "en masse" into EVP storage so that they can be catalogued and 21 searched in various ways, ie. two ways of encrypting with the "des_cbc" 22 algorithm/mode pair are; 23 24 (i) directly; 25 const EVP_CIPHER *cipher = EVP_des_cbc(); 26 EVP_EncryptInit(&ctx, cipher, key, iv); 27 [ ... use EVP_EncryptUpdate() and EVP_EncryptFinal() ...] 28 29 (ii) indirectly; 30 OpenSSL_add_all_ciphers(); 31 cipher = EVP_get_cipherbyname("des_cbc"); 32 EVP_EncryptInit(&ctx, cipher, key, iv); 33 [ ... etc ... ] 34 35 The latter is more generally used because it also allows ciphers/digests to be 36 looked up based on other identifiers which can be useful for automatic cipher 37 selection, eg. in SSL/TLS, or by user-controllable configuration. 38 39 The important point about this is that EVP_CIPHER definitions and structures are 40 passed around with impunity and there is no safe way, without requiring massive 41 rewrites of many applications, to assume that EVP_CIPHERs can be reference 42 counted. One an EVP_CIPHER is exposed to the caller, neither it nor anything it 43 comes from can "safely" be destroyed. Unless of course the way of getting to 44 such ciphers is via entirely distinct API calls that didn't exist before. 45 However existing API usage cannot be made to understand when an EVP_CIPHER 46 pointer, that has been passed to the caller, is no longer being used. 47 48 The other problem with the existing API w.r.t. to hooking EVP_CIPHER support 49 into ENGINE is storage - the OBJ_NAME-based storage used by EVP to register 50 ciphers simultaneously registers cipher *types* and cipher *implementations* - 51 they are effectively the same thing, an "EVP_CIPHER" pointer. The problem with 52 hooking in ENGINEs is that multiple ENGINEs may implement the same ciphers. The 53 solution is necessarily that ENGINE-provided ciphers simply are not registered, 54 stored, or exposed to the caller in the same manner as existing ciphers. This is 55 especially necessary considering the fact ENGINE uses reference counts to allow 56 for cleanup, modularity, and DSO support - yet EVP_CIPHERs, as exposed to 57 callers in the current API, support no such controls. 58 59 Another sticking point for integrating cipher support into ENGINE is linkage. 60 Already there is a problem with the way ENGINE supports RSA, DSA, etc whereby 61 they are available *because* they're part of a giant ENGINE called "openssl". 62 Ie. all implementations *have* to come from an ENGINE, but we get round that by 63 having a giant ENGINE with all the software support encapsulated. This creates 64 linker hassles if nothing else - linking a 1-line application that calls 2 basic 65 RSA functions (eg. "RSA_free(RSA_new());") will result in large quantities of 66 ENGINE code being linked in *and* because of that DSA, DH, and RAND also. If we 67 continue with this approach for EVP_CIPHER support (even if it *was* possible) 68 we would lose our ability to link selectively by selectively loading certain 69 implementations of certain functionality. Touching any part of any kind of 70 crypto would result in massive static linkage of everything else. So the 71 solution is to change the way ENGINE feeds existing "classes", ie. how the 72 hooking to ENGINE works from RSA, DSA, DH, RAND, as well as adding new hooking 73 for EVP_CIPHER, and EVP_MD. 74 75 The way this is now being done is by mostly reverting back to how things used to 76 work prior to ENGINE :-). Ie. RSA now has a "RSA_METHOD" pointer again - this 77 was previously replaced by an "ENGINE" pointer and all RSA code that required 78 the RSA_METHOD would call ENGINE_get_RSA() each time on its ENGINE handle to 79 temporarily get and use the ENGINE's RSA implementation. Apart from being more 80 efficient, switching back to each RSA having an RSA_METHOD pointer also allows 81 us to conceivably operate with *no* ENGINE. As we'll see, this removes any need 82 for a fallback ENGINE that encapsulates default implementations - we can simply 83 have our RSA structure pointing its RSA_METHOD pointer to the software 84 implementation and have its ENGINE pointer set to NULL. 85 86 A look at the EVP_CIPHER hooking is most explanatory, the RSA, DSA (etc) cases 87 turn out to be degenerate forms of the same thing. The EVP storage of ciphers, 88 and the existing EVP API functions that return "software" implementations and 89 descriptions remain untouched. However, the storage takes more meaning in terms 90 of "cipher description" and less meaning in terms of "implementation". When an 91 EVP_CIPHER_CTX is actually initialised with an EVP_CIPHER method and is about to 92 begin en/decryption, the hooking to ENGINE comes into play. What happens is that 93 cipher-specific ENGINE code is asked for an ENGINE pointer (a functional 94 reference) for any ENGINE that is registered to perform the algo/mode that the 95 provided EVP_CIPHER structure represents. Under normal circumstances, that 96 ENGINE code will return NULL because no ENGINEs will have had any cipher 97 implementations *registered*. As such, a NULL ENGINE pointer is stored in the 98 EVP_CIPHER_CTX context, and the EVP_CIPHER structure is left hooked into the 99 context and so is used as the implementation. Pretty much how things work now 100 except we'd have a redundant ENGINE pointer set to NULL and doing nothing. 101 102 Conversely, if an ENGINE *has* been registered to perform the algorithm/mode 103 combination represented by the provided EVP_CIPHER, then a functional reference 104 to that ENGINE will be returned to the EVP_CIPHER_CTX during initialisation. 105 That functional reference will be stored in the context (and released on 106 cleanup) - and having that reference provides a *safe* way to use an EVP_CIPHER 107 definition that is private to the ENGINE. Ie. the EVP_CIPHER provided by the 108 application will actually be replaced by an EVP_CIPHER from the registered 109 ENGINE - it will support the same algorithm/mode as the original but will be a 110 completely different implementation. Because this EVP_CIPHER isn't stored in the 111 EVP storage, nor is it returned to applications from traditional API functions, 112 there is no associated problem with it not having reference counts. And of 113 course, when one of these "private" cipher implementations is hooked into 114 EVP_CIPHER_CTX, it is done whilst the EVP_CIPHER_CTX holds a functional 115 reference to the ENGINE that owns it, thus the use of the ENGINE's EVP_CIPHER is 116 safe. 117 118 The "cipher-specific ENGINE code" I mentioned is implemented in tb_cipher.c but 119 in essence it is simply an instantiation of "ENGINE_TABLE" code for use by 120 EVP_CIPHER code. tb_digest.c is virtually identical but, of course, it is for 121 use by EVP_MD code. Ditto for tb_rsa.c, tb_dsa.c, etc. These instantiations of 122 ENGINE_TABLE essentially provide linker-separation of the classes so that even 123 if ENGINEs implement *all* possible algorithms, an application using only 124 EVP_CIPHER code will link at most code relating to EVP_CIPHER, tb_cipher.c, core 125 ENGINE code that is independant of class, and of course the ENGINE 126 implementation that the application loaded. It will *not* however link any 127 class-specific ENGINE code for digests, RSA, etc nor will it bleed over into 128 other APIs, such as the RSA/DSA/etc library code. 129 130 ENGINE_TABLE is a little more complicated than may seem necessary but this is 131 mostly to avoid a lot of "init()"-thrashing on ENGINEs (that may have to load 132 DSOs, and other expensive setup that shouldn't be thrashed unnecessarily) *and* 133 to duplicate "default" behaviour. Basically an ENGINE_TABLE instantiation, for 134 example tb_cipher.c, implements a hash-table keyed by integer "nid" values. 135 These nids provide the uniquenness of an algorithm/mode - and each nid will hash 136 to a potentially NULL "ENGINE_PILE". An ENGINE_PILE is essentially a list of 137 pointers to ENGINEs that implement that particular 'nid'. Each "pile" uses some 138 caching tricks such that requests on that 'nid' will be cached and all future 139 requests will return immediately (well, at least with minimal operation) unless 140 a change is made to the pile, eg. perhaps an ENGINE was unloaded. The reason is 141 that an application could have support for 10 ENGINEs statically linked 142 in, and the machine in question may not have any of the hardware those 10 143 ENGINEs support. If each of those ENGINEs has a "des_cbc" implementation, we 144 want to avoid every EVP_CIPHER_CTX setup from trying (and failing) to initialise 145 each of those 10 ENGINEs. Instead, the first such request will try to do that 146 and will either return (and cache) a NULL ENGINE pointer or will return a 147 functional reference to the first that successfully initialised. In the latter 148 case it will also cache an extra functional reference to the ENGINE as a 149 "default" for that 'nid'. The caching is acknowledged by a 'uptodate' variable 150 that is unset only if un/registration takes place on that pile. Ie. if 151 implementations of "des_cbc" are added or removed. This behaviour can be 152 tweaked; the ENGINE_TABLE_FLAG_NOINIT value can be passed to 153 ENGINE_set_table_flags(), in which case the only ENGINEs that tb_cipher.c will 154 try to initialise from the "pile" will be those that are already initialised 155 (ie. it's simply an increment of the functional reference count, and no real 156 "initialisation" will take place). 157 158 RSA, DSA, DH, and RAND all have their own ENGINE_TABLE code as well, and the 159 difference is that they all use an implicit 'nid' of 1. Whereas EVP_CIPHERs are 160 actually qualitatively different depending on 'nid' (the "des_cbc" EVP_CIPHER is 161 not an interoperable implementation of "aes_256_cbc"), RSA_METHODs are 162 necessarily interoperable and don't have different flavours, only different 163 implementations. In other words, the ENGINE_TABLE for RSA will either be empty, 164 or will have a single ENGING_PILE hashed to by the 'nid' 1 and that pile 165 represents ENGINEs that implement the single "type" of RSA there is. 166 167 Cleanup - the registration and unregistration may pose questions about how 168 cleanup works with the ENGINE_PILE doing all this caching nonsense (ie. when the 169 application or EVP_CIPHER code releases its last reference to an ENGINE, the 170 ENGINE_PILE code may still have references and thus those ENGINEs will stay 171 hooked in forever). The way this is handled is via "unregistration". With these 172 new ENGINE changes, an abstract ENGINE can be loaded and initialised, but that 173 is an algorithm-agnostic process. Even if initialised, it will not have 174 registered any of its implementations (to do so would link all class "table" 175 code despite the fact the application may use only ciphers, for example). This 176 is deliberately a distinct step. Moreover, registration and unregistration has 177 nothing to do with whether an ENGINE is *functional* or not (ie. you can even 178 register an ENGINE and its implementations without it being operational, you may 179 not even have the drivers to make it operate). What actually happens with 180 respect to cleanup is managed inside eng_lib.c with the "engine_cleanup_***" 181 functions. These functions are internal-only and each part of ENGINE code that 182 could require cleanup will, upon performing its first allocation, register a 183 callback with the "engine_cleanup" code. The other part of this that makes it 184 tick is that the ENGINE_TABLE instantiations (tb_***.c) use NULL as their 185 initialised state. So if RSA code asks for an ENGINE and no ENGINE has 186 registered an implementation, the code will simply return NULL and the tb_rsa.c 187 state will be unchanged. Thus, no cleanup is required unless registration takes 188 place. ENGINE_cleanup() will simply iterate across a list of registered cleanup 189 callbacks calling each in turn, and will then internally delete its own storage 190 (a STACK). When a cleanup callback is next registered (eg. if the cleanup() is 191 part of a gracefull restart and the application wants to cleanup all state then 192 start again), the internal STACK storage will be freshly allocated. This is much 193 the same as the situation in the ENGINE_TABLE instantiations ... NULL is the 194 initialised state, so only modification operations (not queries) will cause that 195 code to have to register a cleanup. 196 197 What else? The bignum callbacks and associated ENGINE functions have been 198 removed for two obvious reasons; (i) there was no way to generalise them to the 199 mechanism now used by RSA/DSA/..., because there's no such thing as a BIGNUM 200 method, and (ii) because of (i), there was no meaningful way for library or 201 application code to automatically hook and use ENGINE supplied bignum functions 202 anyway. Also, ENGINE_cpy() has been removed (although an internal-only version 203 exists) - the idea of providing an ENGINE_cpy() function probably wasn't a good 204 one and now certainly doesn't make sense in any generalised way. Some of the 205 RSA, DSA, DH, and RAND functions that were fiddled during the original ENGINE 206 changes have now, as a consequence, been reverted back. This is because the 207 hooking of ENGINE is now automatic (and passive, it can interally use a NULL 208 ENGINE pointer to simply ignore ENGINE from then on). 209 210 Hell, that should be enough for now ... comments welcome: geoff (a] openssl.org 211 212
