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1 Notes: 2001-09-24
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 :-).
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.
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;
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() ...]
29 (ii) indirectly;
31 cipher = EVP_get_cipherbyname("des_cbc");
32 EVP_EncryptInit(&ctx, cipher, key, iv);
33 [ ... etc ... ]
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.
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.
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.
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.
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.
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.
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
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 independent 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.
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).
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 ENGINE_PILE hashed to by the 'nid' 1 and that pile
165 represents ENGINEs that implement the single "type" of RSA there is.
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 graceful 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.
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 internally use a NULL
208 ENGINE pointer to simply ignore ENGINE from then on).
210 Hell, that should be enough for now ... comments welcome.