1Developing Cipher Algorithms
2============================
3
4Registering And Unregistering Transformation
5--------------------------------------------
6
7There are three distinct types of registration functions in the Crypto
8API. One is used to register a generic cryptographic transformation,
9while the other two are specific to HASH transformations and
10COMPRESSion. We will discuss the latter two in a separate chapter, here
11we will only look at the generic ones.
12
13Before discussing the register functions, the data structure to be
14filled with each, struct crypto_alg, must be considered -- see below
15for a description of this data structure.
16
17The generic registration functions can be found in
18include/linux/crypto.h and their definition can be seen below. The
19former function registers a single transformation, while the latter
20works on an array of transformation descriptions. The latter is useful
21when registering transformations in bulk, for example when a driver
22implements multiple transformations.
23
24::
25
26       int crypto_register_alg(struct crypto_alg *alg);
27       int crypto_register_algs(struct crypto_alg *algs, int count);
28
29
30The counterparts to those functions are listed below.
31
32::
33
34       void crypto_unregister_alg(struct crypto_alg *alg);
35       void crypto_unregister_algs(struct crypto_alg *algs, int count);
36
37
38The registration functions return 0 on success, or a negative errno
39value on failure.  crypto_register_algs() succeeds only if it
40successfully registered all the given algorithms; if it fails partway
41through, then any changes are rolled back.
42
43The unregistration functions always succeed, so they don't have a
44return value.  Don't try to unregister algorithms that aren't
45currently registered.
46
47Single-Block Symmetric Ciphers [CIPHER]
48---------------------------------------
49
50Example of transformations: aes, serpent, ...
51
52This section describes the simplest of all transformation
53implementations, that being the CIPHER type used for symmetric ciphers.
54The CIPHER type is used for transformations which operate on exactly one
55block at a time and there are no dependencies between blocks at all.
56
57Registration specifics
58~~~~~~~~~~~~~~~~~~~~~~
59
60The registration of [CIPHER] algorithm is specific in that struct
61crypto_alg field .cra_type is empty. The .cra_u.cipher has to be
62filled in with proper callbacks to implement this transformation.
63
64See struct cipher_alg below.
65
66Cipher Definition With struct cipher_alg
67~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
68
69Struct cipher_alg defines a single block cipher.
70
71Here are schematics of how these functions are called when operated from
72other part of the kernel. Note that the .cia_setkey() call might happen
73before or after any of these schematics happen, but must not happen
74during any of these are in-flight.
75
76::
77
78             KEY ---.    PLAINTEXT ---.
79                    v                 v
80              .cia_setkey() -> .cia_encrypt()
81                                      |
82                                      '-----> CIPHERTEXT
83
84
85Please note that a pattern where .cia_setkey() is called multiple times
86is also valid:
87
88::
89
90
91      KEY1 --.    PLAINTEXT1 --.         KEY2 --.    PLAINTEXT2 --.
92             v                 v                v                 v
93       .cia_setkey() -> .cia_encrypt() -> .cia_setkey() -> .cia_encrypt()
94                               |                                  |
95                               '---> CIPHERTEXT1                  '---> CIPHERTEXT2
96
97
98Multi-Block Ciphers
99-------------------
100
101Example of transformations: cbc(aes), chacha20, ...
102
103This section describes the multi-block cipher transformation
104implementations. The multi-block ciphers are used for transformations
105which operate on scatterlists of data supplied to the transformation
106functions. They output the result into a scatterlist of data as well.
107
108Registration Specifics
109~~~~~~~~~~~~~~~~~~~~~~
110
111The registration of multi-block cipher algorithms is one of the most
112standard procedures throughout the crypto API.
113
114Note, if a cipher implementation requires a proper alignment of data,
115the caller should use the functions of crypto_skcipher_alignmask() to
116identify a memory alignment mask. The kernel crypto API is able to
117process requests that are unaligned. This implies, however, additional
118overhead as the kernel crypto API needs to perform the realignment of
119the data which may imply moving of data.
120
121Cipher Definition With struct skcipher_alg
122~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
123
124Struct skcipher_alg defines a multi-block cipher, or more generally, a
125length-preserving symmetric cipher algorithm.
126
127Scatterlist handling
128~~~~~~~~~~~~~~~~~~~~
129
130Some drivers will want to use the Generic ScatterWalk in case the
131hardware needs to be fed separate chunks of the scatterlist which
132contains the plaintext and will contain the ciphertext. Please refer
133to the ScatterWalk interface offered by the Linux kernel scatter /
134gather list implementation.
135
136Hashing [HASH]
137--------------
138
139Example of transformations: crc32, md5, sha1, sha256,...
140
141Registering And Unregistering The Transformation
142~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
143
144There are multiple ways to register a HASH transformation, depending on
145whether the transformation is synchronous [SHASH] or asynchronous
146[AHASH] and the amount of HASH transformations we are registering. You
147can find the prototypes defined in include/crypto/internal/hash.h:
148
149::
150
151       int crypto_register_ahash(struct ahash_alg *alg);
152
153       int crypto_register_shash(struct shash_alg *alg);
154       int crypto_register_shashes(struct shash_alg *algs, int count);
155
156
157The respective counterparts for unregistering the HASH transformation
158are as follows:
159
160::
161
162       void crypto_unregister_ahash(struct ahash_alg *alg);
163
164       void crypto_unregister_shash(struct shash_alg *alg);
165       void crypto_unregister_shashes(struct shash_alg *algs, int count);
166
167
168Cipher Definition With struct shash_alg and ahash_alg
169~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
170
171Here are schematics of how these functions are called when operated from
172other part of the kernel. Note that the .setkey() call might happen
173before or after any of these schematics happen, but must not happen
174during any of these are in-flight. Please note that calling .init()
175followed immediately by .finish() is also a perfectly valid
176transformation.
177
178::
179
180       I)   DATA -----------.
181                            v
182             .init() -> .update() -> .final()      ! .update() might not be called
183                         ^    |         |            at all in this scenario.
184                         '----'         '---> HASH
185
186       II)  DATA -----------.-----------.
187                            v           v
188             .init() -> .update() -> .finup()      ! .update() may not be called
189                         ^    |         |            at all in this scenario.
190                         '----'         '---> HASH
191
192       III) DATA -----------.
193                            v
194                        .digest()                  ! The entire process is handled
195                            |                        by the .digest() call.
196                            '---------------> HASH
197
198
199Here is a schematic of how the .export()/.import() functions are called
200when used from another part of the kernel.
201
202::
203
204       KEY--.                 DATA--.
205            v                       v                  ! .update() may not be called
206        .setkey() -> .init() -> .update() -> .export()   at all in this scenario.
207                                 ^     |         |
208                                 '-----'         '--> PARTIAL_HASH
209
210       ----------- other transformations happen here -----------
211
212       PARTIAL_HASH--.   DATA1--.
213                     v          v
214                 .import -> .update() -> .final()     ! .update() may not be called
215                             ^    |         |           at all in this scenario.
216                             '----'         '--> HASH1
217
218       PARTIAL_HASH--.   DATA2-.
219                     v         v
220                 .import -> .finup()
221                               |
222                               '---------------> HASH2
223
224Note that it is perfectly legal to "abandon" a request object:
225- call .init() and then (as many times) .update()
226- _not_ call any of .final(), .finup() or .export() at any point in future
227
228In other words implementations should mind the resource allocation and clean-up.
229No resources related to request objects should remain allocated after a call
230to .init() or .update(), since there might be no chance to free them.
231
232
233Specifics Of Asynchronous HASH Transformation
234~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
235
236Some of the drivers will want to use the Generic ScatterWalk in case the
237implementation needs to be fed separate chunks of the scatterlist which
238contains the input data. The buffer containing the resulting hash will
239always be properly aligned to .cra_alignmask so there is no need to
240worry about this.
241