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1 | Developing Cipher Algorithms |
2 | ============================ | |
3 | ||
4 | Registering And Unregistering Transformation | |
5 | -------------------------------------------- | |
6 | ||
7 | There are three distinct types of registration functions in the Crypto | |
8 | API. One is used to register a generic cryptographic transformation, | |
9 | while the other two are specific to HASH transformations and | |
10 | COMPRESSion. We will discuss the latter two in a separate chapter, here | |
11 | we will only look at the generic ones. | |
12 | ||
13 | Before discussing the register functions, the data structure to be | |
14 | filled with each, struct crypto_alg, must be considered -- see below | |
15 | for a description of this data structure. | |
16 | ||
17 | The generic registration functions can be found in | |
18 | include/linux/crypto.h and their definition can be seen below. The | |
19 | former function registers a single transformation, while the latter | |
20 | works on an array of transformation descriptions. The latter is useful | |
21 | when registering transformations in bulk, for example when a driver | |
22 | implements 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 | ||
30 | The counterparts to those functions are listed below. | |
31 | ||
32 | :: | |
33 | ||
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34 | void crypto_unregister_alg(struct crypto_alg *alg); |
35 | void crypto_unregister_algs(struct crypto_alg *algs, int count); | |
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36 | |
37 | ||
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38 | The registration functions return 0 on success, or a negative errno |
39 | value on failure. crypto_register_algs() succeeds only if it | |
40 | successfully registered all the given algorithms; if it fails partway | |
41 | through, then any changes are rolled back. | |
3b72c814 | 42 | |
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43 | The unregistration functions always succeed, so they don't have a |
44 | return value. Don't try to unregister algorithms that aren't | |
45 | currently registered. | |
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46 | |
47 | Single-Block Symmetric Ciphers [CIPHER] | |
48 | --------------------------------------- | |
49 | ||
4a2abbc6 | 50 | Example of transformations: aes, serpent, ... |
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51 | |
52 | This section describes the simplest of all transformation | |
53 | implementations, that being the CIPHER type used for symmetric ciphers. | |
54 | The CIPHER type is used for transformations which operate on exactly one | |
55 | block at a time and there are no dependencies between blocks at all. | |
56 | ||
57 | Registration specifics | |
58 | ~~~~~~~~~~~~~~~~~~~~~~ | |
59 | ||
60 | The registration of [CIPHER] algorithm is specific in that struct | |
61 | crypto_alg field .cra_type is empty. The .cra_u.cipher has to be | |
62 | filled in with proper callbacks to implement this transformation. | |
63 | ||
64 | See struct cipher_alg below. | |
65 | ||
66 | Cipher Definition With struct cipher_alg | |
67 | ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ | |
68 | ||
69 | Struct cipher_alg defines a single block cipher. | |
70 | ||
71 | Here are schematics of how these functions are called when operated from | |
72 | other part of the kernel. Note that the .cia_setkey() call might happen | |
73 | before or after any of these schematics happen, but must not happen | |
74 | during 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 | ||
85 | Please note that a pattern where .cia_setkey() is called multiple times | |
86 | is 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 | ||
98 | Multi-Block Ciphers | |
99 | ------------------- | |
100 | ||
4a2abbc6 | 101 | Example of transformations: cbc(aes), chacha20, ... |
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102 | |
103 | This section describes the multi-block cipher transformation | |
104 | implementations. The multi-block ciphers are used for transformations | |
105 | which operate on scatterlists of data supplied to the transformation | |
106 | functions. They output the result into a scatterlist of data as well. | |
107 | ||
108 | Registration Specifics | |
109 | ~~~~~~~~~~~~~~~~~~~~~~ | |
110 | ||
111 | The registration of multi-block cipher algorithms is one of the most | |
112 | standard procedures throughout the crypto API. | |
113 | ||
114 | Note, if a cipher implementation requires a proper alignment of data, | |
115 | the caller should use the functions of crypto_skcipher_alignmask() to | |
116 | identify a memory alignment mask. The kernel crypto API is able to | |
117 | process requests that are unaligned. This implies, however, additional | |
118 | overhead as the kernel crypto API needs to perform the realignment of | |
119 | the data which may imply moving of data. | |
120 | ||
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121 | Cipher Definition With struct skcipher_alg |
122 | ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ | |
3b72c814 | 123 | |
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124 | Struct skcipher_alg defines a multi-block cipher, or more generally, a |
125 | length-preserving symmetric cipher algorithm. | |
3b72c814 | 126 | |
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127 | Scatterlist handling |
128 | ~~~~~~~~~~~~~~~~~~~~ | |
3b72c814 | 129 | |
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130 | Some drivers will want to use the Generic ScatterWalk in case the |
131 | hardware needs to be fed separate chunks of the scatterlist which | |
132 | contains the plaintext and will contain the ciphertext. Please refer | |
133 | to the ScatterWalk interface offered by the Linux kernel scatter / | |
134 | gather list implementation. | |
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135 | |
136 | Hashing [HASH] | |
137 | -------------- | |
138 | ||
139 | Example of transformations: crc32, md5, sha1, sha256,... | |
140 | ||
141 | Registering And Unregistering The Transformation | |
142 | ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ | |
143 | ||
144 | There are multiple ways to register a HASH transformation, depending on | |
145 | whether the transformation is synchronous [SHASH] or asynchronous | |
146 | [AHASH] and the amount of HASH transformations we are registering. You | |
147 | can 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 | ||
157 | The respective counterparts for unregistering the HASH transformation | |
158 | are as follows: | |
159 | ||
160 | :: | |
161 | ||
c6d633a9 | 162 | void crypto_unregister_ahash(struct ahash_alg *alg); |
3b72c814 | 163 | |
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164 | void crypto_unregister_shash(struct shash_alg *alg); |
165 | void crypto_unregister_shashes(struct shash_alg *algs, int count); | |
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166 | |
167 | ||
168 | Cipher Definition With struct shash_alg and ahash_alg | |
169 | ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ | |
170 | ||
171 | Here are schematics of how these functions are called when operated from | |
172 | other part of the kernel. Note that the .setkey() call might happen | |
173 | before or after any of these schematics happen, but must not happen | |
174 | during any of these are in-flight. Please note that calling .init() | |
175 | followed immediately by .finish() is also a perfectly valid | |
176 | transformation. | |
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 | ||
199 | Here is a schematic of how the .export()/.import() functions are called | |
200 | when 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 | ||
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224 | Note 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 | ||
228 | In other words implementations should mind the resource allocation and clean-up. | |
229 | No resources related to request objects should remain allocated after a call | |
230 | to .init() or .update(), since there might be no chance to free them. | |
231 | ||
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232 | |
233 | Specifics Of Asynchronous HASH Transformation | |
234 | ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ | |
235 | ||
236 | Some of the drivers will want to use the Generic ScatterWalk in case the | |
237 | implementation needs to be fed separate chunks of the scatterlist which | |
238 | contains the input data. The buffer containing the resulting hash will | |
239 | always be properly aligned to .cra_alignmask so there is no need to | |
240 | worry about this. |