1Kernel Crypto API Architecture
2==============================
3
4Cipher algorithm types
5----------------------
6
7The kernel crypto API provides different API calls for the following
8cipher types:
9
10-  Symmetric ciphers
11
12-  AEAD ciphers
13
14-  Message digest, including keyed message digest
15
16-  Random number generation
17
18-  User space interface
19
20Ciphers And Templates
21---------------------
22
23The kernel crypto API provides implementations of single block ciphers
24and message digests. In addition, the kernel crypto API provides
25numerous "templates" that can be used in conjunction with the single
26block ciphers and message digests. Templates include all types of block
27chaining mode, the HMAC mechanism, etc.
28
29Single block ciphers and message digests can either be directly used by
30a caller or invoked together with a template to form multi-block ciphers
31or keyed message digests.
32
33A single block cipher may even be called with multiple templates.
34However, templates cannot be used without a single cipher.
35
36See /proc/crypto and search for "name". For example:
37
38-  aes
39
40-  ecb(aes)
41
42-  cmac(aes)
43
44-  ccm(aes)
45
46-  rfc4106(gcm(aes))
47
48-  sha1
49
50-  hmac(sha1)
51
52-  authenc(hmac(sha1),cbc(aes))
53
54In these examples, "aes" and "sha1" are the ciphers and all others are
55the templates.
56
57Synchronous And Asynchronous Operation
58--------------------------------------
59
60The kernel crypto API provides synchronous and asynchronous API
61operations.
62
63When using the synchronous API operation, the caller invokes a cipher
64operation which is performed synchronously by the kernel crypto API.
65That means, the caller waits until the cipher operation completes.
66Therefore, the kernel crypto API calls work like regular function calls.
67For synchronous operation, the set of API calls is small and
68conceptually similar to any other crypto library.
69
70Asynchronous operation is provided by the kernel crypto API which
71implies that the invocation of a cipher operation will complete almost
72instantly. That invocation triggers the cipher operation but it does not
73signal its completion. Before invoking a cipher operation, the caller
74must provide a callback function the kernel crypto API can invoke to
75signal the completion of the cipher operation. Furthermore, the caller
76must ensure it can handle such asynchronous events by applying
77appropriate locking around its data. The kernel crypto API does not
78perform any special serialization operation to protect the caller's data
79integrity.
80
81Crypto API Cipher References And Priority
82-----------------------------------------
83
84A cipher is referenced by the caller with a string. That string has the
85following semantics:
86
87::
88
89        template(single block cipher)
90
91
92where "template" and "single block cipher" is the aforementioned
93template and single block cipher, respectively. If applicable,
94additional templates may enclose other templates, such as
95
96::
97
98        template1(template2(single block cipher)))
99
100
101The kernel crypto API may provide multiple implementations of a template
102or a single block cipher. For example, AES on newer Intel hardware has
103the following implementations: AES-NI, assembler implementation, or
104straight C. Now, when using the string "aes" with the kernel crypto API,
105which cipher implementation is used? The answer to that question is the
106priority number assigned to each cipher implementation by the kernel
107crypto API. When a caller uses the string to refer to a cipher during
108initialization of a cipher handle, the kernel crypto API looks up all
109implementations providing an implementation with that name and selects
110the implementation with the highest priority.
111
112Now, a caller may have the need to refer to a specific cipher
113implementation and thus does not want to rely on the priority-based
114selection. To accommodate this scenario, the kernel crypto API allows
115the cipher implementation to register a unique name in addition to
116common names. When using that unique name, a caller is therefore always
117sure to refer to the intended cipher implementation.
118
119The list of available ciphers is given in /proc/crypto. However, that
120list does not specify all possible permutations of templates and
121ciphers. Each block listed in /proc/crypto may contain the following
122information -- if one of the components listed as follows are not
123applicable to a cipher, it is not displayed:
124
125-  name: the generic name of the cipher that is subject to the
126   priority-based selection -- this name can be used by the cipher
127   allocation API calls (all names listed above are examples for such
128   generic names)
129
130-  driver: the unique name of the cipher -- this name can be used by the
131   cipher allocation API calls
132
133-  module: the kernel module providing the cipher implementation (or
134   "kernel" for statically linked ciphers)
135
136-  priority: the priority value of the cipher implementation
137
138-  refcnt: the reference count of the respective cipher (i.e. the number
139   of current consumers of this cipher)
140
141-  selftest: specification whether the self test for the cipher passed
142
143-  type:
144
145   -  skcipher for symmetric key ciphers
146
147   -  cipher for single block ciphers that may be used with an
148      additional template
149
150   -  shash for synchronous message digest
151
152   -  ahash for asynchronous message digest
153
154   -  aead for AEAD cipher type
155
156   -  compression for compression type transformations
157
158   -  rng for random number generator
159
160   -  kpp for a Key-agreement Protocol Primitive (KPP) cipher such as
161      an ECDH or DH implementation
162
163-  blocksize: blocksize of cipher in bytes
164
165-  keysize: key size in bytes
166
167-  ivsize: IV size in bytes
168
169-  seedsize: required size of seed data for random number generator
170
171-  digestsize: output size of the message digest
172
173-  geniv: IV generator (obsolete)
174
175Key Sizes
176---------
177
178When allocating a cipher handle, the caller only specifies the cipher
179type. Symmetric ciphers, however, typically support multiple key sizes
180(e.g. AES-128 vs. AES-192 vs. AES-256). These key sizes are determined
181with the length of the provided key. Thus, the kernel crypto API does
182not provide a separate way to select the particular symmetric cipher key
183size.
184
185Cipher Allocation Type And Masks
186--------------------------------
187
188The different cipher handle allocation functions allow the specification
189of a type and mask flag. Both parameters have the following meaning (and
190are therefore not covered in the subsequent sections).
191
192The type flag specifies the type of the cipher algorithm. The caller
193usually provides a 0 when the caller wants the default handling.
194Otherwise, the caller may provide the following selections which match
195the aforementioned cipher types:
196
197-  CRYPTO_ALG_TYPE_CIPHER Single block cipher
198
199-  CRYPTO_ALG_TYPE_COMPRESS Compression
200
201-  CRYPTO_ALG_TYPE_AEAD Authenticated Encryption with Associated Data
202   (MAC)
203
204-  CRYPTO_ALG_TYPE_BLKCIPHER Synchronous multi-block cipher
205
206-  CRYPTO_ALG_TYPE_ABLKCIPHER Asynchronous multi-block cipher
207
208-  CRYPTO_ALG_TYPE_KPP Key-agreement Protocol Primitive (KPP) such as
209   an ECDH or DH implementation
210
211-  CRYPTO_ALG_TYPE_DIGEST Raw message digest
212
213-  CRYPTO_ALG_TYPE_HASH Alias for CRYPTO_ALG_TYPE_DIGEST
214
215-  CRYPTO_ALG_TYPE_SHASH Synchronous multi-block hash
216
217-  CRYPTO_ALG_TYPE_AHASH Asynchronous multi-block hash
218
219-  CRYPTO_ALG_TYPE_RNG Random Number Generation
220
221-  CRYPTO_ALG_TYPE_AKCIPHER Asymmetric cipher
222
223-  CRYPTO_ALG_TYPE_PCOMPRESS Enhanced version of
224   CRYPTO_ALG_TYPE_COMPRESS allowing for segmented compression /
225   decompression instead of performing the operation on one segment
226   only. CRYPTO_ALG_TYPE_PCOMPRESS is intended to replace
227   CRYPTO_ALG_TYPE_COMPRESS once existing consumers are converted.
228
229The mask flag restricts the type of cipher. The only allowed flag is
230CRYPTO_ALG_ASYNC to restrict the cipher lookup function to
231asynchronous ciphers. Usually, a caller provides a 0 for the mask flag.
232
233When the caller provides a mask and type specification, the caller
234limits the search the kernel crypto API can perform for a suitable
235cipher implementation for the given cipher name. That means, even when a
236caller uses a cipher name that exists during its initialization call,
237the kernel crypto API may not select it due to the used type and mask
238field.
239
240Internal Structure of Kernel Crypto API
241---------------------------------------
242
243The kernel crypto API has an internal structure where a cipher
244implementation may use many layers and indirections. This section shall
245help to clarify how the kernel crypto API uses various components to
246implement the complete cipher.
247
248The following subsections explain the internal structure based on
249existing cipher implementations. The first section addresses the most
250complex scenario where all other scenarios form a logical subset.
251
252Generic AEAD Cipher Structure
253~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
254
255The following ASCII art decomposes the kernel crypto API layers when
256using the AEAD cipher with the automated IV generation. The shown
257example is used by the IPSEC layer.
258
259For other use cases of AEAD ciphers, the ASCII art applies as well, but
260the caller may not use the AEAD cipher with a separate IV generator. In
261this case, the caller must generate the IV.
262
263The depicted example decomposes the AEAD cipher of GCM(AES) based on the
264generic C implementations (gcm.c, aes-generic.c, ctr.c, ghash-generic.c,
265seqiv.c). The generic implementation serves as an example showing the
266complete logic of the kernel crypto API.
267
268It is possible that some streamlined cipher implementations (like
269AES-NI) provide implementations merging aspects which in the view of the
270kernel crypto API cannot be decomposed into layers any more. In case of
271the AES-NI implementation, the CTR mode, the GHASH implementation and
272the AES cipher are all merged into one cipher implementation registered
273with the kernel crypto API. In this case, the concept described by the
274following ASCII art applies too. However, the decomposition of GCM into
275the individual sub-components by the kernel crypto API is not done any
276more.
277
278Each block in the following ASCII art is an independent cipher instance
279obtained from the kernel crypto API. Each block is accessed by the
280caller or by other blocks using the API functions defined by the kernel
281crypto API for the cipher implementation type.
282
283The blocks below indicate the cipher type as well as the specific logic
284implemented in the cipher.
285
286The ASCII art picture also indicates the call structure, i.e. who calls
287which component. The arrows point to the invoked block where the caller
288uses the API applicable to the cipher type specified for the block.
289
290::
291
292
293    kernel crypto API                                |   IPSEC Layer
294                                                     |
295    +-----------+                                    |
296    |           |            (1)
297    |   aead    | <-----------------------------------  esp_output
298    |  (seqiv)  | ---+
299    +-----------+    |
300                     | (2)
301    +-----------+    |
302    |           | <--+                (2)
303    |   aead    | <-----------------------------------  esp_input
304    |   (gcm)   | ------------+
305    +-----------+             |
306          | (3)               | (5)
307          v                   v
308    +-----------+       +-----------+
309    |           |       |           |
310    |  skcipher |       |   ahash   |
311    |   (ctr)   | ---+  |  (ghash)  |
312    +-----------+    |  +-----------+
313                     |
314    +-----------+    | (4)
315    |           | <--+
316    |   cipher  |
317    |   (aes)   |
318    +-----------+
319
320
321
322The following call sequence is applicable when the IPSEC layer triggers
323an encryption operation with the esp_output function. During
324configuration, the administrator set up the use of seqiv(rfc4106(gcm(aes)))
325as the cipher for ESP. The following call sequence is now depicted in
326the ASCII art above:
327
3281. esp_output() invokes crypto_aead_encrypt() to trigger an
329   encryption operation of the AEAD cipher with IV generator.
330
331   The SEQIV generates the IV.
332
3332. Now, SEQIV uses the AEAD API function calls to invoke the associated
334   AEAD cipher. In our case, during the instantiation of SEQIV, the
335   cipher handle for GCM is provided to SEQIV. This means that SEQIV
336   invokes AEAD cipher operations with the GCM cipher handle.
337
338   During instantiation of the GCM handle, the CTR(AES) and GHASH
339   ciphers are instantiated. The cipher handles for CTR(AES) and GHASH
340   are retained for later use.
341
342   The GCM implementation is responsible to invoke the CTR mode AES and
343   the GHASH cipher in the right manner to implement the GCM
344   specification.
345
3463. The GCM AEAD cipher type implementation now invokes the SKCIPHER API
347   with the instantiated CTR(AES) cipher handle.
348
349   During instantiation of the CTR(AES) cipher, the CIPHER type
350   implementation of AES is instantiated. The cipher handle for AES is
351   retained.
352
353   That means that the SKCIPHER implementation of CTR(AES) only
354   implements the CTR block chaining mode. After performing the block
355   chaining operation, the CIPHER implementation of AES is invoked.
356
3574. The SKCIPHER of CTR(AES) now invokes the CIPHER API with the AES
358   cipher handle to encrypt one block.
359
3605. The GCM AEAD implementation also invokes the GHASH cipher
361   implementation via the AHASH API.
362
363When the IPSEC layer triggers the esp_input() function, the same call
364sequence is followed with the only difference that the operation starts
365with step (2).
366
367Generic Block Cipher Structure
368~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
369
370Generic block ciphers follow the same concept as depicted with the ASCII
371art picture above.
372
373For example, CBC(AES) is implemented with cbc.c, and aes-generic.c. The
374ASCII art picture above applies as well with the difference that only
375step (4) is used and the SKCIPHER block chaining mode is CBC.
376
377Generic Keyed Message Digest Structure
378~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
379
380Keyed message digest implementations again follow the same concept as
381depicted in the ASCII art picture above.
382
383For example, HMAC(SHA256) is implemented with hmac.c and
384sha256_generic.c. The following ASCII art illustrates the
385implementation:
386
387::
388
389
390    kernel crypto API            |       Caller
391                                 |
392    +-----------+         (1)    |
393    |           | <------------------  some_function
394    |   ahash   |
395    |   (hmac)  | ---+
396    +-----------+    |
397                     | (2)
398    +-----------+    |
399    |           | <--+
400    |   shash   |
401    |  (sha256) |
402    +-----------+
403
404
405
406The following call sequence is applicable when a caller triggers an HMAC
407operation:
408
4091. The AHASH API functions are invoked by the caller. The HMAC
410   implementation performs its operation as needed.
411
412   During initialization of the HMAC cipher, the SHASH cipher type of
413   SHA256 is instantiated. The cipher handle for the SHA256 instance is
414   retained.
415
416   At one time, the HMAC implementation requires a SHA256 operation
417   where the SHA256 cipher handle is used.
418
4192. The HMAC instance now invokes the SHASH API with the SHA256 cipher
420   handle to calculate the message digest.
421