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   -  givcipher for cipher with associated IV generator (see the geniv
161      entry below for the specification of the IV generator type used by
162      the cipher implementation)
163
164   -  kpp for a Key-agreement Protocol Primitive (KPP) cipher such as
165      an ECDH or DH implementation
166
167-  blocksize: blocksize of cipher in bytes
168
169-  keysize: key size in bytes
170
171-  ivsize: IV size in bytes
172
173-  seedsize: required size of seed data for random number generator
174
175-  digestsize: output size of the message digest
176
177-  geniv: IV generation type:
178
179   -  eseqiv for encrypted sequence number based IV generation
180
181   -  seqiv for sequence number based IV generation
182
183   -  chainiv for chain iv generation
184
185   -  <builtin> is a marker that the cipher implements IV generation and
186      handling as it is specific to the given cipher
187
188Key Sizes
189---------
190
191When allocating a cipher handle, the caller only specifies the cipher
192type. Symmetric ciphers, however, typically support multiple key sizes
193(e.g. AES-128 vs. AES-192 vs. AES-256). These key sizes are determined
194with the length of the provided key. Thus, the kernel crypto API does
195not provide a separate way to select the particular symmetric cipher key
196size.
197
198Cipher Allocation Type And Masks
199--------------------------------
200
201The different cipher handle allocation functions allow the specification
202of a type and mask flag. Both parameters have the following meaning (and
203are therefore not covered in the subsequent sections).
204
205The type flag specifies the type of the cipher algorithm. The caller
206usually provides a 0 when the caller wants the default handling.
207Otherwise, the caller may provide the following selections which match
208the aforementioned cipher types:
209
210-  CRYPTO_ALG_TYPE_CIPHER Single block cipher
211
212-  CRYPTO_ALG_TYPE_COMPRESS Compression
213
214-  CRYPTO_ALG_TYPE_AEAD Authenticated Encryption with Associated Data
215   (MAC)
216
217-  CRYPTO_ALG_TYPE_BLKCIPHER Synchronous multi-block cipher
218
219-  CRYPTO_ALG_TYPE_ABLKCIPHER Asynchronous multi-block cipher
220
221-  CRYPTO_ALG_TYPE_GIVCIPHER Asynchronous multi-block cipher packed
222   together with an IV generator (see geniv field in the /proc/crypto
223   listing for the known IV generators)
224
225-  CRYPTO_ALG_TYPE_KPP Key-agreement Protocol Primitive (KPP) such as
226   an ECDH or DH implementation
227
228-  CRYPTO_ALG_TYPE_DIGEST Raw message digest
229
230-  CRYPTO_ALG_TYPE_HASH Alias for CRYPTO_ALG_TYPE_DIGEST
231
232-  CRYPTO_ALG_TYPE_SHASH Synchronous multi-block hash
233
234-  CRYPTO_ALG_TYPE_AHASH Asynchronous multi-block hash
235
236-  CRYPTO_ALG_TYPE_RNG Random Number Generation
237
238-  CRYPTO_ALG_TYPE_AKCIPHER Asymmetric cipher
239
240-  CRYPTO_ALG_TYPE_PCOMPRESS Enhanced version of
241   CRYPTO_ALG_TYPE_COMPRESS allowing for segmented compression /
242   decompression instead of performing the operation on one segment
243   only. CRYPTO_ALG_TYPE_PCOMPRESS is intended to replace
244   CRYPTO_ALG_TYPE_COMPRESS once existing consumers are converted.
245
246The mask flag restricts the type of cipher. The only allowed flag is
247CRYPTO_ALG_ASYNC to restrict the cipher lookup function to
248asynchronous ciphers. Usually, a caller provides a 0 for the mask flag.
249
250When the caller provides a mask and type specification, the caller
251limits the search the kernel crypto API can perform for a suitable
252cipher implementation for the given cipher name. That means, even when a
253caller uses a cipher name that exists during its initialization call,
254the kernel crypto API may not select it due to the used type and mask
255field.
256
257Internal Structure of Kernel Crypto API
258---------------------------------------
259
260The kernel crypto API has an internal structure where a cipher
261implementation may use many layers and indirections. This section shall
262help to clarify how the kernel crypto API uses various components to
263implement the complete cipher.
264
265The following subsections explain the internal structure based on
266existing cipher implementations. The first section addresses the most
267complex scenario where all other scenarios form a logical subset.
268
269Generic AEAD Cipher Structure
270~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
271
272The following ASCII art decomposes the kernel crypto API layers when
273using the AEAD cipher with the automated IV generation. The shown
274example is used by the IPSEC layer.
275
276For other use cases of AEAD ciphers, the ASCII art applies as well, but
277the caller may not use the AEAD cipher with a separate IV generator. In
278this case, the caller must generate the IV.
279
280The depicted example decomposes the AEAD cipher of GCM(AES) based on the
281generic C implementations (gcm.c, aes-generic.c, ctr.c, ghash-generic.c,
282seqiv.c). The generic implementation serves as an example showing the
283complete logic of the kernel crypto API.
284
285It is possible that some streamlined cipher implementations (like
286AES-NI) provide implementations merging aspects which in the view of the
287kernel crypto API cannot be decomposed into layers any more. In case of
288the AES-NI implementation, the CTR mode, the GHASH implementation and
289the AES cipher are all merged into one cipher implementation registered
290with the kernel crypto API. In this case, the concept described by the
291following ASCII art applies too. However, the decomposition of GCM into
292the individual sub-components by the kernel crypto API is not done any
293more.
294
295Each block in the following ASCII art is an independent cipher instance
296obtained from the kernel crypto API. Each block is accessed by the
297caller or by other blocks using the API functions defined by the kernel
298crypto API for the cipher implementation type.
299
300The blocks below indicate the cipher type as well as the specific logic
301implemented in the cipher.
302
303The ASCII art picture also indicates the call structure, i.e. who calls
304which component. The arrows point to the invoked block where the caller
305uses the API applicable to the cipher type specified for the block.
306
307::
308
309
310    kernel crypto API                                |   IPSEC Layer
311                                                     |
312    +-----------+                                    |
313    |           |            (1)
314    |   aead    | <-----------------------------------  esp_output
315    |  (seqiv)  | ---+
316    +-----------+    |
317                     | (2)
318    +-----------+    |
319    |           | <--+                (2)
320    |   aead    | <-----------------------------------  esp_input
321    |   (gcm)   | ------------+
322    +-----------+             |
323          | (3)               | (5)
324          v                   v
325    +-----------+       +-----------+
326    |           |       |           |
327    |  skcipher |       |   ahash   |
328    |   (ctr)   | ---+  |  (ghash)  |
329    +-----------+    |  +-----------+
330                     |
331    +-----------+    | (4)
332    |           | <--+
333    |   cipher  |
334    |   (aes)   |
335    +-----------+
336
337
338
339The following call sequence is applicable when the IPSEC layer triggers
340an encryption operation with the esp_output function. During
341configuration, the administrator set up the use of rfc4106(gcm(aes)) as
342the cipher for ESP. The following call sequence is now depicted in the
343ASCII art above:
344
3451. esp_output() invokes crypto_aead_encrypt() to trigger an
346   encryption operation of the AEAD cipher with IV generator.
347
348   In case of GCM, the SEQIV implementation is registered as GIVCIPHER
349   in crypto_rfc4106_alloc().
350
351   The SEQIV performs its operation to generate an IV where the core
352   function is seqiv_geniv().
353
3542. Now, SEQIV uses the AEAD API function calls to invoke the associated
355   AEAD cipher. In our case, during the instantiation of SEQIV, the
356   cipher handle for GCM is provided to SEQIV. This means that SEQIV
357   invokes AEAD cipher operations with the GCM cipher handle.
358
359   During instantiation of the GCM handle, the CTR(AES) and GHASH
360   ciphers are instantiated. The cipher handles for CTR(AES) and GHASH
361   are retained for later use.
362
363   The GCM implementation is responsible to invoke the CTR mode AES and
364   the GHASH cipher in the right manner to implement the GCM
365   specification.
366
3673. The GCM AEAD cipher type implementation now invokes the SKCIPHER API
368   with the instantiated CTR(AES) cipher handle.
369
370   During instantiation of the CTR(AES) cipher, the CIPHER type
371   implementation of AES is instantiated. The cipher handle for AES is
372   retained.
373
374   That means that the SKCIPHER implementation of CTR(AES) only
375   implements the CTR block chaining mode. After performing the block
376   chaining operation, the CIPHER implementation of AES is invoked.
377
3784. The SKCIPHER of CTR(AES) now invokes the CIPHER API with the AES
379   cipher handle to encrypt one block.
380
3815. The GCM AEAD implementation also invokes the GHASH cipher
382   implementation via the AHASH API.
383
384When the IPSEC layer triggers the esp_input() function, the same call
385sequence is followed with the only difference that the operation starts
386with step (2).
387
388Generic Block Cipher Structure
389~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
390
391Generic block ciphers follow the same concept as depicted with the ASCII
392art picture above.
393
394For example, CBC(AES) is implemented with cbc.c, and aes-generic.c. The
395ASCII art picture above applies as well with the difference that only
396step (4) is used and the SKCIPHER block chaining mode is CBC.
397
398Generic Keyed Message Digest Structure
399~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
400
401Keyed message digest implementations again follow the same concept as
402depicted in the ASCII art picture above.
403
404For example, HMAC(SHA256) is implemented with hmac.c and
405sha256_generic.c. The following ASCII art illustrates the
406implementation:
407
408::
409
410
411    kernel crypto API            |       Caller
412                                 |
413    +-----------+         (1)    |
414    |           | <------------------  some_function
415    |   ahash   |
416    |   (hmac)  | ---+
417    +-----------+    |
418                     | (2)
419    +-----------+    |
420    |           | <--+
421    |   shash   |
422    |  (sha256) |
423    +-----------+
424
425
426
427The following call sequence is applicable when a caller triggers an HMAC
428operation:
429
4301. The AHASH API functions are invoked by the caller. The HMAC
431   implementation performs its operation as needed.
432
433   During initialization of the HMAC cipher, the SHASH cipher type of
434   SHA256 is instantiated. The cipher handle for the SHA256 instance is
435   retained.
436
437   At one time, the HMAC implementation requires a SHA256 operation
438   where the SHA256 cipher handle is used.
439
4402. The HMAC instance now invokes the SHASH API with the SHA256 cipher
441   handle to calculate the message digest.
442