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_HASH Raw message digest 212 213- CRYPTO_ALG_TYPE_SHASH Synchronous multi-block hash 214 215- CRYPTO_ALG_TYPE_AHASH Asynchronous multi-block hash 216 217- CRYPTO_ALG_TYPE_RNG Random Number Generation 218 219- CRYPTO_ALG_TYPE_AKCIPHER Asymmetric cipher 220 221- CRYPTO_ALG_TYPE_PCOMPRESS Enhanced version of 222 CRYPTO_ALG_TYPE_COMPRESS allowing for segmented compression / 223 decompression instead of performing the operation on one segment 224 only. CRYPTO_ALG_TYPE_PCOMPRESS is intended to replace 225 CRYPTO_ALG_TYPE_COMPRESS once existing consumers are converted. 226 227The mask flag restricts the type of cipher. The only allowed flag is 228CRYPTO_ALG_ASYNC to restrict the cipher lookup function to 229asynchronous ciphers. Usually, a caller provides a 0 for the mask flag. 230 231When the caller provides a mask and type specification, the caller 232limits the search the kernel crypto API can perform for a suitable 233cipher implementation for the given cipher name. That means, even when a 234caller uses a cipher name that exists during its initialization call, 235the kernel crypto API may not select it due to the used type and mask 236field. 237 238Internal Structure of Kernel Crypto API 239--------------------------------------- 240 241The kernel crypto API has an internal structure where a cipher 242implementation may use many layers and indirections. This section shall 243help to clarify how the kernel crypto API uses various components to 244implement the complete cipher. 245 246The following subsections explain the internal structure based on 247existing cipher implementations. The first section addresses the most 248complex scenario where all other scenarios form a logical subset. 249 250Generic AEAD Cipher Structure 251~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 252 253The following ASCII art decomposes the kernel crypto API layers when 254using the AEAD cipher with the automated IV generation. The shown 255example is used by the IPSEC layer. 256 257For other use cases of AEAD ciphers, the ASCII art applies as well, but 258the caller may not use the AEAD cipher with a separate IV generator. In 259this case, the caller must generate the IV. 260 261The depicted example decomposes the AEAD cipher of GCM(AES) based on the 262generic C implementations (gcm.c, aes-generic.c, ctr.c, ghash-generic.c, 263seqiv.c). The generic implementation serves as an example showing the 264complete logic of the kernel crypto API. 265 266It is possible that some streamlined cipher implementations (like 267AES-NI) provide implementations merging aspects which in the view of the 268kernel crypto API cannot be decomposed into layers any more. In case of 269the AES-NI implementation, the CTR mode, the GHASH implementation and 270the AES cipher are all merged into one cipher implementation registered 271with the kernel crypto API. In this case, the concept described by the 272following ASCII art applies too. However, the decomposition of GCM into 273the individual sub-components by the kernel crypto API is not done any 274more. 275 276Each block in the following ASCII art is an independent cipher instance 277obtained from the kernel crypto API. Each block is accessed by the 278caller or by other blocks using the API functions defined by the kernel 279crypto API for the cipher implementation type. 280 281The blocks below indicate the cipher type as well as the specific logic 282implemented in the cipher. 283 284The ASCII art picture also indicates the call structure, i.e. who calls 285which component. The arrows point to the invoked block where the caller 286uses the API applicable to the cipher type specified for the block. 287 288:: 289 290 291 kernel crypto API | IPSEC Layer 292 | 293 +-----------+ | 294 | | (1) 295 | aead | <----------------------------------- esp_output 296 | (seqiv) | ---+ 297 +-----------+ | 298 | (2) 299 +-----------+ | 300 | | <--+ (2) 301 | aead | <----------------------------------- esp_input 302 | (gcm) | ------------+ 303 +-----------+ | 304 | (3) | (5) 305 v v 306 +-----------+ +-----------+ 307 | | | | 308 | skcipher | | ahash | 309 | (ctr) | ---+ | (ghash) | 310 +-----------+ | +-----------+ 311 | 312 +-----------+ | (4) 313 | | <--+ 314 | cipher | 315 | (aes) | 316 +-----------+ 317 318 319 320The following call sequence is applicable when the IPSEC layer triggers 321an encryption operation with the esp_output function. During 322configuration, the administrator set up the use of seqiv(rfc4106(gcm(aes))) 323as the cipher for ESP. The following call sequence is now depicted in 324the ASCII art above: 325 3261. esp_output() invokes crypto_aead_encrypt() to trigger an 327 encryption operation of the AEAD cipher with IV generator. 328 329 The SEQIV generates the IV. 330 3312. Now, SEQIV uses the AEAD API function calls to invoke the associated 332 AEAD cipher. In our case, during the instantiation of SEQIV, the 333 cipher handle for GCM is provided to SEQIV. This means that SEQIV 334 invokes AEAD cipher operations with the GCM cipher handle. 335 336 During instantiation of the GCM handle, the CTR(AES) and GHASH 337 ciphers are instantiated. The cipher handles for CTR(AES) and GHASH 338 are retained for later use. 339 340 The GCM implementation is responsible to invoke the CTR mode AES and 341 the GHASH cipher in the right manner to implement the GCM 342 specification. 343 3443. The GCM AEAD cipher type implementation now invokes the SKCIPHER API 345 with the instantiated CTR(AES) cipher handle. 346 347 During instantiation of the CTR(AES) cipher, the CIPHER type 348 implementation of AES is instantiated. The cipher handle for AES is 349 retained. 350 351 That means that the SKCIPHER implementation of CTR(AES) only 352 implements the CTR block chaining mode. After performing the block 353 chaining operation, the CIPHER implementation of AES is invoked. 354 3554. The SKCIPHER of CTR(AES) now invokes the CIPHER API with the AES 356 cipher handle to encrypt one block. 357 3585. The GCM AEAD implementation also invokes the GHASH cipher 359 implementation via the AHASH API. 360 361When the IPSEC layer triggers the esp_input() function, the same call 362sequence is followed with the only difference that the operation starts 363with step (2). 364 365Generic Block Cipher Structure 366~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 367 368Generic block ciphers follow the same concept as depicted with the ASCII 369art picture above. 370 371For example, CBC(AES) is implemented with cbc.c, and aes-generic.c. The 372ASCII art picture above applies as well with the difference that only 373step (4) is used and the SKCIPHER block chaining mode is CBC. 374 375Generic Keyed Message Digest Structure 376~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 377 378Keyed message digest implementations again follow the same concept as 379depicted in the ASCII art picture above. 380 381For example, HMAC(SHA256) is implemented with hmac.c and 382sha256_generic.c. The following ASCII art illustrates the 383implementation: 384 385:: 386 387 388 kernel crypto API | Caller 389 | 390 +-----------+ (1) | 391 | | <------------------ some_function 392 | ahash | 393 | (hmac) | ---+ 394 +-----------+ | 395 | (2) 396 +-----------+ | 397 | | <--+ 398 | shash | 399 | (sha256) | 400 +-----------+ 401 402 403 404The following call sequence is applicable when a caller triggers an HMAC 405operation: 406 4071. The AHASH API functions are invoked by the caller. The HMAC 408 implementation performs its operation as needed. 409 410 During initialization of the HMAC cipher, the SHASH cipher type of 411 SHA256 is instantiated. The cipher handle for the SHA256 instance is 412 retained. 413 414 At one time, the HMAC implementation requires a SHA256 operation 415 where the SHA256 cipher handle is used. 416 4172. The HMAC instance now invokes the SHASH API with the SHA256 cipher 418 handle to calculate the message digest. 419
