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