1.\" $OpenBSD: crypto.9,v 1.19 2002/07/16 06:31:57 angelos Exp $ 2.\" 3.\" The author of this manual page is Angelos D. Keromytis (angelos@cis.upenn.edu) 4.\" 5.\" Copyright (c) 2000, 2001 Angelos D. Keromytis 6.\" 7.\" Permission to use, copy, and modify this software with or without fee 8.\" is hereby granted, provided that this entire notice is included in 9.\" all source code copies of any software which is or includes a copy or 10.\" modification of this software. 11.\" 12.\" THIS SOFTWARE IS BEING PROVIDED "AS IS", WITHOUT ANY EXPRESS OR 13.\" IMPLIED WARRANTY. IN PARTICULAR, NONE OF THE AUTHORS MAKES ANY 14.\" REPRESENTATION OR WARRANTY OF ANY KIND CONCERNING THE 15.\" MERCHANTABILITY OF THIS SOFTWARE OR ITS FITNESS FOR ANY PARTICULAR 16.\" PURPOSE. 17.\" 18.\" $FreeBSD$ 19.\" 20.Dd July 17, 2018 21.Dt CRYPTO 9 22.Os 23.Sh NAME 24.Nm crypto 25.Nd API for cryptographic services in the kernel 26.Sh SYNOPSIS 27.In opencrypto/cryptodev.h 28.Ft int32_t 29.Fn crypto_get_driverid device_t size_t int 30.Ft int 31.Fn crypto_register uint32_t int uint16_t uint32_t "int \*[lp]*\*[rp]\*[lp]void *, uint32_t *, struct cryptoini *\*[rp]" "int \*[lp]*\*[rp]\*[lp]void *, uint64_t\*[rp]" "int \*[lp]*\*[rp]\*[lp]void *, struct cryptop *\*[rp]" "void *" 32.Ft int 33.Fn crypto_kregister uint32_t int uint32_t "int \*[lp]*\*[rp]\*[lp]void *, struct cryptkop *\*[rp]" "void *" 34.Ft int 35.Fn crypto_unregister uint32_t int 36.Ft int 37.Fn crypto_unregister_all uint32_t 38.Ft void 39.Fn crypto_done "struct cryptop *" 40.Ft void 41.Fn crypto_kdone "struct cryptkop *" 42.Ft int 43.Fn crypto_find_driver "const char *" 44.Ft int 45.Fn crypto_newsession "crypto_session_t *" "struct cryptoini *" int 46.Ft int 47.Fn crypto_freesession crypto_session_t 48.Ft int 49.Fn crypto_dispatch "struct cryptop *" 50.Ft int 51.Fn crypto_kdispatch "struct cryptkop *" 52.Ft int 53.Fn crypto_unblock uint32_t int 54.Ft "struct cryptop *" 55.Fn crypto_getreq int 56.Ft void 57.Fn crypto_freereq void 58.Bd -literal 59#define CRYPTO_SYMQ 0x1 60#define CRYPTO_ASYMQ 0x2 61 62#define EALG_MAX_BLOCK_LEN 16 63 64struct cryptoini { 65 int cri_alg; 66 int cri_klen; 67 int cri_mlen; 68 caddr_t cri_key; 69 uint8_t cri_iv[EALG_MAX_BLOCK_LEN]; 70 struct cryptoini *cri_next; 71}; 72 73struct cryptodesc { 74 int crd_skip; 75 int crd_len; 76 int crd_inject; 77 int crd_flags; 78 struct cryptoini CRD_INI; 79#define crd_iv CRD_INI.cri_iv 80#define crd_key CRD_INI.cri_key 81#define crd_alg CRD_INI.cri_alg 82#define crd_klen CRD_INI.cri_klen 83 struct cryptodesc *crd_next; 84}; 85 86struct cryptop { 87 TAILQ_ENTRY(cryptop) crp_next; 88 crypto_session_t crp_session; 89 int crp_ilen; 90 int crp_olen; 91 int crp_etype; 92 int crp_flags; 93 caddr_t crp_buf; 94 caddr_t crp_opaque; 95 struct cryptodesc *crp_desc; 96 int (*crp_callback) (struct cryptop *); 97 caddr_t crp_mac; 98}; 99 100struct crparam { 101 caddr_t crp_p; 102 u_int crp_nbits; 103}; 104 105#define CRK_MAXPARAM 8 106 107struct cryptkop { 108 TAILQ_ENTRY(cryptkop) krp_next; 109 u_int krp_op; /* ie. CRK_MOD_EXP or other */ 110 u_int krp_status; /* return status */ 111 u_short krp_iparams; /* # of input parameters */ 112 u_short krp_oparams; /* # of output parameters */ 113 uint32_t krp_hid; 114 struct crparam krp_param[CRK_MAXPARAM]; 115 int (*krp_callback)(struct cryptkop *); 116}; 117.Ed 118.Sh DESCRIPTION 119.Nm 120is a framework for drivers of cryptographic hardware to register with 121the kernel so 122.Dq consumers 123(other kernel subsystems, and 124users through the 125.Pa /dev/crypto 126device) are able to make use of it. 127Drivers register with the framework the algorithms they support, 128and provide entry points (functions) the framework may call to 129establish, use, and tear down sessions. 130Sessions are used to cache cryptographic information in a particular driver 131(or associated hardware), so initialization is not needed with every request. 132Consumers of cryptographic services pass a set of 133descriptors that instruct the framework (and the drivers registered 134with it) of the operations that should be applied on the data (more 135than one cryptographic operation can be requested). 136.Pp 137Keying operations are supported as well. 138Unlike the symmetric operators described above, 139these sessionless commands perform mathematical operations using 140input and output parameters. 141.Pp 142Since the consumers may not be associated with a process, drivers may 143not 144.Xr sleep 9 . 145The same holds for the framework. 146Thus, a callback mechanism is used 147to notify a consumer that a request has been completed (the 148callback is specified by the consumer on a per-request basis). 149The callback is invoked by the framework whether the request was 150successfully completed or not. 151An error indication is provided in the latter case. 152A specific error code, 153.Er EAGAIN , 154is used to indicate that a session handle has changed and that the 155request may be re-submitted immediately with the new session. 156Errors are only returned to the invoking function if not 157enough information to call the callback is available (meaning, there 158was a fatal error in verifying the arguments). 159For session initialization and teardown no callback mechanism is used. 160.Pp 161The 162.Fn crypto_find_driver 163function may be called to return the specific id of the provided name. 164If the specified driver could not be found, the returned id is -1. 165.Pp 166The 167.Fn crypto_newsession 168routine is called by consumers of cryptographic services (such as the 169.Xr ipsec 4 170stack) that wish to establish a new session with the framework. 171The second argument contains all the necessary information for 172the driver to establish the session. 173The third argument is either a specific driver id, or one or both 174of 175.Dv CRYPTOCAP_F_HARDWARE , 176to select hardware devices, 177or 178.Dv CRYPTOCAP_F_SOFTWARE , 179to select software devices. 180If both are specified, a hardware device will be returned 181before a software device will be. 182On success, the value pointed to by the first argument will be the opaque 183session handle. 184The various fields in the 185.Vt cryptoini 186structure are: 187.Bl -tag -width ".Va cri_next" 188.It Va cri_alg 189Contains an algorithm identifier. 190Currently supported algorithms are: 191.Pp 192.Bl -tag -width ".Dv CRYPTO_RIPEMD160_HMAC" -compact 193.It Dv CRYPTO_AES_128_NIST_GMAC 194.It Dv CRYPTO_AES_192_NIST_GMAC 195.It Dv CRYPTO_AES_256_NIST_GMAC 196.It Dv CRYPTO_AES_CBC 197.It Dv CRYPTO_AES_ICM 198.It Dv CRYPTO_AES_NIST_GCM_16 199.It Dv CRYPTO_AES_NIST_GMAC 200.It Dv CRYPTO_AES_XTS 201.It Dv CRYPTO_ARC4 202.It Dv CRYPTO_BLF_CBC 203.It Dv CRYPTO_CAMELLIA_CBC 204.It Dv CRYPTO_CAST_CBC 205.It Dv CRYPTO_DEFLATE_COMP 206.It Dv CRYPTO_DES_CBC 207.It Dv CRYPTO_3DES_CBC 208.It Dv CRYPTO_MD5 209.It Dv CRYPTO_MD5_HMAC 210.It Dv CRYPTO_MD5_KPDK 211.It Dv CRYPTO_NULL_HMAC 212.It Dv CRYPTO_NULL_CBC 213.It Dv CRYPTO_RIPEMD160_HMAC 214.It Dv CRYPTO_SHA1 215.It Dv CRYPTO_SHA1_HMAC 216.It Dv CRYPTO_SHA1_KPDK 217.It Dv CRYPTO_SHA2_256_HMAC 218.It Dv CRYPTO_SHA2_384_HMAC 219.It Dv CRYPTO_SHA2_512_HMAC 220.It Dv CRYPTO_SKIPJACK_CBC 221.El 222.It Va cri_klen 223Specifies the length of the key in bits, for variable-size key 224algorithms. 225.It Va cri_mlen 226Specifies how many bytes from the calculated hash should be copied back. 2270 means entire hash. 228.It Va cri_key 229Contains the key to be used with the algorithm. 230.It Va cri_iv 231Contains an explicit initialization vector (IV), if it does not prefix 232the data. 233This field is ignored during initialization 234.Pq Nm crypto_newsession . 235If no IV is explicitly passed (see below on details), a random IV is used 236by the device driver processing the request. 237.It Va cri_next 238Contains a pointer to another 239.Vt cryptoini 240structure. 241Multiple such structures may be linked to establish multi-algorithm sessions 242.Xr ( ipsec 4 243is an example consumer of such a feature). 244.El 245.Pp 246The 247.Vt cryptoini 248structure and its contents will not be modified by the framework (or 249the drivers used). 250.Pp 251.Fn crypto_freesession 252is called with the session handle returned by 253.Fn crypto_newsession 254to free the session. 255.Pp 256.Fn crypto_dispatch 257is called to process a request. 258The various fields in the 259.Vt cryptop 260structure are: 261.Bl -tag -width ".Va crp_callback" 262.It Va crp_session 263Contains the session handle. 264.It Va crp_ilen 265Indicates the total length in bytes of the buffer to be processed. 266.It Va crp_olen 267On return, contains the total length of the result. 268For symmetric crypto operations, this will be the same as the input length. 269This will be used if the framework needs to allocate a new 270buffer for the result (or for re-formatting the input). 271.It Va crp_callback 272This routine is invoked upon completion of the request, whether 273successful or not. 274It is invoked through the 275.Fn crypto_done 276routine. 277If the request was not successful, an error code is set in the 278.Va crp_etype 279field. 280It is the responsibility of the callback routine to set the appropriate 281.Xr spl 9 282level. 283.It Va crp_etype 284Contains the error type, if any errors were encountered, or zero if 285the request was successfully processed. 286If the 287.Er EAGAIN 288error code is returned, the session handle has changed (and has been recorded 289in the 290.Va crp_session 291field). 292The consumer should record the new session handle and use it in all subsequent 293requests. 294In this case, the request may be re-submitted immediately. 295This mechanism is used by the framework to perform 296session migration (move a session from one driver to another, because 297of availability, performance, or other considerations). 298.Pp 299Note that this field only makes sense when examined by 300the callback routine specified in 301.Va crp_callback . 302Errors are returned to the invoker of 303.Fn crypto_process 304only when enough information is not present to call the callback 305routine (i.e., if the pointer passed is 306.Dv NULL 307or if no callback routine was specified). 308.It Va crp_flags 309Is a bitmask of flags associated with this request. 310Currently defined flags are: 311.Bl -tag -width ".Dv CRYPTO_F_CBIFSYNC" 312.It Dv CRYPTO_F_IMBUF 313The buffer pointed to by 314.Va crp_buf 315is an mbuf chain. 316.It Dv CRYPTO_F_IOV 317The buffer pointed to by 318.Va crp_buf 319is an 320.Vt uio 321structure. 322.It Dv CRYPTO_F_BATCH 323Batch operation if possible. 324.It Dv CRYPTO_F_CBIMM 325Do callback immediately instead of doing it from a dedicated kernel thread. 326.It Dv CRYPTO_F_DONE 327Operation completed. 328.It Dv CRYPTO_F_CBIFSYNC 329Do callback immediately if operation is synchronous (that the driver 330specified the 331.Dv CRYPTOCAP_F_SYNC 332flag). 333.It Dv CRYPTO_F_ASYNC 334Try to do the crypto operation in a pool of workers 335if the operation is synchronous (that is, if the driver specified the 336.Dv CRYPTOCAP_F_SYNC 337flag). 338It aims to speed up processing by dispatching crypto operations 339on different processors. 340.It Dv CRYPTO_F_ASYNC_KEEPORDER 341Dispatch callbacks in the same order they are posted. 342Only relevant if the 343.Dv CRYPTO_F_ASYNC 344flag is set and if the operation is synchronous. 345.El 346.It Va crp_buf 347Points to the input buffer. 348On return (when the callback is invoked), 349it contains the result of the request. 350The input buffer may be an mbuf 351chain or a contiguous buffer, 352depending on 353.Va crp_flags . 354.It Va crp_opaque 355This is passed through the crypto framework untouched and is 356intended for the invoking application's use. 357.It Va crp_desc 358This is a linked list of descriptors. 359Each descriptor provides 360information about what type of cryptographic operation should be done 361on the input buffer. 362The various fields are: 363.Bl -tag -width ".Va crd_inject" 364.It Va crd_iv 365When the flag 366.Dv CRD_F_IV_EXPLICIT 367is set, this field contains the IV. 368.It Va crd_key 369When the 370.Dv CRD_F_KEY_EXPLICIT 371flag is set, the 372.Va crd_key 373points to a buffer with encryption or authentication key. 374.It Va crd_alg 375An algorithm to use. 376Must be the same as the one given at newsession time. 377.It Va crd_klen 378The 379.Va crd_key 380key length. 381.It Va crd_skip 382The offset in the input buffer where processing should start. 383.It Va crd_len 384How many bytes, after 385.Va crd_skip , 386should be processed. 387.It Va crd_inject 388The 389.Va crd_inject 390field specifies an offset in bytes from the beginning of the buffer. 391For encryption algorithms, this may be where the IV will be inserted 392when encrypting or where the IV may be found for 393decryption (subject to 394.Va crd_flags ) . 395For MAC algorithms, this is where the result of the keyed hash will be 396inserted. 397.It Va crd_flags 398The following flags are defined: 399.Bl -tag -width 3n 400.It Dv CRD_F_ENCRYPT 401For encryption algorithms, this bit is set when encryption is required 402(when not set, decryption is performed). 403.It Dv CRD_F_IV_PRESENT 404.\" This flag name has nothing to do w/ it's behavior, fix the name. 405For encryption, if this bit is not set the IV used to encrypt the packet 406will be written at the location pointed to by 407.Va crd_inject . 408The IV length is assumed to be equal to the blocksize of the 409encryption algorithm. 410For encryption, if this bit is set, nothing is done. 411For decryption, this flag has no meaning. 412Applications that do special 413.Dq "IV cooking" , 414such as the half-IV mode in 415.Xr ipsec 4 , 416can use this flag to indicate that the IV should not be written on the packet. 417This flag is typically used in conjunction with the 418.Dv CRD_F_IV_EXPLICIT 419flag. 420.It Dv CRD_F_IV_EXPLICIT 421This bit is set when the IV is explicitly 422provided by the consumer in the 423.Va crd_iv 424field. 425Otherwise, for encryption operations the IV is provided for by 426the driver used to perform the operation, whereas for decryption 427operations the offset of the IV is provided by the 428.Va crd_inject 429field. 430This flag is typically used when the IV is calculated 431.Dq "on the fly" 432by the consumer, and does not precede the data (some 433.Xr ipsec 4 434configurations, and the encrypted swap are two such examples). 435.It Dv CRD_F_KEY_EXPLICIT 436For encryption and authentication (MAC) algorithms, this bit is set when the key 437is explicitly provided by the consumer in the 438.Va crd_key 439field for the given operation. 440Otherwise, the key is taken at newsession time from the 441.Va cri_key 442field. 443As calculating the key schedule may take a while, it is recommended that often 444used keys are given their own session. 445.It Dv CRD_F_COMP 446For compression algorithms, this bit is set when compression is required (when 447not set, decompression is performed). 448.El 449.It Va CRD_INI 450This 451.Vt cryptoini 452structure will not be modified by the framework or the device drivers. 453Since this information accompanies every cryptographic 454operation request, drivers may re-initialize state on-demand 455(typically an expensive operation). 456Furthermore, the cryptographic 457framework may re-route requests as a result of full queues or hardware 458failure, as described above. 459.It Va crd_next 460Point to the next descriptor. 461Linked operations are useful in protocols such as 462.Xr ipsec 4 , 463where multiple cryptographic transforms may be applied on the same 464block of data. 465.El 466.El 467.Pp 468.Fn crypto_getreq 469allocates a 470.Vt cryptop 471structure with a linked list of as many 472.Vt cryptodesc 473structures as were specified in the argument passed to it. 474.Pp 475.Fn crypto_freereq 476deallocates a structure 477.Vt cryptop 478and any 479.Vt cryptodesc 480structures linked to it. 481Note that it is the responsibility of the 482callback routine to do the necessary cleanups associated with the 483opaque field in the 484.Vt cryptop 485structure. 486.Pp 487.Fn crypto_kdispatch 488is called to perform a keying operation. 489The various fields in the 490.Vt cryptkop 491structure are: 492.Bl -tag -width ".Va krp_callback" 493.It Va krp_op 494Operation code, such as 495.Dv CRK_MOD_EXP . 496.It Va krp_status 497Return code. 498This 499.Va errno Ns -style 500variable indicates whether lower level reasons 501for operation failure. 502.It Va krp_iparams 503Number if input parameters to the specified operation. 504Note that each operation has a (typically hardwired) number of such parameters. 505.It Va krp_oparams 506Number if output parameters from the specified operation. 507Note that each operation has a (typically hardwired) number of such parameters. 508.It Va krp_kvp 509An array of kernel memory blocks containing the parameters. 510.It Va krp_hid 511Identifier specifying which low-level driver is being used. 512.It Va krp_callback 513Callback called on completion of a keying operation. 514.El 515.Sh DRIVER-SIDE API 516The 517.Fn crypto_get_driverid , 518.Fn crypto_get_driver_session , 519.Fn crypto_register , 520.Fn crypto_kregister , 521.Fn crypto_unregister , 522.Fn crypto_unblock , 523and 524.Fn crypto_done 525routines are used by drivers that provide support for cryptographic 526primitives to register and unregister with the kernel crypto services 527framework. 528.Pp 529Drivers must first use the 530.Fn crypto_get_driverid 531function to acquire a driver identifier, specifying the 532.Fa flags 533as an argument. 534One of 535.Dv CRYPTOCAP_F_SOFTWARE 536or 537.Dv CRYPTOCAP_F_HARDWARE 538must be specified. 539The 540.Dv CRYPTOCAP_F_SYNC 541may also be specified, and should be specified if the driver does all of 542it's operations synchronously. 543Drivers must pass the size of their session struct as the second argument. 544An appropriately sized memory will be allocated by the framework, zeroed, and 545passed to the driver's 546.Fn newsession 547method. 548.Pp 549For each algorithm the driver supports, it must then call 550.Fn crypto_register . 551The first two arguments are the driver and algorithm identifiers. 552The next two arguments specify the largest possible operator length (in bits, 553important for public key operations) and flags for this algorithm. 554The last four arguments must be provided in the first call to 555.Fn crypto_register 556and are ignored in all subsequent calls. 557They are pointers to three 558driver-provided functions that the framework may call to establish new 559cryptographic context with the driver, free already established 560context, and ask for a request to be processed (encrypt, decrypt, 561etc.); and an opaque parameter to pass when calling each of these routines. 562.Pp 563.Fn crypto_unregister 564is called by drivers that wish to withdraw support for an algorithm. 565The two arguments are the driver and algorithm identifiers, respectively. 566Typically, drivers for 567PCMCIA 568crypto cards that are being ejected will invoke this routine for all 569algorithms supported by the card. 570.Fn crypto_unregister_all 571will unregister all algorithms registered by a driver 572and the driver will be disabled (no new sessions will be allocated on 573that driver, and any existing sessions will be migrated to other 574drivers). 575The same will be done if all algorithms associated with a driver are 576unregistered one by one. 577After a call to 578.Fn crypto_unregister_all 579there will be no threads in either the newsession or freesession function 580of the driver. 581.Pp 582The calling convention for the driver-supplied routines are: 583.Pp 584.Bl -item -compact 585.It 586.Ft int 587.Fn \*[lp]*newsession\*[rp] "device_t" "crypto_session_t" "struct cryptoini *" ; 588.It 589.Ft void 590.Fn \*[lp]*freesession\*[rp] "device_t" "crypto_session_t" ; 591.It 592.Ft int 593.Fn \*[lp]*process\*[rp] "device_t" "struct cryptop *" "int" ; 594.It 595.Ft int 596.Fn \*[lp]*kprocess\*[rp] "device_t" "struct cryptkop *" "int" ; 597.El 598.Pp 599On invocation, the first argument to 600all routines is the 601.Fa device_t 602that was provided to 603.Fn crypto_get_driverid . 604The second argument to 605.Fn newsession 606is the opaque session handle for the new session. 607The third argument is identical to that of 608.Fn crypto_newsession . 609.Pp 610Drivers obtain a pointer to their session memory by invoking 611.Fn crypto_get_driver_session 612on the opaque 613.Vt crypto_session_t 614handle. 615.Pp 616The 617.Fn freesession 618routine takes as arguments the opaque data value and the session handle. 619It should clear any context associated with the session (clear hardware 620registers, memory, etc.). 621If no resources need to be released other than the contents of session memory, 622the method is optional. 623The 624.Nm 625framework will zero and release the allocated session memory (after running the 626.Fn freesession 627method, if one exists). 628.Pp 629The 630.Fn process 631routine is invoked with a request to perform crypto processing. 632This routine must not block or sleep, but should queue the request and return 633immediately or process the request to completion. 634In case of an unrecoverable error, the error indication must be placed in the 635.Va crp_etype 636field of the 637.Vt cryptop 638structure. 639When the request is completed, or an error is detected, the 640.Fn process 641routine must invoke 642.Fn crypto_done . 643Session migration may be performed, as mentioned previously. 644.Pp 645In case of a temporary resource exhaustion, the 646.Fn process 647routine may return 648.Er ERESTART 649in which case the crypto services will requeue the request, mark the driver 650as 651.Dq blocked , 652and stop submitting requests for processing. 653The driver is then responsible for notifying the crypto services 654when it is again able to process requests through the 655.Fn crypto_unblock 656routine. 657This simple flow control mechanism should only be used for short-lived 658resource exhaustion as it causes operations to be queued in the crypto 659layer. 660Doing so is preferable to returning an error in such cases as 661it can cause network protocols to degrade performance by treating the 662failure much like a lost packet. 663.Pp 664The 665.Fn kprocess 666routine is invoked with a request to perform crypto key processing. 667This routine must not block, but should queue the request and return 668immediately. 669Upon processing the request, the callback routine should be invoked. 670In case of an unrecoverable error, the error indication must be placed in the 671.Va krp_status 672field of the 673.Vt cryptkop 674structure. 675When the request is completed, or an error is detected, the 676.Fn kprocess 677routine should invoked 678.Fn crypto_kdone . 679.Sh RETURN VALUES 680.Fn crypto_register , 681.Fn crypto_kregister , 682.Fn crypto_unregister , 683.Fn crypto_newsession , 684.Fn crypto_freesession , 685and 686.Fn crypto_unblock 687return 0 on success, or an error code on failure. 688.Fn crypto_get_driverid 689returns a non-negative value on error, and \-1 on failure. 690.Fn crypto_getreq 691returns a pointer to a 692.Vt cryptop 693structure and 694.Dv NULL 695on failure. 696.Fn crypto_dispatch 697returns 698.Er EINVAL 699if its argument or the callback function was 700.Dv NULL , 701and 0 otherwise. 702The callback is provided with an error code in case of failure, in the 703.Va crp_etype 704field. 705.Sh FILES 706.Bl -tag -width ".Pa sys/opencrypto/crypto.c" 707.It Pa sys/opencrypto/crypto.c 708most of the framework code 709.El 710.Sh SEE ALSO 711.Xr crypto 4 , 712.Xr ipsec 4 , 713.Xr crypto 7 , 714.Xr malloc 9 , 715.Xr sleep 9 716.Sh HISTORY 717The cryptographic framework first appeared in 718.Ox 2.7 719and was written by 720.An Angelos D. Keromytis Aq Mt angelos@openbsd.org . 721.Sh BUGS 722The framework currently assumes that all the algorithms in a 723.Fn crypto_newsession 724operation must be available by the same driver. 725If that is not the case, session initialization will fail. 726.Pp 727The framework also needs a mechanism for determining which driver is 728best for a specific set of algorithms associated with a session. 729Some type of benchmarking is in order here. 730.Pp 731Multiple instances of the same algorithm in the same session are not 732supported. 733