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CRYPTO(9) Kernel Developer's Manual CRYPTO(9)

NAME

crypto — API for cryptographic services in the kernel

SYNOPSIS

#include <opencrypto/cryptodev.h>

int32_t
crypto_get_driverid(device_t, int);

int
crypto_register(uint32_t, int, uint16_t, uint32_t, int (*)(void *, uint32_t *, struct cryptoini *), int (*)(void *, uint64_t), int (*)(void *, struct cryptop *), void *);

int
crypto_kregister(uint32_t, int, uint32_t, int (*)(void *, struct cryptkop *), void *);

int
crypto_unregister(uint32_t, int);

int
crypto_unregister_all(uint32_t);

void
crypto_done(struct cryptop *);

void
crypto_kdone(struct cryptkop *);

int
crypto_find_driver(const char *);

int
crypto_newsession(uint64_t *, struct cryptoini *, int);

int
crypto_freesession(uint64_t);

int
crypto_dispatch(struct cryptop *);

int
crypto_kdispatch(struct cryptkop *);

int
crypto_unblock(uint32_t, int);

struct cryptop *
crypto_getreq(int);

void
crypto_freereq(void);

#define	CRYPTO_SYMQ	0x1
#define	CRYPTO_ASYMQ	0x2

#define EALG_MAX_BLOCK_LEN      16

struct cryptoini {
	int                cri_alg;
	int                cri_klen;
	int                cri_mlen;
	caddr_t            cri_key;
	uint8_t            cri_iv[EALG_MAX_BLOCK_LEN];
	struct cryptoini  *cri_next;
};

struct cryptodesc {
	int                crd_skip;
	int                crd_len;
	int                crd_inject;
	int                crd_flags;
	struct cryptoini   CRD_INI;
#define crd_iv          CRD_INI.cri_iv
#define crd_key         CRD_INI.cri_key
#define crd_alg         CRD_INI.cri_alg
#define crd_klen        CRD_INI.cri_klen
	struct cryptodesc *crd_next;
};

struct cryptop {
	TAILQ_ENTRY(cryptop) crp_next;
	uint64_t           crp_sid;
	int                crp_ilen;
	int                crp_olen;
	int                crp_etype;
	int                crp_flags;
	caddr_t            crp_buf;
	caddr_t            crp_opaque;
	struct cryptodesc *crp_desc;
	int              (*crp_callback) (struct cryptop *);
	caddr_t            crp_mac;
};

struct crparam {
        caddr_t         crp_p;
        u_int           crp_nbits;
};

#define CRK_MAXPARAM    8

struct cryptkop {
        TAILQ_ENTRY(cryptkop) krp_next;
        u_int              krp_op;         /* ie. CRK_MOD_EXP or other */
        u_int              krp_status;     /* return status */
        u_short            krp_iparams;    /* # of input parameters */
        u_short            krp_oparams;    /* # of output parameters */
        uint32_t           krp_hid;
        struct crparam     krp_param[CRK_MAXPARAM];
        int               (*krp_callback)(struct cryptkop *);
};

DESCRIPTION

crypto is a framework for drivers of cryptographic hardware to register with the kernel so “consumers” (other kernel subsystems, and users through the /dev/crypto device) are able to make use of it. Drivers register with the framework the algorithms they support, and provide entry points (functions) the framework may call to establish, use, and tear down sessions. Sessions are used to cache cryptographic information in a particular driver (or associated hardware), so initialization is not needed with every request. Consumers of cryptographic services pass a set of descriptors that instruct the framework (and the drivers registered with it) of the operations that should be applied on the data (more than one cryptographic operation can be requested).

Keying operations are supported as well. Unlike the symmetric operators described above, these sessionless commands perform mathematical operations using input and output parameters.

Since the consumers may not be associated with a process, drivers may not sleep(9). The same holds for the framework. Thus, a callback mechanism is used to notify a consumer that a request has been completed (the callback is specified by the consumer on a per-request basis). The callback is invoked by the framework whether the request was successfully completed or not. An error indication is provided in the latter case. A specific error code, EAGAIN, is used to indicate that a session number has changed and that the request may be re-submitted immediately with the new session number. Errors are only returned to the invoking function if not enough information to call the callback is available (meaning, there was a fatal error in verifying the arguments). For session initialization and teardown there is no callback mechanism used.

The () function may be called to return the specific id of the provided name. If the specified driver could not be found, the returned id is -1.

The () routine is called by consumers of cryptographic services (such as the ipsec(4) stack) that wish to establish a new session with the framework. The second argument contains all the necessary information for the driver to establish the session. The third argument is either a specific driver id, or one or both of CRYPTOCAP_F_HARDWARE, to select hardware devices, or CRYPTOCAP_F_SOFTWARE, to select software devices. If both are specified, a hardware device will be returned before a software device will be. On success, the value pointed to by the first argument will be the Session IDentifier (SID). The various fields in the cryptoini structure are:

cri_alg
Contains an algorithm identifier. Currently supported algorithms are:

 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
cri_klen
Specifies the length of the key in bits, for variable-size key algorithms.
cri_mlen
Specifies how many bytes from the calculated hash should be copied back. 0 means entire hash.
cri_key
Contains the key to be used with the algorithm.
cri_iv
Contains an explicit initialization vector (IV), if it does not prefix the data. This field is ignored during initialization (crypto_newsession). If no IV is explicitly passed (see below on details), a random IV is used by the device driver processing the request.
cri_next
Contains a pointer to another cryptoini structure. Multiple such structures may be linked to establish multi-algorithm sessions (ipsec(4) is an example consumer of such a feature).

The cryptoini structure and its contents will not be modified by the framework (or the drivers used). Subsequent requests for processing that use the SID returned will avoid the cost of re-initializing the hardware (in essence, SID acts as an index in the session cache of the driver).

() is called with the SID returned by crypto_newsession() to disestablish the session.

() is called to process a request. The various fields in the cryptop structure are:

crp_sid
Contains the SID.
crp_ilen
Indicates the total length in bytes of the buffer to be processed.
crp_olen
On return, contains the total length of the result. For symmetric crypto operations, this will be the same as the input length. This will be used if the framework needs to allocate a new buffer for the result (or for re-formatting the input).
crp_callback
This routine is invoked upon completion of the request, whether successful or not. It is invoked through the () routine. If the request was not successful, an error code is set in the crp_etype field. It is the responsibility of the callback routine to set the appropriate spl(9) level.
crp_etype
Contains the error type, if any errors were encountered, or zero if the request was successfully processed. If the EAGAIN error code is returned, the SID has changed (and has been recorded in the crp_sid field). The consumer should record the new SID and use it in all subsequent requests. In this case, the request may be re-submitted immediately. This mechanism is used by the framework to perform session migration (move a session from one driver to another, because of availability, performance, or other considerations).

Note that this field only makes sense when examined by the callback routine specified in crp_callback. Errors are returned to the invoker of () only when enough information is not present to call the callback routine (i.e., if the pointer passed is NULL or if no callback routine was specified).

crp_flags
Is a bitmask of flags associated with this request. Currently defined flags are:
The buffer pointed to by crp_buf is an mbuf chain.
The buffer pointed to by crp_buf is an uio structure.
Batch operation if possible.
Do callback immediately instead of doing it from a dedicated kernel thread.
Operation completed.
Do callback immediately if operation is synchronous (that the driver specified the CRYPTOCAP_F_SYNC flag).
crp_buf
Points to the input buffer. On return (when the callback is invoked), it contains the result of the request. The input buffer may be an mbuf chain or a contiguous buffer, depending on crp_flags.
crp_opaque
This is passed through the crypto framework untouched and is intended for the invoking application's use.
crp_desc
This is a linked list of descriptors. Each descriptor provides information about what type of cryptographic operation should be done on the input buffer. The various fields are:
crd_iv
When the flag CRD_F_IV_EXPLICIT is set, this field contains the IV.
crd_key
When the CRD_F_KEY_EXPLICIT flag is set, the crd_key points to a buffer with encryption or authentication key.
crd_alg
An algorithm to use. Must be the same as the one given at newsession time.
crd_klen
The crd_key key length.
crd_skip
The offset in the input buffer where processing should start.
crd_len
How many bytes, after crd_skip, should be processed.
crd_inject
The crd_inject field specifies an offset in bytes from the beginning of the buffer. For encryption algorithms, this may be where the IV will be inserted when encrypting or where the IV may be found for decryption (subject to crd_flags). For MAC algorithms, this is where the result of the keyed hash will be inserted.
crd_flags
The following flags are defined:
For encryption algorithms, this bit is set when encryption is required (when not set, decryption is performed).
For encryption, if this bit is not set the IV used to encrypt the packet will be written at the location pointed to by crd_inject. The IV length is assumed to be equal to the blocksize of the encryption algorithm. For encryption, if this bit is set, nothing is done. For decryption, this flag has no meaning. Applications that do special “IV cooking”, such as the half-IV mode in ipsec(4), can use this flag to indicate that the IV should not be written on the packet. This flag is typically used in conjunction with the CRD_F_IV_EXPLICIT flag.
This bit is set when the IV is explicitly provided by the consumer in the crd_iv field. Otherwise, for encryption operations the IV is provided for by the driver used to perform the operation, whereas for decryption operations the offset of the IV is provided by the crd_inject field. This flag is typically used when the IV is calculated “on the fly” by the consumer, and does not precede the data (some ipsec(4) configurations, and the encrypted swap are two such examples).
For encryption and authentication (MAC) algorithms, this bit is set when the key is explicitly provided by the consumer in the crd_key field for the given operation. Otherwise, the key is taken at newsession time from the cri_key field. As calculating the key schedule may take a while, it is recommended that often used keys are given their own session.
For compression algorithms, this bit is set when compression is required (when not set, decompression is performed).
CRD_INI
This cryptoini structure will not be modified by the framework or the device drivers. Since this information accompanies every cryptographic operation request, drivers may re-initialize state on-demand (typically an expensive operation). Furthermore, the cryptographic framework may re-route requests as a result of full queues or hardware failure, as described above.
crd_next
Point to the next descriptor. Linked operations are useful in protocols such as ipsec(4), where multiple cryptographic transforms may be applied on the same block of data.

() allocates a cryptop structure with a linked list of as many cryptodesc structures as were specified in the argument passed to it.

() deallocates a structure cryptop and any cryptodesc structures linked to it. Note that it is the responsibility of the callback routine to do the necessary cleanups associated with the opaque field in the cryptop structure.

() is called to perform a keying operation. The various fields in the cryptkop structure are:

krp_op
Operation code, such as CRK_MOD_EXP.
krp_status
Return code. This errno-style variable indicates whether lower level reasons for operation failure.
krp_iparams
Number if input parameters to the specified operation. Note that each operation has a (typically hardwired) number of such parameters.
krp_oparams
Number if output parameters from the specified operation. Note that each operation has a (typically hardwired) number of such parameters.
krp_kvp
An array of kernel memory blocks containing the parameters.
krp_hid
Identifier specifying which low-level driver is being used.
krp_callback
Callback called on completion of a keying operation.

DRIVER-SIDE API

The crypto_get_driverid(), crypto_register(), (), crypto_unregister(), crypto_unblock(), and crypto_done() routines are used by drivers that provide support for cryptographic primitives to register and unregister with the kernel crypto services framework.

Drivers must first use the () function to acquire a driver identifier, specifying the flags as an argument. One of CRYPTOCAP_F_SOFTWARE or CRYPTOCAP_F_HARDWARE must be specified. The CRYPTOCAP_F_SYNC may also be specified, and should be specified if the driver does all of it's operations synchronously.

For each algorithm the driver supports, it must then call (). The first two arguments are the driver and algorithm identifiers. The next two arguments specify the largest possible operator length (in bits, important for public key operations) and flags for this algorithm. The last four arguments must be provided in the first call to crypto_register() and are ignored in all subsequent calls. They are pointers to three driver-provided functions that the framework may call to establish new cryptographic context with the driver, free already established context, and ask for a request to be processed (encrypt, decrypt, etc.); and an opaque parameter to pass when calling each of these routines.

() is called by drivers that wish to withdraw support for an algorithm. The two arguments are the driver and algorithm identifiers, respectively. Typically, drivers for PCMCIA crypto cards that are being ejected will invoke this routine for all algorithms supported by the card. () will unregister all algorithms registered by a driver and the driver will be disabled (no new sessions will be allocated on that driver, and any existing sessions will be migrated to other drivers). The same will be done if all algorithms associated with a driver are unregistered one by one. After a call to crypto_unregister_all() there will be no threads in either the newsession or freesession function of the driver.

The calling convention for the three driver-supplied routines are:

  • int (*newsession)(device_t, uint32_t *, struct cryptoini *);
  • int (*freesession)(device_t, uint64_t);
  • int (*process)(device_t, struct cryptop *, int);
  • int (*kprocess)(device_t, struct cryptkop *, int);

On invocation, the first argument to all routines is the device_t that was provided to (). The second argument to () contains the driver identifier obtained via crypto_get_driverid(). On successful return, it should contain a driver-specific session identifier. The third argument is identical to that of crypto_newsession().

The () routine takes as arguments the opaque data value and the SID (which is the concatenation of the driver identifier and the driver-specific session identifier). It should clear any context associated with the session (clear hardware registers, memory, etc.).

The () routine is invoked with a request to perform crypto processing. This routine must not block or sleep, but should queue the request and return immediately or process the request to completion. In case of an unrecoverable error, the error indication must be placed in the crp_etype field of the cryptop structure. When the request is completed, or an error is detected, the process() routine must invoke crypto_done(). Session migration may be performed, as mentioned previously.

In case of a temporary resource exhaustion, the () routine may return ERESTART in which case the crypto services will requeue the request, mark the driver as “blocked”, and stop submitting requests for processing. The driver is then responsible for notifying the crypto services when it is again able to process requests through the () routine. This simple flow control mechanism should only be used for short-lived resource exhaustion as it causes operations to be queued in the crypto layer. Doing so is preferable to returning an error in such cases as it can cause network protocols to degrade performance by treating the failure much like a lost packet.

The () routine is invoked with a request to perform crypto key processing. This routine must not block, but should queue the request and return immediately. Upon processing the request, the callback routine should be invoked. In case of an unrecoverable error, the error indication must be placed in the krp_status field of the cryptkop structure. When the request is completed, or an error is detected, the kprocess() routine should invoked ().

RETURN VALUES

crypto_register(), crypto_kregister(), crypto_unregister(), crypto_newsession(), crypto_freesession(), and crypto_unblock() return 0 on success, or an error code on failure. crypto_get_driverid() returns a non-negative value on error, and -1 on failure. crypto_getreq() returns a pointer to a cryptop structure and NULL on failure. crypto_dispatch() returns EINVAL if its argument or the callback function was NULL, and 0 otherwise. The callback is provided with an error code in case of failure, in the crp_etype field.

FILES

sys/opencrypto/crypto.c
most of the framework code

SEE ALSO

crypto(4), ipsec(4), crypto(7), malloc(9), sleep(9)

HISTORY

The cryptographic framework first appeared in OpenBSD 2.7 and was written by Angelos D. Keromytis <angelos@openbsd.org>.

BUGS

The framework currently assumes that all the algorithms in a crypto_newsession() operation must be available by the same driver. If that is not the case, session initialization will fail.

The framework also needs a mechanism for determining which driver is best for a specific set of algorithms associated with a session. Some type of benchmarking is in order here.

Multiple instances of the same algorithm in the same session are not supported. Note that 3DES is considered one algorithm (and not three instances of DES). Thus, 3DES and DES could be mixed in the same request.

July 10, 2015 Debian