opencrypto is a framework for drivers of cryptographic hardware to register with the kernel so “consumers” (other kernel subsystems, and eventually users through an appropriate 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 use condition variables:
condvar(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 an 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). No callback mechanism is used for session initialization and teardown.
The
crypto_newsession() routine is called by consumers of cryptographic services (such as the
ipsec(4) stack) that wish to establish a new session with the framework. On success, the first argument will contain the Session Identifier (SID). The second argument contains all the necessary information for the driver to establish the session. The third argument indicates whether a hardware driver should be used (1) or not (0). The various fields in the
cryptoini structure are:
cri_alg
Contains an algorithm identifier. Currently supported algorithms are:
CRYPTO_DES_CBC
CRYPTO_3DES_CBC
CRYPTO_BLF_CBC
CRYPTO_CAST_CBC
CRYPTO_SKIPJACK_CBC
CRYPTO_MD5_HMAC
CRYPTO_SHA1_HMAC
CRYPTO_RIPEMD160_HMAC
CRYPTO_MD5_KPDK
CRYPTO_SHA1_KPDK
CRYPTO_AES_CBC
CRYPTO_ARC4
CRYPTO_MD5
CRYPTO_SHA1
cri_klen
Specifies the length of the key in bits, for variable-size key algorithms.
cri_rnd
Specifies the number of rounds to be used with the algorithm, for variable-round algorithms.
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. 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).
crypto_freesession() is called with the SID returned by
crypto_newsession() to disestablish the session.
crypto_dispatch() 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 length of the result, not including crd_skip. For symmetric crypto operations, this will be the same as the input length.
crp_alloctype
Indicates the type of buffer, as used in the kernel
malloc(9) routine. 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
crypto_done() 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
crypto_process() 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:
CRYPTO_F_IMBUF
The buffer pointed to by crp_buf is an mbuf chain.
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 (of a type identified by crp_alloctype), 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_skip
The offset in the input buffer where processing should start.
crd_len
How many bytes, after crd_skip, should be processed.
crd_inject
Offset from the beginning of the buffer to insert any results. For encryption algorithms, this is where the initialization vector (IV) will be inserted when encrypting or where it can be found when decrypting (subject to crd_flags). For MAC algorithms, this is where the result of the keyed hash will be inserted.
crd_flags
For adjusting general operation from userland, the following flags are defined:
CRD_F_ENCRYPT
For encryption algorithms, this bit is set when encryption is required (when not set, decryption is performed).
CRD_F_IV_PRESENT
For encryption algorithms, this bit is set when the IV already precedes the data, so the
crd_inject value will be ignored and no IV will be written in the buffer. Otherwise, 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. Some 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.
CRD_F_IV_EXPLICIT
For encryption algorithms, this bit is set when the IV is explicitly provided by the consumer in the
crd_iv fields. Otherwise, for encryption operations the IV is provided for by the driver used to perform the operation, whereas for decryption operations it is pointed to 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).
CRD_F_COMP
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.
crypto_getreq() allocates a
cryptop structure with a linked list of as many
cryptodesc structures as were specified in the argument passed to it.
crypto_freereq() 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.
crypto_kdispatch() is called to perform a keying operation. The various fields in the
crytokop structure are:
krp_op
Operation code, such as CRK_MOD_EXP.
krp_status
Return code. This errno-style variable indicates whether there were lower level reasons for operation failure.
krp_iparams
Number of input parameters to the specified operation. Note that each operation has a (typically hardwired) number of such parameters.
krp_oparams
Number of 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.
The following sysctl entries exist to adjust the behaviour of the system from userland:
kern.usercrypto
Allow (1) or forbid (0) userland access to /dev/crypto.
kern.userasymcrypto
Allow (1) or forbid (0) userland access to do asymmetric crypto requests.
kern.cryptodevallowsoft
Enable/disable access to hardware versus software operations:
< 0
Force userlevel requests to use software operations, always.
= 0
Use hardware if present, grant userlevel requests for non-accelerated operations (handling the latter in software).
> 0
Allow user requests only for operations which are hardware-accelerated.