Asynchronous block cipher API is used with the ciphers of type CRYPTO_ALG_TYPE_ABLKCIPHER (listed as type “ablkcipher” in /proc/crypto).

Asynchronous cipher operations imply that the function invocation for a cipher request returns immediately before the completion of the operation. The cipher request is scheduled as a separate kernel thread and therefore load-balanced on the different CPUs via the process scheduler. To allow the kernel crypto API to inform the caller about the completion of a cipher request, the caller must provide a callback function. That function is invoked with the cipher handle when the request completes.

To support the asynchronous operation, additional information than just the cipher handle must be supplied to the kernel crypto API. That additional information is given by filling in the ablkcipher_request data structure.

For the asynchronous block cipher API, the state is maintained with the tfm cipher handle. A single tfm can be used across multiple calls and in parallel. For asynchronous block cipher calls, context data supplied and only used by the caller can be referenced the request data structure in addition to the IV used for the cipher request. The maintenance of such state information would be important for a crypto driver implementer to have, because when calling the callback function upon completion of the cipher operation, that callback function may need some information about which operation just finished if it invoked multiple in parallel. This state information is unused by the kernel crypto API.