The operation is very similar to the symmetric cipher discussion. During initialization, the struct sockaddr data structure must be filled as follows:

struct sockaddr_alg sa = {
	.salg_family = AF_ALG,
	.salg_type = "aead", /* this selects the symmetric cipher */
	.salg_name = "gcm(aes)" /* this is the cipher name */
};
    

Before data can be sent to the kernel using the write/send system call family, the consumer must set the key. The key setting is described with the setsockopt invocation below.

In addition, before data can be sent to the kernel using the write/send system call family, the consumer must set the authentication tag size. To set the authentication tag size, the caller must use the setsockopt invocation described below.

Using the sendmsg() system call, the application provides the data that should be processed for encryption or decryption. In addition, the IV is specified with the data structure provided by the sendmsg() system call.

The sendmsg system call parameter of struct msghdr is embedded into the struct cmsghdr data structure. See recv(2) and cmsg(3) for more information on how the cmsghdr data structure is used together with the send/recv system call family. That cmsghdr data structure holds the following information specified with a separate header instances:

The send system call family allows the following flag to be specified:

Note: The kernel reports -EINVAL for any unexpected data. The caller must make sure that all data matches the constraints given in /proc/crypto for the selected cipher.

With the recv() system call, the application can read the result of the cipher operation from the kernel crypto API. The output buffer must be at least as large as defined with the memory structure below. If the output data size is smaller, the cipher operation is not performed.

The authenticated decryption operation may indicate an integrity error. Such breach in integrity is marked with the -EBADMSG error code.