When using the one-shot API for symmetric ciphers, AEAD ciphers, as well as message digests, the library uses the zero copy interface to provide the input data to the kernel. That means, the kernel operates on the user space pages.

To ensure the efficiency of this zero copy approach, the caller should use a page-aligned data buffer for the input data. Non-aligned buffers would work also, but the kernel would need to perform more page accesses, lowering the throughput. Such an aligned buffer can be created, for example, using the following call - the value 4096 should be the size of one page on the system:

    unsigned char buf[4096] __attribute__((__aligned__(4096)));
   

    unsigned char *buf;
    posix_memalign((void *)&buf, PAGE_SIZE, buflen);