Crypto API is a cryptography framework in the Linux kernel, for various parts of the kernel that deal with cryptography, such as IPsec and dm-crypt. It was introduced in kernel version 2.5.45 and has since expanded to include essentially all popular block ciphers and hash functions.
Userspace interfaces
Many platforms that provide hardware acceleration of AES encryption expose this to programs through an extension of the instruction set architecture (ISA) of the various chipsets (e.g. AES instruction set for x86). With this sort of implementation any program (kernel-mode or user-space) may utilize these features directly.
Some platforms, such as the ARM Kirkwood SheevaPlug and AMD Geode processors, however, are not implemented as ISA extensions, and are only accessible through kernel-mode drivers. In order for user-mode applications that utilize encryption, such as OpenSSL or GnuTLS, to take advantage of such acceleration, they must interface with the kernel.