SSL acceleration

SSL acceleration (TLS acceleration) is a method of offloading processor-intensive public-key encryption for Transport Layer Security (TLS) and its predecessor Secure Sockets Layer (SSL) to a hardware accelerator.

Typically this means having a separate card that plugs into a PCI slot in a computer that contains one or more co-processors able to handle much of the SSL processing.

SSL accelerators may use off the shelf CPUs, but most use custom ASICs and RISC chips to do most of the difficult computational work.

The most computationally expensive part of an SSL session is the SSL handshake, where the SSL server (usually an SSL webserver) and the SSL client (usually a web browser) agree on a number of parameters that establish the security of the connection.

Part of the role of the SSL handshake is to agree on session keys (symmetric keys, used for the duration of a given session), but the encryption and signature of the SSL handshake messages itself is done using asymmetric keys (contained in the certificates), which requires more computational power than the symmetric cryptography used for the encryption/decryption of the session data.

Typically a hardware SSL accelerator will offload processing of the SSL handshake while leaving it to the server software to process the less intense symmetric cryptography of the actual SSL data exchange, but some accelerators handle all SSL operations and terminate the SSL connection, thus leaving the server seeing only unencrypted connections.

Modern x86 CPUs support Advanced Encryption Standard (AES) encoding and decoding in hardware, using the AES instruction set proposed by Intel in March 2008.

Allwinner Technology provides a hardware cryptographic accelerator in its A10, A20, A30 and A80 ARM system-on-chip series, and all ARM CPUs have acceleration in the later ARMv8 architecture. The accelerator provides the RSA public-key algorithm, several widely used symmetric-key algorithms, cryptographic hash functions, and a cryptographically secure pseudo-random number generator.

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