High Quality Content by WIKIPEDIA articles! Galois/Counter Mode (GCM) is a mode of operation for symmetric key cryptographic block ciphers that has been widely adopted because of its efficiency and performance. GCM throughput rates for state of the art, high speed communication channels can be achieved with reasonable hardware resources . It is an authenticated encryption algorithm designed to provide both data authenticity (integrity) and confidentiality. GCM mode is defined for block ciphers with a block size of 128 bits. GMAC is an authentication-only variant of the GCM which can be used as an incremental message authentication code. Both GCM and GMAC can accept initialization vectors of arbitrary length.