Galois/Counter Mode

In <a href="/facts/Cryptography/e94PEVau">cryptography</a>, Galois/Counter Mode (GCM) is a <a href="/facts/Block_cipher_mode_of_operation/vIUabB2z">mode of operation</a> for <a href="/facts/Symmetric-key_algorithm/5zfe0b76">symmetric-key</a> cryptographic <a href="/facts/Block_cipher/MJywAyfQ">block ciphers</a> which is widely adopted for its performance. GCM throughput rates for state-of-the-art, high-speed communication channels can be achieved with inexpensive hardware resources. 
The GCM algorithm provides both data authenticity (integrity) and confidentiality and belongs to the class of <a href="/facts/Authenticated_encryption/u3229wI2">authenticated encryption with associated data (AEAD)</a> methods. This means that as input it takes a key K, some plaintext P, and some associated data AD; it then encrypts the plaintext using the key to produce ciphertext C, and computes an authentication tag T from the ciphertext and the associated data (which remains unencrypted). A recipient with knowledge of K, upon reception of AD, C and T, can decrypt the ciphertext to recover the plaintext P and can check the tag T to ensure that neither ciphertext nor associated data were tampered with.
GCM uses a block cipher with block size 128 bits (commonly <a href="/facts/AES-128/HdXGBY2Q">AES-128</a>) operated in <a href="/facts/Counter_mode/vIUabB2z">counter mode</a> for encryption, and uses arithmetic in the <a href="/facts/Galois_field/UkMGJo3m">Galois field</a> GF(2128) to compute the authentication tag; hence the name.
Galois Message Authentication Code (GMAC) is an authentication-only variant of the GCM which can form an incremental <a href="/facts/Message_authentication_code/mXxCP5Yl">message authentication code</a>. Both GCM and GMAC can accept <a href="/facts/Initialization_vector/qlVQjCS3">initialization vectors</a> of arbitrary length.
Different block cipher modes of operation can have significantly different performance and efficiency characteristics, even when used with the same block cipher. GCM can take full advantage of <a href="/facts/Parallel_processing_(computing)/nQzcpzQt">parallel processing</a> and implementing GCM can make efficient use of an <a href="/facts/Instruction_pipeline/3zy6CcX1">instruction pipeline</a> or a hardware pipeline. By contrast, the <a href="/facts/Cipher_block_chaining/vIUabB2z">cipher block chaining</a> (CBC) mode of operation incurs <a href="/facts/Pipeline_stall/e3Ee0yJg">pipeline stalls</a> that hamper its efficiency and performance.

Galois/Counter Mode open-in-new

Galois/Counter Mode