What is the Galois/Counter Mode (GCM)?

The Galois/Counter Mode (GCM) is a mode of operation for symmetric key block ciphers that provides both confidentiality and authenticity of data. It combines the Counter (CTR) mode of encryption with the Galois Field (GF) multiplication to achieve these security goals.

In GCM, a unique nonce (number used once) and a counter are combined to generate a unique encryption key for each block of data. This ensures that even if the same plaintext is encrypted multiple times, the resulting ciphertext will be different. The counter is incremented for each block, allowing parallel processing and efficient encryption of large amounts of data.

GCM also incorporates an authentication mechanism called the Galois Message Authentication Code (GMAC). GMAC uses the Galois Field multiplication to generate a tag, which is appended to the ciphertext. This tag provides integrity and authenticity of the data, ensuring that it has not been tampered with during transmission or storage.

The advantages of GCM include its efficiency, as it can be parallelized and implemented in hardware, and its ability to provide both confidentiality and authenticity in a single operation. It is widely used in modern cryptographic protocols and applications, such as secure communication protocols (e.g., TLS) and disk encryption.