📝Galois/Counter Mode (GCM) and GMAC

tags

§ Cryptography Block cipher mode of operation

RFC

RFC 5288 - AES Galois Counter Mode (GCM) Cipher Suites for TLS

  • Encryption is the same as Counter (CTR) mode of operation mode but adds Galois field multiplication to compute message authentication code.
  • MAC algorithm
    • encrypt 128 bits of zeros, run through GHASH
    • xor/ghash with non-encrypted AD (Authentication Data) (optional)
    • xor/ghash with ciphertext
    • xor/ghash with concatenated length of AD (64 bit) and Plaintext (64 bit)
    • finally, xor with encrypted initialization vector + counter 0
    • the result is authentication tag
  • GHASH is a multiplication (in Galois Field) by H (= E_k(0^128) ) (encrypted zeros)

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