Galois/Counter Mode

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GCM mode (Galois/Counter Mode) is a mode of operation for symmetric key cryptographic block ciphers. It is an authenticated encryption algorithm designed to provide both authentication and privacy. GCM mode is defined for block ciphers with a block size of 128 bits. GMAC is an authentication-only variant of the GCM.

Contents 1 Encryption and authentication 2 Performance 3 Patents 4 See also 5 External links 6 Notes 7 References

[edit] Encryption and authentication

As the name suggests, GCM mode combines the well-known counter mode of encryption with the new Galois mode of authentication. The key feature is that the Galois field multiplication used for authentication can be easily computed in parallel thus permitting higher throughput than the authentication algorithms that use chaining modes, like CBC. The GF(2¹²⁸) field used is defined by the polynomial x¹²⁸ + x⁷ + x² + x + 1.

The GHASH function is defined by GHASH(H,A,C) = X_{m + n + 1}, where the inputs A and C, and the variables X_i for i = 0,...,m + n + 1 are defined as^[1]

GCM mode was designed by John Viega and David A. McGrew as an improvement to Carter-Wegman Counter CWC mode.

GCM mode is used in the IEEE 802.1AE (MACsec) Ethernet security, ANSI (INCITS) Fibre Channel Security Protocols (FC-SP), IEEE P1619.1 tape storage, and IETF IPSec standards.

On November 26, 2007 NIST announced the release of NIST Special Publication 800-38D Recommendation for Block Cipher Modes of Operation: Galois/Counter Mode (GCM) and GMAC making GCM and GMAC official standards.

[edit] Performance

GCM requires one block cipher operation and one 128-bit multiplication in the Galois field per each block (128 bit) of encrypted and authenticated data.

[edit] Patents

According to the authors' statement, GCM is unencumbered by patents.

[edit] See also

• Block cipher modes of operation

[edit] External links

• NIST Special Publication SP800-38D defining GCM and GMAC
• RFC 4106: The Use of Galois/Counter Mode (GCM) in IPsec Encapsulating Security Payload (ESP)
• RFC 4543: The Use of Galois Message Authentication Code (GMAC) in IPsec ESP and AH
• IEEE 802.1AE - Media Access Control (MAC) Security
• IEEE Security in Storage Working Group works on P1619.1 standard; the latest draft can be obtained from the mailing list archives.
• INCITS T11 Technical Committee works on Fiber Channel - Security Protocols project.

[edit] Notes

[edit] References

• NIST Special Publication 800-38D (November, 2007) Recommendation for Block Cipher Modes of Operation: Galois/Counter Mode (GCM) for Confidentiality and Authentication
• McGrew, David A. & Viega, John; : "The Galois/Counter Mode of Operation (GCM)", page 5. 2005

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