M6 (cipher)

M6
General
Designers	Hitachi
First published	1997
Successors	M8
Cipher detail
Key sizes	40–64 bits
Block sizes	64 bits
Structure	Feistel network
Rounds	10
Best public cryptanalysis
	Mod n cryptanalysis: 1 known plaintext allows recovering the key with about 235 trial encryptions; "a few dozen" known plaintexts reduces this to about 231

In cryptography, M6 is a block cipher proposed by Hitachi in 1997 for use in the IEEE 1394 FireWire standard. The design allows some freedom in choosing a few of the cipher's operations, so M6 is considered a family of ciphers.

The algorithm operates on blocks of 64 bits using a 10-round Feistel network structure. The key size is 40 bits by default, but can be up to 64 bits. The key schedule is very simple, producing two 32-bit subkeys: the high 32 bits of the key, and the sum mod 2³² of this and the low 32 bits.

Because its round function is based on rotation and addition, M6 was one of the first ciphers attacked by mod n cryptanalysis. Mod 5, about 100 known plaintexts suffice to distinguish the output from a pseudorandom permutation. Mod 257, information about the secret key itself is revealed. One known plaintext reduces the complexity of a brute force attack to about 2³⁵ trial encryptions; "a few dozen" known plaintexts lowers this number to about 2³¹. Due to its simple key schedule, M6 is also vulnerable to a slide attack, which requires more known plaintext but less computation.

References[]

John Kelsey, Bruce Schneier, David Wagner (March 1999). Mod n Cryptanalysis, with Applications Against RC5P and M6 (PDF/PostScript). 6th International Workshop on Fast Software Encryption (FSE '99). Rome: Springer-Verlag. pp. 139–155. Retrieved 25 January 2007.CS1 maint: multiple names: authors list (link)
David Wagner, Alex Biryukov (1999). "Slide Attacks" (PostScript). Retrieved 25 January 2007. Cite journal requires |journal= (help)

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