User:Matt Crypto/MediaWiki:Block ciphers

Two rounds of the XTEA block cipher

In cryptography, XTEA (eXtended TEA) is a block cipher designed to correct weaknesses in TEA. The cipher's designers were David Wheeler and Roger Needham of the Cambridge Computer Laboratory, and the algorithm was presented in an unpublished technical report in 1997 (Needham and Wheeler, 1997). It is not subject to any patents.

Like TEA, XTEA is a 64-bit block Feistel network with a 128-bit key and a suggested 64 rounds. Several differences from TEA are apparent, including a somewhat more complex key-schedule and a rearrangement of the shifts, XORs and additions.

Presented along with XTEA was a variable-width block cipher termed Block TEA, which uses the XTEA round function but applies it cyclically across an entire message for several iterations. Because it operates on the entire message, Block TEA has the property that it does not need a mode of operation. An attack on the full Block TEA was described in (Saarinen, 1998), which also details a weakness in Block TEA's successor, XXTEA.

As of 2004, the best attack reported on XTEA is a related-key differential attack on 26 out of 64 rounds of XTEA, requiring 2^20.5 chosen plaintexts and a time complexity of 2^115.15 (Ko et al, 2004).

References

Youngdai Ko, Seokhie Hong, Wonil Lee, Sangjin Lee, and Jongin Lim. Related key differential attacks on 26 rounds of XTEA and full rounds of GOST. In Proceedings of FSE '04, Lecture Notes in Computer Science, 2004. Springer-Verlag.
Roger M. Needham and David J. Wheeler. Tea extensions. Technical report, Computer Laboratory, University of Cambridge, October 1997.
Markku-Juhani Saarinen. Cryptanalysis of block tea. Unpublished manuscript, October 1998. URL [1].

External Links

Block ciphers