A note on Cook's elastic block cipher

ABSTRACT

"VIL Block Cipher" is a kind of block cipher that supports Variable Input-Lengths. It was first proposed by Bellare, etc. and since then several constructions have been given, among which Cook's elastic block cipher is a special one. In this paper we present a security model called "MIL model" for VIL block ciphers, investigating their security when a fixed secret key is used for multiple input-lengths. Our results show that if the key schedule is not well designed, Cook's elastic block cipher is vulnerable when processing multiple-length inputs under a fixed secret key. Thus, further considerations are needed to use Cook's elastic block cipher safely in practice.

REFERENCES

• 1: Mihir Bellare , Phillip Rogaway, On the Construction of Variable-Input-Length Ciphers, Proceedings of the 6th International Workshop on Fast Software Encryption, p.231-244, March 24-26, 1999
• 2: Debra Lee Cook , Angelos Keromytis, Elastic block ciphers, Columbia University, New York, NY, 2006
• 3: Debra Cook , Angelos Keromytis , Moti Yung, Elastic block ciphers: the basic design, Proceedings of the 2nd ACM symposium on Information, computer and communications security, March 20-22, 2007, Singapore [doi>10.1145/1229285.1229324]
• 4: D. Cook, M. Yung, and A. Keromytis. Elastic Block Ciphers in Practice: Constructions and Modes of Encyption. In European Conference on Computer Network Defense 2007. http://2007.ec2nd.org/abstracts/cook-abstract.html.
• 5: Debra L. Cook , Moti Yung , Angelos D. Keromytis, The Security of Elastic Block Ciphers Against Key-Recovery Attacks, Proceedings of the 10th international conference on Information Security, October 09-12, 2007, Valparafso, Chile [doi>10.1007/978-3-540-75496-1_6]
• 6: Debra L. Cook , Moti Yung , Angelos D. Keromytis, Methods for Linear and Differential Cryptanalysis of Elastic Block Ciphers, Proceedings of the 13th Australasian conference on Information Security and Privacy, p.187-202, July 07-09, 2008, Wollongong, Australia [doi>10.1007/978-3-540-70500-0_14]
• 7: Michael Luby , Charles Rackoff, How to construct pseudorandom permutations from pseudorandom functions, SIAM Journal on Computing, v.17 n.2, p.373-386, April 1988 [doi>10.1137/0217022]
• 8: S. Patel, Z. Ramzan, and G. S. Sundaram. Efficient constructions of variable-input-length block ciphers. In Proceedings of Selected Areas in Cryptography, LNCS 3357, Springer-Verlag, 2004.

INDEX TERMS

Primary Classification:
  E. Data
  E.0 GENERAL

General Terms:
Algorithms, Security

Keywords:
block cipher, elastic block cipher, encryption, pseudorandomness, security model, variable-input-length block cipher