Abstract

1 Introduction With its 56-bit keys, the susceptibility of DES to exhaustive key search has been a concern and a complaint since the cipher was first made public; see, for example, [6]. The problem has escalated to the point that the Electronic Frontier Foundation has now built a DES cracking machine, at a cost of less than 250,000 USD, that can find the right key in about three days. There have been many approaches suggested for reducing DES's vulnerability to exhaustive key search. One is to construct a DES-based block cipher which employs a longer key. Triple DES (typically in "EDE mode") is the best-known algorithm in this vein. It seems to be quite secure, but efficiency considerations make triple DES a rather painful way to solve the exhaustive key-search problem. Specifically, triple-DES encryption/decryption requires multiple DES encryptions/decryptions. This paper analyzes a much cheaper alternative.

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