NICK HOFFMAN

Abstract. In this paper, a simpliﬁed version of the International Data Encryption Algorithm (IDEA) is described. This simpliﬁed version, like simpliﬁed versions of DES [8] [12] and AES [6] [7] that have appeared in print, is intended to help students understand the algorithm by providing a version that permits examples to be worked by hand. IDEA is useful teaching tool to help students bridge the gap between DES and AES.

1. Introduction The International Data Encryption Algorithm (IDEA) is a symmetric-key, block cipher. It was published in 1991 by Lai, Massey, and Murphy [3]. IDEA is a modiﬁcation of the Proposed Encryption Standard (PES) that was published in 1990 by Lai and Massy [1]; PES was designed as a replacement for the Data Encryption Standard (DES). The algorithm was modiﬁed and published in 1991 after Biham and Shamir described the technique of diﬀerential cryptanalysis. The new algorithm was called the Improved Proposed Encryption Standard (IPES); its name changed to IDEA in 1992. IDEA is a candidate block cipher to the NESSIE Project. NESSIE is a project within the Information Societies Technology (IST) Program of the European Commission [3]. In the Second Edition (1996) of Applied Cryptography Bruce Schneier [9] describes IDEA as “... the best and most secure block algorithm available to the public at this time;” however, in 1999 [10] he began to recommend newer algorithms because IDEA “...isn’t very fast ... [and] IDEA is patented.” Although IDEA did not replace DES, it was incorporated into Pretty Good Privacy (PGP). The algorithm is patented and licensed by MediaCrypt. MediaCrypt now oﬀers a successor algorithm IDEA NXT. 2. Description of the Encryption Algorithm IDEA encrypts a 64-bit block of plaintext to 64-bit block of ciphertext. It uses a 128-bit key. The algorithm consists of eight identical rounds and a “half” round ﬁnal transformation. Today, because of 128-bit cryptosystems like AES, IDEA...

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