International Data Encryption Algorithm
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An encryption round of IDEA 

General  

Designers  Xuejia Lai and James Massey 
Derived from  PES 
Successors  MMB, MESH, Akelarre, IDEA NXT (FOX) 
Cipher detail  
Key sizes  128 bits 
Block sizes  64 bits 
Structure  Substitutionpermutation network 
Rounds  8.5 
Best public cryptanalysis  
A highorder differentiallinear attack requiring 2^{64}2^{52} chosen plaintexts breaks 6 rounds with a complexity of 2^{126.8} encryptions (Biham et al, 2007). 
In cryptography, the International Data Encryption Algorithm (IDEA) is a block cipher designed by Xuejia Lai and James Massey of ETH Zurich and was first described in 1991. The algorithm was intended as a replacement for the Data Encryption Standard. IDEA is a minor revision of an earlier cipher, PES (Proposed Encryption Standard); IDEA was originally called IPES (Improved PES).
The cipher was designed under a research contract with the Hasler Foundation, which became part of AscomTech AG. The cipher is patented in a number of countries but is freely available for noncommercial use. The name "IDEA" is also a trademark. The patents will expire in 2010–2011. Today, IDEA is licensed in all the countries where it is patented by MediaCrypt.
IDEA was used in Pretty Good Privacy (PGP) v2.0, and was incorporated after the original cipher used in v1.0, BassOmatic, was found to be insecure.^{[1]} IDEA is an optional algorithm in the OpenPGP standard.
Contents 
[edit] Operation
IDEA operates on 64bit blocks using a 128bit key, and consists of a series of eight identical transformations (a round, see the illustration) and an output transformation (the halfround). The processes for encryption and decryption are similar. IDEA derives much of its security by interleaving operations from different groups — modular addition and multiplication, and bitwise eXclusive OR (XOR) — which are algebraically "incompatible" in some sense. In more detail, these operators, which all deal with 16bit quantities, are:
 Bitwise eXclusive OR (denoted with a blue ⊕).
 Addition modulo 2^{16} (denoted with a green ).
 Multiplication modulo 2^{16}+1, where the allzero word (0x0000) is interpreted as 2^{16} (denoted by a red ).
After the eight rounds comes a final "half round", the output transformation illustrated below:
[edit] Key schedule
Each round uses six subkeys, while the halfround uses four. Each subkey is a sixteenbit value. The first eight subkeys are extracted directly from the key, with K1 from the first round being the lower sixteen bits; further groups of eight keys are created by rotating the main key left 25 bits between each group of eight. This means that it is rotated less than once per round, on average, for a total of six rotations.
[edit] Security
The designers analysed IDEA to measure its strength against differential cryptanalysis and concluded that it is immune under certain assumptions. No successful linear or algebraic weaknesses have been reported. Some classes of weak keys have been found — E.g. (Daemen et al, 1994) — but these are of little concern in practice, being so rare as to be unnecessary to avoid explicitly. As of 2007^{[update]}, the best attack which applies to all keys can break IDEA reduced to 6 rounds (the full IDEA cipher uses 8.5 rounds)^{[2]}.
Bruce Schneier thought highly of IDEA in 1996, writing, "In my opinion, it is the best and most secure block algorithm available to the public at this time." (Applied Cryptography, 2nd ed.) However, by 1999 he was no longer recommending IDEA due to the availability of faster algorithms, some progress in its cryptanalysis, and the issue of patents [1].
IDEA is patented in Austria, France, Germany, Italy, Netherlands, Spain, Sweden, Switzerland, United Kingdom, (European patent EPB0482154), the United States (US patent #5,214,703, issued May 25, 1993) and Japan (JP 3225440).
MediaCrypt is now also offering a successor to IDEA and focuses on its new cipher (official release on May 2005) IDEA NXT, which was previously called FOX.
[edit] References
 ^ Garfinkel, Simson (December 1, 1994). PGP: Pretty Good Privacy. O'Reilly Media. pp. pp.101–102. ISBN 9781565920989.
 ^ Biham, E. and Dunkelman, O. and Keller, N. "A New Attack on 6Round IDEA". SpringerVerlag.
 J. Daemen, R. Govaerts, and J. Vandewalle, Weak keys for IDEA, CRYPTO '93. pp224–231.
 Hüseyin Demirci, Erkan Türe, Ali Aydin Selçuk, A New Meet in the Middle Attack on The IDEA Block Cipher, 10th Annual Workshop on Selected Areas in Cryptography, 2004.
 Xuejia Lai and James L. Massey, A Proposal for a New Block Encryption Standard, EUROCRYPT 1990, pp389–404
 Xuejia Lai and James L. Massey and S. Murphy, Markov ciphers and differential cryptanalysis, Advances in Cryptology — Eurocrypt '91, SpringerVerlag (1992), pp17–38.
 Eli Biham, Orr Dunkelman, Nathan Keller, A New Attack on 6round IDEA, Fast Software Encryption Workshop, 2007.