# International Data Encryption Algorithm

IDEA
 Missing imageInternational_Data_Encryption_Algorithm_InfoBox_Diagram.png An encryption round of IDEA
General
Designer(s) James Massey, Xuejia Lai
First published 1991
Derived from PES
Cipher(s) based on this design MESH, Akelarre
Algorithm detail
Block size(s) 64 bits
Key size(s) 128 bits
Structure Substitution-permutation network
Number of rounds 8.5
Best cryptanalysis
A collision attack requiring 224 chosen plaintexts breaks 5 rounds with a complexity of 2126 (Demirci et al, 2003).

In cryptography, the International Data Encryption Algorithm (IDEA) is a block cipher designed by Xuejia Lai and James L. Massey of ETH-Zürich 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 Ascom-Tech AG. The cipher is patented in a number of countries but is freely available for non-commercial use. The name "IDEA" is also a trademark. The patents will expire in 20102011.

IDEA was used in Pretty Good Privacy (PGP) V2.0, and was incorporated after the original cipher used in v1.0 ("Bass-O-Matic") was found to be insecure. It is an optional algorithm in OpenPGP.

 Contents

## Operation

IDEA operates on 64-bit blocks using a 128-bit key, and consists of eight identical transformations (a round, see the illustration) and an output transformation (the half-round). The processes for encryption and decryption are similar. IDEA derives much of its security by interleaving operations from different groupsmodular addition and multiplication, and bitwise eXclusive OR (XOR) — which are algebraically "incompatible" in some sense. In more detail, these operators, which all deal with 16-bit quantities, are:

• Bitwise eXclusive OR (denoted with a blue ⊕).
• Addition modulo 216 (denoted with a green ).
• Multiplication modulo 216+1, where the all-zero word (0x0000) is interpreted as 216 (denoted by a red Missing image
Odot.png
odot

).

## 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 2004, the best attack which applies to all keys breaks 5 out of 8.5 rounds (Demirci et al, 2003).

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] (http://slashdot.org/interviews/99/10/29/0832246.shtml).

IDEA is patented in Austria, France, Germany, Italy, Netherlands, Spain, Sweden, Switzerland, United Kingdom, (European patent EP-B-0482154), the United States (US patent #5,214,703 (http://patft.uspto.gov/netacgi/nph-Parser?u=/netahtml/srchnum.htm&Sect1=PTO1&Sect2=HITOFF&p=1&r=1&l=50&f=G&d=PALL&s1=5214703.WKU.&OS=PN/5214703&RS=PN/5214703)) and Japan (JP 3225440).

## References

• 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, 2003.
• 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, Springer-Verlag (1992), pp17–38.

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