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IDEA
An encryption round of IDEA
General
Designer(s):	James Massey, Xuejia Lai
First published:	1991
Derived from:	PES
Successor(s):	MMB, MESH, Akelarre, IDEA NXT (FOX)
Cipher detail
Key size(s):	128 bits
Block size(s):	64 bits
Structure:	Substitution-permutation network
Rounds:	8.5
Best public 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 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 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 2010–2011. Today, IDEA is licensed worldwide 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.

IDEA operates on 64-bit blocks using a 128-bit key, and consists of a series 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 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 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 ).

After the eight rounds comes a final "half round", the output transformation illustrated below:

The first 6 keys-parts (K₁ through K₆) for the first round are taken directly as the first 6 consecutive blocks of 16 bits (MSB being the first). This means that only 96 of the 128 bits are used in each round. After each round, including before the concluding half-round, the 128 bit key undergoes a 25 bit rotation to the left, i.e. the LSB becomes the 25th LSB. Then the 6 key-parts are again taken from the first 6 consecutive blocks of 16 bits.^[2]

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 can break IDEA reduced to 5 rounds (the full IDEA cipher uses 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].

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, 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.

1. ^ Garfinkel, Simson (December 1, 1994). PGP: Pretty Good Privacy. O'Reilly Media, pp.101–102. ISBN 978-1565920989. 
2. ^ Schneier, Bruce (1996). Applied Cryptography, Second, John Wiley & Sons. 

• 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.

• RSA FAQ on Block Ciphers
• Mediacrypt homepage — IDEA licensor
• SCAN entry for IDEA
• Helger Lipmaa's links for IDEA
• IDEA in 448 bytes of 80x86