International Data Encryption Algorithm

IDEA
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
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. It is a minor revision of an earlier cipher, PES (Proposed Encryption Standard); IDEA was originally called IPES (Improved PES). IDEA was used as the symmetric cypher in early versions of the Pretty Good Privacy cryptosystem.

IDEA was designed under a research contract with the Hasler Foundation, which became part of Ascom-Tech AG. IDEA is patented (US patent 5,214,703) but is free for non-commercial use. The patents will expire in 2010–2011.

How it works

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 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 '+' in a circle).
• Addition modulo 2¹⁶ (denoted with a green '+' in a box).
• Multiplication modulo 2¹⁶+1, where the all-zero word (0x0000) is interpreted as 2¹⁶ (denoted by a red dotted circle).

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 .

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.
• X. Lai, J.L. Massey and S. Murphy, Markov ciphers and differential cryptanalysis, Advances in Cryptology — Eurocrypt '91, Springer-Verlag (1992), pp17–38.