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. As of 2007, the best attack which applies to all keys can break IDEA reduced to 6 rounds (the full IDEA cipher uses 8.5 rounds). Note that a "break" is any attack which requires less than 2128 operations; the 6-round attack requires 264 known plaintexts and 2126.8 operations.
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.
Read more about this topic: International Data Encryption Algorithm
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