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The International Data Encryption Algorithm (IDEA) is a block cipher developed in 1991 by James Massey and Xuejia Lai. It was intended as a replacement for the venerable DES block cipher, whose low key size was starting to become an issue.

IDEA operates on 64-bit blocks and has a 128-bit key size. Internally, the cipher consists of 8 rounds and an additional "half-round", with each round mixing bitwise XOR operations with addition and multiplication modulo 216.

Due to its simple key schedule, IDEA has known classes of weak keys which reduce the security of the cipher if used. However, such weak keys are rare enough that IDEA remains secure if keys are chosen randomly.

IDEA used to be patented in several countries, but the last patents expired in 2012. Even before that, the patent holders had granted a free license for non-commercial use. IDEA was adopted for PGP v2.0 to replace the insecure BassOMatic cipher of PGP v1, and remains an optional cipher in the OpenPGP standard.

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