Differential-Linear Weak Key Classes of IDEA

Large weak key classes of IDEA are found for which membership is tested with a differential-linear test while encrypting with a single key. In particular, one in every 265 keys for 8.5-round IDEA is weak. A related-key differential-linear attack on 4-round IDEA is presented which is successful for all keys. Large weak key classes are found for 4.5- to 6.5-round and 8-round IDEA for which membership of these classes is tested using similar related-key differential-linear tests.

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