Weak keys of IDEA
Joan Daemen, René Govaerts, Joos P. L. Vandewalle · 1993
Abstract. Large classes of weak keys have been found for the block cipher algorithm IDEA, previously known as IPES [2]. IDEA has a 128-bit key and encrypts blocks of 64 bits. For a class of 223 keys IDEA exhibits a linear factor. For a certain class of 235 keys the cipher has a global characteristic with probability 1. For another class of 251 keys only two encryptions and solving a set of 16 nonlinear boolean equations with 12 variables is su±cient to test if the used key belongs to this class. If it does, its particular value can be calculated e±ciently. It is shown that the problem of weak keys can be eliminated by slightly modifying the key schedule of IDEA. 1