Square is one of the latest Multivariate Public Key Cryptosystem (MPKC) for encryption, proposed by Clough, et al. [1]. Its trapdoor is based on the discovery of Ding et al. [2] that the time complexity of cryptanalysis of the Hidden Field Encryption (HFE) increases steeply as the characteristic of the field grows. After it was attacked, Clough et al. [3] proposed to reinforce it into the "Double-Layer Square." Now we show that it is possible to eliminate the reinforcement by Rank Attack.
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