Scramble algorithms，which are also called pure-position permutation algorithms，are commonly used as an image ancryption，are investigated in this paper．Unfortunately, they are frail under known-text attack．According to the weakness of pure position permutation algorithm,We put forward an effective decryption algorithm for aIl pure-position permutation algorithms．First,a summary of the pure position permutation image encryption algorithms is given by introducing the concept ofergodic nieces．Secondly,by using probability theory and algebra knowledge，the declyption probabilit：of pure-position permutation algorithms is verified theoretically． Then，by defining operation system of fuzzy ergodic matrices，we improve a specific decryption algorithm．Finally some simulation results are shown.

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