a key security level gradation method is proposed which is helpful to counter IEA (Iterative Encryption Attack) and provides RSA with favorable immunity against it. Firstly, we analyze the course of IEA in detail, and then present the concept of security grade of key-pair and the hierarchy of grades, and propose an algorithm that grades security grade of key-pairs. Secondly, we define the concept of attack cost, and then launch a series of experiments for the purpose of exploring the relationship between attack cost and key security grade. At last, it is demonstrated that if to choose keys properly by this method RSA is improved to be effective to counter IEA in aspect of computation power.

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In this paper, we pioneer a key security level gradation scheme which is proved to efficient to counteract Iterative-Encrypting-Attack against RSA. And we make it clear that the bug which hides after the traditional key generation algorithm is exploited by Iterative-Encrypting-Attack and weakens RSA security, and that the case can be improved if the traditional key generation algorithm is modified delicately. After analyzing the preliminary and practical steps of Iterative Encrypting Attack in detail, we propose the concept of security grade of key-pair and depict the hierarchy of grades systematically, and bring forward an algorithm that grades security grade of key pairs. Furthermore, we introduce the concept of attack cost into the gradation prototype, then program for grading algorithm and set in motion a series of experiments for surveying the relationship between attack-cost and key-security-grade. At last, by the attained result from experimental statistics, we point it out  that if key-pair is properly chosen RSA system can acquire a satisfying immunity from Iterative Encrypting Attack