This paper presents an optimal method for recovering the secret keys of the light encryption device (LED) by combining the impossible differential fault attack with the algebraic differential fault attack. The proposed optimal method effectively improves the performance of fault attacks. A fault attack model, named the redundant random nibble fault model (RRNFM), is proposed to simulate a multiple fault injection in a real-world environment. Using the optimal method and the RRNFM, the 64-bit

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... ret key of LED can be recovered by injecting no more than six faults in 1300 experiments. We establish an equation of inverse proportionality between the number of faults injected and the remaining amount of the secret key. Using the equation, the number of fault injection can be accurately predicted, providing an effective way of evaluating fault attacks. INDEX TERMS Impossible differential fault attack, LED, algebraic differential attack, redundant faults.