Despite their widespread usage in block cipher security, linear and differential cryptanalysis still lack a robust treatment of their success probability, and the success chances of these attacks have commonly been estimated in a rather ad hoc fashion. In this paper, we present an analytical calculation of the success probability of linear and differential cryptanalytic attacks. The results apply to an extended sense of the term "success" where the correct key is found not necessarily as the highest-ranking candidate but within a set of high-ranking candidates. Experimental results show that the analysis provides accurate results in most cases, especially in linear cryptanalysis. In cases where the results are less accurate, as in certain cases of differential cryptanalysis, the results are useful to provide approximate estimates of the success probability and the necessary plaintext requirement. The analysis also reveals that the attacked key length in differential cryptanalysis is one of the factors that affect the success probability directly besides the signal-to-noise ratio and the available plaintext amount. © 2007 International Association for Cryptologic Research.