Calculating the number of rounds required for block cipher with known differential Suppose we have a 128-bit block cipher for which the best two-round differential holds with probability $p = 2^7$. Suppose as well that this differential is iterative and that the best differential attack will be formed when using this differential.

How many rounds would the block cipher need in order to be resistant to differential cryptanalysis?
I think that you typed the probability of two rounds incorrect and it should be $2^{-7}$. By assuming that, you should choose the numbers of rounds in a way that the number of (chosen plaintexts, chosen ciphertext), exceeds of the number of  (plaintext, ciphertext) that are available(for example with today's computational devices is bounded). As you told the probability for two rounds, if you choose 15 rounds, the number of (chosen plaintexts, chosen Ciphertexts) will be about $15 \times 7 = 105$, that we can use it as a bound for this. You should be considered that it is a theoretical bound and choosing the number of rounds based on 2 rounds estimation is somehow not precious.