AbstractThe zipper hash utilizes two‐pass hashing to strengthen the iterated hash functions against the generic attack. In this paper, we analyze the features of zipper hash and several existing generic attacks on hash functions. A new tree structure called inverse‐diamond, which starts from one fixed point and ends with many points, is exploited to guarantee that the corresponding message blocks in the two passes be identical. Then, combining the inverse‐diamond structure of depth l, with the multicollision of length n – l (n is bit number of the hash value) and the (k, 2k + k − 1)‐expandable message together, we firstly present a second preimage attack on zipper hash of which the time complexity is about O((2 k + n)2n/2 + 2n − k + (n − l)2n − l + 2l + 1), less than O(2n), and the memory complexity is about O(2k + 1 + 3*2l). Specially, if k = l = n/2, then the time complexity is about O(n2n/2), and the memory complexity is about O(2n/2). Copyright © 2015 John Wiley & Sons, Ltd.