Some embodiments provide a method for performing a block cryptographic operation that includes a plurality of rounds. The method receives a message that includes several blocks. The method selects a set of the blocks. The set has a particular number of blocks. The method applies a cryptographic operation to the selected set of blocks. A particular round of the cryptographic operation for a first block in the set is performed after a later round than the particular round for a second block in the set, while a different particular round for the first block is performed before an earlier round than the different particular round for the second block. In some embodiments, at least two rounds for the first block are performed one after the other without any intervening rounds for any other blocks in the set.