Method and system for cryptographic transformation of a structured data set. The structured data set is partitioned into a first subset and a plurality of further subsets for encryption in parallel. The subsets are divided into a plurality of blocks of predetermined size. A first block for each subset is identified as well as a location of each further block in said subset relative to said first block of its subset. Cryptographic transformation of the data subsets is performed using a key according to a block chain process and an offset value for the first block of each subset from the first block of the first subset is logged. The process allows a block chain to be broken into part way in the chain. The process may allow different partitioning to be used for decryption than was used for encryption, thereby allowing parallel processing on varying numbers of computational cores.