A method and apparatus for cryptographically converting a digital input data block into a digital output data block. The apparatus has an input for supplying the input data block and a further input for supplying a code conversion digital key K1. Cryptographic processing merges a selected part M1 of the digital input data block with the key K1 to produce a data block B1 that is non-linearly dependent on M1 and K1. The merging is performed in one sequentially inseparable step. The digital output block is derived from a selected part of the data block B1.