A data processing apparatus includes a processing circuit configured to segment an original bit sequence into a plurality of input blocks each having a first block length, create a random bit sequence, encrypt respective ones of the plurality of input blocks with a block cipher scheme using a key (having a key length larger than the first block length) based on the random bit sequence to derive intermediate blocks, concatenate every predetermined number of intermediate blocks to form one or more output blocks, compute an invertible transformation function on the one or more output blocks and the random bit sequence to derive a further output block, and output an encrypted bit sequence including those output blocks.