In accordance with the present invention, computer implemented methods and systems are provided for converting an elastic block cipher from an original block cipher. The elastic block cipher accepts variable length inputs where the computation workload increases in proportion to the block size. To generate the elastic block cipher, the number of rounds in the block cipher is increased such that the round function is applied to each bit position at least the same number of times as the original block cipher. In addition, bits that are beyond the normal block size are left out of the round function and XORed and swapped with bits output from the round function to become part of the input to the next round.