In this paper, a novel block cipher [5,8,13] based on a microprocessor system has been proposed where the encryption is done through Rotational Addition technique (RAT). The original message is considered as a stream of bits, which is then divided into a number of blocks, each containing n bits, where n is any one of 2, 4, 8, 16, 32, 64, 128, 256. The two adjacent blocks are then added where the modulus of addition is 2n. The result replaces the second block, first block remaining unchanged. The modulo addition has been implemented in a very simple manner where the carry out of the MSB is discarded to get the result. The technique is applied in a cascaded manner by varying the block size from 2 to 256. The whole technique has been implemented through a microprocessor-based system by using a modulo subtraction technique for decryption.