Abstract: A cryptographic method and apparatus are disclosed which transform a message or arbitrary length into a block of fixed length (128 bits) defined modification detection code (MDC). Although there are a large number of messages which result in the same MDC, because the MDC is a many-to-one function of the input, it is required that it is practically not feasible for an opponent to find them. In analyzing the methods, a distinction is made between two types of attacks, i.e., insiders (who have access to the system) and outsiders (who do not). The first method employs four encryption steps per DEA block and provides the higher degree of security. Coupling between the different DEA operations is provided by using the input keys also as data in two of the four encryption steps. In addition, there is cross coupling by interchanging half of the internal keys.

Abstract: Code modification circuitry alters the end portion of each block in a sequence of code blocks, and also inserts additional bits at the junction between contiguous blocks. The codes to be processed are run-length limited (RLL) codes having a DC component which is to be removed for certain applications such as magnetic recording. The modification circuitry retains the RLL format. Charge (or the integral of the waveform) accumulated by the sequence of bits of one block is compensated by selecting the sense of charge accumulation in next block to be of opposite sense. This is accomplished by the code modification circuitry using a relatively small set of possible combinations of digital words at the junctions of the blocks.

Abstract: A method and an apparatus are disclosed for converting error syndromes of an error-correcting code to pointers which identify the positions of the erroneous bits. Each syndrome is converted by a plurality of hashing functions into a plurality of hash words, which in turn are used to address a plurality of read-only stores. The outputs of the read-only stores are logically combined to obtain the respective error pointer. A preferred embodiment uses three hashing functions (41, 43, 45) and three read-only stores (53, 55, 57) and combines their outputs by an Exclusive-Or function (59). The storage capacity and the processing time required for syndrome-to-error pointer conversion are reduced by the disclosed scheme.