1. Field of the Invention
The present invention relates to the field of cryptography. More specifically, the present invention relates to the robustness of stream ciphers.
2. Background Information
Crytographic ciphers can be broadly divided into block ciphers and stream ciphers. Block ciphers cipher a block of plain text into ciphered text by applying multiple successive rounds of transformation to the plain text, using a cipher key. An example of a block cipher is the well known DES cipher. Stream ciphers cipher a stream of plain data into ciphered data by combining the stream of plain data with a pseudo random sequence dynamically generated using a cipher key. An example of a stream cipher is the well known XPF/KPD cipher.
Most stream ciphers employ one or more linear feedback shift registers (LFSR). In various applications, it is desirable to employ multiple LFSRs to increase the robustness of a stream cipher. However, employment of multiple LFSRs requires employment of a combiner function to recombine the multiple data bits output by the LFSRS. Most combiner functions known in the art are inefficient in their real estate requirement for hardware implementations. Thus, a robust stream cipher with a more efficient combiner function is desired.