Parallel stream cipher for secure high-speed communications

Abstract

Due to ongoing improvements in high-speed communications, the speed of data encryption must also increase. Accordingly, this paper proposes an PS-LFSR with an m(⩾2)-times faster shifting during one clock interval and a parallel stream cipher that is faster by paralleling many similar keystream generators using the PS-LFSRs. Finally, an m-parallel SUM-BSG with 8-parallel for detail is proposed as a design example of the proposed parallel stream cipher. When compared with a conventional stream cipher, the properties of the proposed cipher exhibited the same crypto-degree with m-times faster processing.

Introduction

Due to ongoing improvements in high-speed communications, the speed of data encryption must also increase. Cryptography is the only known practical method for protecting information transmitted through communication networks that use land lines, communication satellites, and microwave facilities. Cryptographic methods can be divided into block ciphers, stream ciphers, and public-key cryptosystems [4], [8]. There are four application modes of block ciphers: the ECB (electronic codebook) mode, CFB (cipher feedback) mode, CBC (cipher-block chaining) mode, and OFB (output feedback) mode [8]. The ECB mode outputs ciphertext blocks from plaintext blocks via a complex transformation controlled by a secret key. The CFB mode autonomously establishes communication synchronization using feedback from the ciphertext to the input block. The CBC mode is useful for a general-purpose block-oriented transmission or for authentication with block-chaining. The OFB mode is similar to a stream cipher, in which a block cipher generates random sequence blocks from an initial value block [8]. However, all four modes have weaknesses in their application to an erroneous channel as in, for example, a wireless channel. In an erroneous channel, a one-bit error in a ciphertext will propagate to many blocks of recovered plaintext in the receiver. In the ECB mode, a one-bit channel error in a ciphertext will propagate to the full range of the recovered plaintext block in the receiver. Accordingly, a channel with a 10⁻⁶ BER (bit error rate) will be degraded to a channel with a $\text{10}{}^{\mathrm{-4}}\text{(≈128×10}{}^{\mathrm{-6}}\text{)}$ BER if a block cipher with a 128-bit block size is applied. In terms of error propagation, cases using the CFB and CBC modes will be more seriously affected than those using the ECB mode. In contrast, the OFB mode offers a unique solution to the block cipher problem, however, it needs a faster encryption speed. For example, a DES [7], [8] with 16 rounds will generally output 64 bits in 16 system-clock intervals, therefore, the concept of repetition (round) decreases the processing speed.

Public-key cryptosystems are not useful for data-encryption because of their slow processing rate and the problem of bit-error propagation as in the ECB mode. Stream ciphers exhibit good properties including no error propagation, security levels properly selectable according to certain security criteria, and a higher processing ability than block ciphers, however, new high-speed communication systems are requiring faster data encryption.

This paper focuses on the following three problems in designing a cryptosystem: security, fast enciphering/deciphering, and error propagation persistence in channels including mobile communication. As a result, a parallel stream cipher is proposed that combines the strengths of stream and block ciphers, that is, the security and freedom from error propagation of a stream cipher and the parallel processing ability of a block cipher. Normally, all LFSRs in a stream cipher shift/output 1-bit for one clock-time interval, whereas, in the proposed cipher the LFSRs are elevated to a high-speed type, PS-LFSRs, which shifts/output m(⩾2)-bits for one clock interval. Plus, as an improved version of the (single) nonlinear combine function, an m-parallel nonlinear combine function (general type) is introduced, which generates m-bit keystream sequences for the proposed parallel stream cipher. Finally, an m-parallel SUM-BSG is presented as a design example, arranged with many Rueppel's summation generators [1] in parallel and m=8 for details. Its performance is analyzed in terms of cryptographic security and the processing speed compared with a conventional stream cipher.