Secure encoder designs based on turbo codes

Secure encoders are schemes aiming at providing both reliability and security in a lightweight fashion. In this paper, a secure channel encoder based on turbo codes is constructed by using the techniques of puncturing and trellis pruning. Puncturing is employed to downgrade the performance of the code and thus increase the error probability experienced by an eavesdropper at a given SNR. This has the advantage that various cryptanalytic attacks, whose complexity depends on the error probability, will become infeasible. On the other hand, trellis pruning is implemented in a secret fashion to enable legitimate users communicate reliably. An algorithm that, based on EXIT analysis, computes the corresponding puncturing and pruning rates is proposed.

I. Introduction

Various secure encoder constructions have been presented in the past few years [1]–[6]. A secure encoder is a channel encoder that apart from reliability provides security as well. This is usually achieved by keeping secret suitable parameters of the encoding process. These constructions are not intended to replace sophisticated cryptographic algorithms at higher layers, but to enable the use of lightweight systems [7]. This is very important, particularly for small terminals with limited computational capabilities. Designs based on turbo codes that assume the puncturing pattern and/or the interleaver to be secret are proposed in [1]–[3]. Secure coding schemes have also been obtained in [4] by randomly puncturing quasi-cyclic low density parity check (LDPC) codes, and in [5] by hiding the structure of the generator matrices of polar codes.