Soft Value
Strategies and Performances of Soft Input Decryption
Abstract:
This paper analyzes performance aspects of Soft Input Decryption and L-values. Soft Input Decryption uses L-values (soft output) of a SISO channel decoder for the correction of input of Soft Input Decryption (SID blocks) which have been modified during the transmission over a noisy channel. The algorithm, strategies and scenarios of Soft Input Decryption are described. The results of the tested performance of L-values show how many L-values are necessary for the correction of SID blocks. This number is higher than the number of wrong bits. This difference is presented. The number of L-values is estimated for different lengths of SID blocks as well as for different Eb/N0 ratios. The time performance of Soft Input Decryption depends on the used cryptographic mechanism for the verification of the cryptographic check values (digital signatures (based on Elliptic Curve Cryptography), MACs (based on CBC-DES) and H-MACs (based on SHA-1)). The combination of the performance of L-values and time performance gives an estimation, if and when Soft Input Decryption can be performed in practice. Further optimizations are outlined.
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1. Introduction
The combination of a SISO decoder and Decryptor, which is analyzed in this paper, can be considered as a concatenation of codes: en-/decryptor has the role of an outer en-/decoder and the channel encoder/SISO channel decoder has the role of an inner en-/decoder has (Fig. 1).
Concatenation of codes, presented as an outer and inner code was already devised by Forney in 1966 [1]. In literature, it is known as concatenated codes [2], general concatenated codes [3] or super channel codes. The inner code is generally short and decoded with a soft decision decoding algorithm, while the outer code is generally longer and decoded with an algebraic decoding method [2].
In most cases a convolutional code is used as an inner code in combination with a Reed Solomon code or another convolutional code as an outer code. Such a type of concatenated codes can be compared to the combination of codes investigated in this work (Fig.1). Two good characteristics are the result of such a concatenated schema: good error performance because of the use of SISO principle and good security performance as the result of the use of the cryptographic mechanisms.
The next common point of this work with previous works in coding is the idea of the use of reliability in decoding. There are several works which explore the reliability-based soft-decision decoding algorithms for linear block codes, using the concept of error correction by ordering the decoded bits by their reliability values. The values of soft outputs of the decoder have been used as reliability values.
The idea of inversion of the least probable bits (with the lowest reliability values) originated from Chase decoding algorithms [4] in 1972, which were the generalization of the GMD (Generalized Minimum Distance) algorithms from 1966 [1]. These algorithms have been applied to a binary (n, k) linear block code and are referenced as LRP (Least Reliability Positions) algorithms.
Chase algorithms generate a list of candidate code words by complementing all possible combinations of bits with the lowest reliability values. The candidate with the best metric is the decoded solution.
The similarity to the method of the Soft Input Decryption, is the use of L-values reordered and iteratively tested. The difference is that Soft Input Decryption uses two decoders (inner and outer) and a non-linear block code (cryptographic algorithms). Two codes enable the use of feedback from the outer to inner code.
About the Author
The International Journal of Computer Science and Information Security (IJCSIS) is one of the leading international journals for publishing novel ideas, state-of-the-art research results and fundamental advances in all aspects of computer science and information & communication security. IJCSIS is a peer reviewed international journal with a key objective to provide the academic and industrial community a medium for presenting original research and applications related to Computer Science and Information Security.
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