Two classes of 3rd order correlation immune symmetric Boolean functions have been constructed respectively in [1] and [2], in which some interesting phenomena of the algebraic degree have been observed as well. However, a good explanation has not been given. In this paper, we obtain the formulas for the degree of these functions, which can well explain the behavior of their degree.

Proving the security of cancelable biometrics and other template protection techniques is a key prerequisite for the widespread deployment of biometric technologies. BioEncoding is a cancelable biometrics scheme that has been proposed recently to protect biometric templates represented as binary strings like iris codes. Unlike other template protection schemes, BioEncoding does not require user-specific keys or tokens. Moreover, it satisfies the requirements of untraceable biometrics without sacrificing the matching accuracy. However, the security of BioEncoding against smart attacks, such as correlation and optimization-based attacks, has to be proved before recommending it for practical deployment. In this paper, the security of BioEncopding, in terms of both non-invertibility and privacy protection, is analyzed. First, resistance of protected templates generated using BioEncoding against brute-force search attacks is revisited rigorously. Then, vulnerabilities of BioEncoding with respect to correlation attacks and optimization based attacks are identified and explained. Furthermore, an important modification to the BioEncoding algorithm is proposed to enhance its security against correlation attacks. The effect of integrating this modification into BioEncoding is validated and its impact on the matching accuracy is investigated empirically using CASIA-IrisV3-Interval dataset. Experimental results confirm the efficacy of the proposed modification and show that it has no negative impact on the matching accuracy.

A Boolean function is said to be correlation immune if its output leaks no information about its input values. Such functions have many applications in computer security practices including the construction of key stream generators from a set of shift registers. Finding methods for easy construction of correlation immune Boolean functions has been an active research area since the introduction of the notion by Siegenthaler. In this paper, we present several constructions of nonpalindromic correlation immune symmetric Boolean functions. Our methods involve finding binomial coefficient identities and obtaining new correlation immune functions from known correlation immune functions. We also consider the construction of higher order correlation immunity symmetric functions and propose a class of third order correlation immune symmetric functions on n variables, where n+1(≥ 9) is a perfect square.

A nonlinear combiner random number generator is a general keystream generator for certain stream ciphers. The generator is composed of several linear feedback shift registers and a nonlinear function; the output is used as a keystream. A fast correlation attack is a typical attack for such keystream generators. Mihaljevi, Fossorier, and Imai have proposed an improved fast correlation attack. The attack is based on error correction of information bits only in the corresponding binary linear block code; APP threshold decoding is employed for the error correction procedure. In this letter, we propose a method which improves the success rate of their attacks with similar complexity. The method adds some intentional error to original parity check equations. Those equations are then used in APP threshold decoding.

Various attacks against RC5 have been analyzed intensively. A known plaintext attack has not been reported that it works on so higher round as a chosen plaintext attack, but it can work more efficiently and practically. In this paper, we investigate a known plaintext attack against RC5 by improving a correlation attack. As for a known plaintext attack against RC5, the best known result is a linear cryptanalysis. They have reported that RC5-32 with 10 rounds can be broken by 264 plaintexts under the heuristic assumption: RC5-32 with r rounds can be broken with a success probability of 90% by using 26r+4 plaintexts. However, their assumption seems to be highly optimistic. Our known plaintext correlation attack can break RC5-32 with 10 rounds (20 half-rounds) in a more strict sense with a success probability of 90% by using 263.67 plaintexts. Furthermore, our attack can break RC5-32 with 21 half-rounds in a success probability of 30% by using 263.07 plaintexts.

In this paper, important methods for cryptanalysis of the stream cipher based on a class of keystream generators are discussed. These methods employ an approach called the fast correlation attack. This cryptographic problem is treated by considering its equivalent channel coding approach, namely decoding of certain very low rate codes in presence of very high noise. A novel family of algorithms for the fast correlation attack is presented. The algorithms are based on the iterative decoding principle in conjunction with a novel method for constructing the parity-checks. A goal of this paper is to summarize reported results and to compare some of the recent ones. Accordingly, the family is compared with recently proposed improved fast correlation attacks based on iterative decoding methods. An analysis of the algorithms performances and complexities is presented. The corresponding trade-offs between performance, complexity and required inputs are pointed out.

An algorithm for cryptanalysis of certain keystream generators is proposed. The developed algorithm has the following two advantages over other reported ones: it is more powerful, and it can be implemented by a high-speed software or a simple hardware suitable for high parallel architectures. The algorithm is based on error-correction of information bits only (of the corresponding binary block code) with a novel method for construction of the parity-checks, and the employed error-correction procedure is an APP based threshold decoding. Experimental and theoretical analyses of the algorithm performance are presented, and its complexity is evaluated. The proposed algorithm is compared with recently proposed improved fast correlation attacks based on convolutional codes and turbo decoding. The underlying principles, performance and complexity are compared, and the gain obtained with the novel approach is pointed out.

Two classes of nonlinear feedforward logic (NLFFL) pseudonoise (PN) code generators based on the use of AND and majority logic (ML) gates are compared. Cross-correlation and code-division multiple-access (CDMA) properties of properly designed NLFFL sequences are found to be comparable with the properties of well-known linear PN codes. It is determined that code design employing ML gates with an odd number of inputs is easier compared with designing with AND gates. This is especially true when the degree of nonlinearity is large, since the nonbalance problem, e. g. , at the output of an AND gate, can be avoided. ML type sequences are less vulnerable to correlation attack and jamming by the m-sequence of an NLFFL generator