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Communication model in cryptography and algebraic attacks on stream ciphers
# 12, December 2013 УДК: 003.26 There are two entities in communication model: sender and receiver. They communicate by using the secret and public channel. Sender and receiver can use symmetric or asymmetric cryptosystems. One of the symmetric cryptosystems is stream ciphers. Construction the stream ciphers method derived from the onetime pad is used in transactions, where high speed and continuity are required. Onetime pad cryptosystem is perfectly secure if key is used only once. System of equations is used to analyze the stream ciphers. These equations are constructed for each element of set keystream known elements. Linearization and fast algebraic attacks are methods to speed up the algebraic attack. The first method reduces the problem of solving the system of nonlinear equations to computing the kernel space of the matrix of system of equations coefficients. Fast algebraic attacks make degree of equations low.
Analysis of stream ciphers by solving a system of algebraic equations
Engineering Education # 03, March 2013 DOI: 10.7463/0313.0546388 The authors research the method of algebraic attack on stream ciphers based on solution of algebraic equations. The method is applied to a stream cipher constructed according to the disposable pad scheme. The authors consider types of key stream generators needed to construct stream ciphers. The key stream is a pseudorandom bit sequence which is combined with the original message to get the ciphertext. The algebraic method of finding the key of the key stream is described in the article. A program that generates a key stream and a system of equations to analyze the stream cipher was developed. The authors found maximum parameters of the stream cipher, the analysis of which can be carried out by software in several first elements of the key stream, its output function and changes of memory state.



