Cryptanalysis of RSA-type cryptosystems based on Lucas sequences, Gaussian integers and elliptic curves
Abstract
In 1995, Kuwakado, Koyama and Tsuruoka presented a new RSA-type scheme based on singular cubic curves y^2 ≡ x^3 + bx^2 (mod N) where N = pq is an RSA modulus. Then, in 2002, Elkamchouchi, Elshenawy and Shaban introduced an extension of the RSA scheme to the field of Gaussian integers using a modulus N = P Q where P and Q are Gaussian primes such that p = |P | and q = |Q| are ordinary primes. Later, in 2007, Castagnos proposed a scheme over quadratic field quotients with an RSA modulus N = pq based on Lucas sequences. In the three schemes, the public exponent e is an integer satisfying the key equation ed − k (p^2 − 1)(q^2 − 1 )= 1. In this paper, we apply the continued fraction method to launch an attack on the three schemes when the private exponent d is sufficiently small. Our experiments demonstrate that for a 1024-bit modulus, our method works for values of d of up to 520 bits. We also examine the effect of dropping the usual assumption that p and q have the same bit size.
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