Abstract
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We consider four variants of the RSA cryptosystem with an RSA modulus N= pq where the public exponent e and the private exponent d satisfy an equation of the form ed- k(p2- 1 ) (q2- 1 ) = 1. We show that, if the prime numbers p and q share most significant bits, that is, if the prime difference | p- q| is sufficiently small, then one can solve the equation for larger values of d, and factor the RSA modulus, which makes the systems insecure.