Paper 2012/106

More on Correcting Errors in RSA Private Keys: Breaking CRT-RSA with Low Weight Decryption Exponents

Santanu Sarkar and Subhamoy Maitra

Abstract

Several schemes have been proposed towards the fast encryption and decryption in RSA and its variants. One popular idea is to use integers having low Hamming weight in the preparation of the decryption exponents. This is to reduce the multiplication effort in the square and multiply method in the exponentiation routine, both in encryption and decryption. In this paper we show that such schemes are insecure in CRT-RSA when the encryption exponent is small (e.g., $e = 2^{16}+1$). In particular, we show that the CRT-RSA schemes presented in SAC 1996 and ACISP 2005 with low weight decryption exponents can be broken in a few minutes in certain cases. Further, the scheme of CT-RSA 2010, where the decryption exponents are not of low weight but they have large low weight factors, can also be cryptanalysed. To mount the attack, we exploit the heuristic proposed by Henecka et al (Crypto 2010) that is capable of correcting errors in the secret parameters when the encryption exponent is small. In the process, we identify a few modifications of the error correction strategy that provides significantly improved experimental outcome and also beats the theoretical bounds given in the work of Henecka et al.

Note: Presented at Indo-US workshop on "Mathematical and Statistical Aspects of Cryptography" at Indian Statistical Institute, Kolkata, January 12-14, 2012.