Paper 2009/165

Securing RSA against Fault Analysis by Double Addition Chain Exponentiation

Matthieu Rivain

Abstract

Fault Analysis is a powerful cryptanalytic technique that enables to break cryptographic implementations embedded in portable devices more efficiently than any other technique. For an RSA implemented with the Chinese Remainder Theorem method, one faulty execution suffices to factorize the public modulus and fully recover the private key. It is therefore mandatory to protect embedded implementations of RSA against fault analysis. This paper provides a new countermeasure against fault analysis for exponentiation and RSA. It consists in a {\em self-secure} exponentiation algorithm, namely an exponentiation algorithm that provides a direct way to check the result coherence. An RSA implemented with our solution hence avoids the use of an extended modulus (which slows down the computation) as in several other countermeasures. Moreover, our exponentiation algorithm involves $1.65$ multiplications per bit of the exponent which is significantly less than the $2$ required by other self-secure exponentiations.