A Reject Timing Attackon an IND-CCA2 Public-Key Cryptosystem

摘要

EPOC-2 is a public-key cryptosystem that can be proved IND-CCA2 under the factoring assumption in the random oracle model. It was written into a standard specification P1363 of IEEE, and it has been a candidate of the public-key cryptosystem in several international standards (or portfolio) on cryptography, e.g. NESSIE, CRYPTREC, ISO, etc. In this paper we propose a chosen ciphertext attack against EPOC-2 from NESSIE by observing the timing of the reject signs from the decryption oracle. We construct an algorithm, which can factor the public modulus using the difference of the reject symbols. For random 384-bit primes, the modulus can be factored with probability at least 1/2 by invoking about 385 times to the decryption oracle.