Index Calculation Attacks on RSA Signature and Encryption

Abstract. At Crypto ’85, Desmedt and Odlyzko described a chosen-ciphertext attack against plain RSA encryption. The technique can also be applied to RSA signatures and enables an existential forgery under a chosen-message attack. The potential of this attack remained untapped until a twitch in the technique made it effective against two very popular RSA signature standards, namely iso/iec 9796-1 and iso/iec 9796-2. Following the attack iso/iec 9796-1 was withdrawn and iso/iec 9796-2 amended. In this paper, we recall Desmedt and Odlyzko’s attack as well as its application to the cryptanalysis of iso/iec 9796-2.