International Association for Cryptologic Research

International Association
for Cryptologic Research


Ari Singer


Modified Parameter Attacks: Practical Attacks against CCA2 Secure Cryptosystems and Countermeasures
We introduce the concept of Modified Parameter Attacks, a natural extension of the idea of Adapative Chosen Ciphertext Attacks (CCA2) under which some CCA2 secure systems can be shown to be insecure. These insecurities can be addressed at the application level, but can also be addressed when cryptographic schemes are being designed. We survey some existing CCA2 secure systems which are vulnerable to this attack and suggest practical countermeasures.
NAEP: Provable Security in the Presence of Decryption Failures
We consider the impact of the possibility of decryption failures in proofs of security for padding schemes, where these failures are both message and key dependent. We explain that an average case failure analysis is not necessarily sufficient to achieve provable security with existing CCA2-secure schemes. On a positive note, we introduce NAEP, an efficient padding scheme similar to PSS-E designed especially for the NTRU one-way function. We show that with this padding scheme we can prove security in the presence of decryption failures, under certain explicitly stated assumptions. We also discuss the applicability of proofs of security to instantiated cryptosystems in general, introducing a more practical notion of cost to describe the power of an adversary.