The notion of non-malleable cryptography, an extension of semantically secure cryptography, is defined. Informally, in the context of encryption the additional requirement is that given the ciphertext it is impossible to generate a different ciphertext so that the respective plaintexts are related. The same concept makes sense in the contexts of string commitment and zero-knowledge proofs of possession of knowledge. Non-malleable schemes for each of these three problems are presented. The schemes do not assume a trusted center; a user need not know anything about the number or identity of other system users. Our cryptosystem is the first proven to be secure against a strong type of chosen ciphertext attack proposed by Rackoff and Simon, in which the attacker knows the ciphertext she wishes to break and can query the decryption oracle on any ciphertext other than the target.

Abstract. We compare the relative strengths of popular notions of security for public key encryption schemes. We consider the goals of privacy and non-malleability, each under chosen plaintext attack and two kinds of chosen ciphertext attack. For each of the resulting pairs of definitions we prove either an implication (every scheme meeting one notion must meet the other) or a separation (there is a scheme meeting one notion but not the other, assuming the first notion can be met at all). We similarly treat plaintext awareness, a notion of security in the random oracle model. An additional contribution of this paper is a new definition of non-malleability which we believe is simpler than the previous one.

We study notions and schemes for symmetric (ie. private key) encryption in a concrete security framework. We give four di erent notions of security against chosen plaintext attack and analyze the concrete complexity ofreductions among them, providing both upper and lower bounds, and obtaining tight relations. In this way we classify notions (even though polynomially reducible to each other) as stronger or weaker in terms of concrete security. Next we provide concrete security analyses of methods to encrypt using a block cipher, including the most popular encryption method, CBC. We establish tight bounds (meaning

We introduce the notion of non-malleable noninteractive zero-knowledge (NIZK) proof systems. We show how to transform any ordinary NIZK proof system into one that has strong non-malleability properties. We then show that the elegant encryption scheme of Naor and Yung [NY] can be made secure against the strongest form of chosen-ciphertext attack by using a non-malleable NIZK proof instead of a standard NIZK proof. Our encryption scheme is simple to describe and works in the standard cryptographic model under general assumptions. The encryption scheme can be realized assuming the existence of trapdoor permutations. 1 Introduction Modern cryptography provides us with several fundamental tools, from encryption schemes to zeroknowledge proofs. For each of these tools, we have some intuition about what they "should" achieve. But we must be careful to understand the gap between our intuition and what we can actually achieve. Indeed, a major goal of cryptography is to refine our tools to br...

Recently Victor Shoup noted that there is a gap in the widely-believed security result of OAEP against adaptive chosen-ciphertext attacks. Moreover, he showed that, presumably, OAEP cannot be proven secure from the one-wayness of the underlying trapdoor permutation. This paper establishes another result on the security of OAEP. It proves that OAEP oers semantic security against adaptive chosen-ciphertext attacks, in the random oracle model, under the partial-domain one-wayness of the underlying permutation. Therefore, this uses a formally stronger assumption. Nevertheless, since partial-domain one-wayness of the RSA function is equivalent to its (full-domain) one-wayness, it follows that the security of RSA{OAEP can actually be proven under the sole RSA assumption, although the reduction is not tight.

Abstract. In this paper we present a new kind of cryptanalytic attack which utilizes bugs in the hardware implementation of computer instructions. The best known example of such a bug is the Intel division bug, which resulted in slightly inaccurate results for extremely rare inputs. Whereas in most applications such bugs can be viewed as a minor nuisance, we show that in the case of RSA (even when protected by OAEP), Pohlig-Hellman, elliptic curve cryptography, and several other schemes, such bugs can be a security disaster: Decrypting ciphertexts on any computer which multiplies even one pair of numbers incorrectly can lead to full leakage of the secret key, sometimes with a single well-chosen ciphertext. Keywords: Bug attack, Fault attack, RSA, Pohlig-Hellman, ECC. 1

We study protocols for strong authentication and key exchange in asymmetric scenarios where the authentication server possesses a pair of private and public keys while the client has only a weak human-memorizable password as its authentication key. We present and analyze several simple password protocols in this scenario, and show that the security of these protocols can be formally proven based on standard cryptographic assumptions. Remarkably, our analysis shows optimal resistance to off-line password guessing attacks under the choice of suitable public key encryption functions. In addition to user authentication, we enhance our protocols to provide two-way authentication, authenticated key exchange, defense against server's compromise, and user anonymity. We complement these results with a proof that public key techniques are unavoidable for password protocols that resist off-line guessing attacks. As a further contribution, we introduce the notion of public passwords that...

We consider a novel security requirement of encryption schemes that we call “key-privacy” or “anonymity”.It asks that an eavesdropper in possession of a ciphertext not be able to tell which specific key, out of a set of known public keys, is the one under which the ciphertext was created, meaning the receiver is anonymous from the point of view of the adversary.We investigate the anonymity of known encryption schemes.We prove that the El Gamal scheme provides anonymity under chosen-plaintext attack assuming the Decision Diffie-Hellman problem is hard and that the Cramer-Shoup scheme provides anonymity under chosen-ciphertext attack under the same assumption.We also consider anonymity for trapdoor permutations.Known attacks indicate that the RSA trapdoor permutation is not anonymous and neither are the standard encryption schemes based on it.We provide a variant of RSA-OAEP that provides anonymity in the random oracle model assuming RSA is one-way.We also give constructions of anonymous trapdoor permutations, assuming RSA is one-way, which yield anonymous encryption schemes in the standard model.

Abstract. Seven years after the optimal asymmetric encryption padding (OAEP) which makes chosen-ciphertext secure encryption scheme from any trapdoor one-way permutation (but whose unique application is RSA), this paper presents REACT, a new conversion which applies to any weakly secure cryptosystem, in the random oracle model: it is optimal from both the computational and the security points of view. Indeed, the overload is negligible, since it just consists of two more hashings for both encryption and decryption, and the reduction is very tight. Furthermore, advantages of REACT beyond OAEP are numerous: 1. it is more general since it applies to any partially trapdoor one-way function (a.k.a. weakly secure public-key encryption scheme) and therefore provides security relative to RSA but also to the Diffie-Hellman problem or the factorization; 2. it is possible to integrate symmetric encryption (block and stream ciphers) to reach very high speed rates; 3. it provides a key distribution with session key encryption, whose overall scheme achieves chosen-ciphertext security even with weakly secure symmetric scheme. Therefore, REACT could become a new alternative to OAEP, and even reach security relative to factorization, while allowing symmetric integration.

Keywords: Asymmetric encryption, Non-malleability, Indistinguishability, equivalence between notions, semantic security.