Naor-Yung paradigm with shared randomness and applications

The Naor-Yung paradigm (Naor and Yung, STOC’90) allows to generically boost security under chosen-plaintext attacks (CPA) to security against chosen-ciphertext attacks (CCA) for public-key encryption (PKE) schemes. The main idea is to encrypt the plaintext twice (under independent public keys), and to append a non-interactive zero-knowledge (NIZK) proof that the two ciphertexts indeed encrypt the same message. Later work by Camenisch, Chandran, and Shoup (Eurocrypt’09) and Naor and Segev (Crypto’09 and SIAM J. Comput.’12) established that the very same techniques can also be used in the settings of key-dependent message (KDM) and key-leakage attacks (respectively). In this paper we study the conditions under which the two ciphertexts in the Naor-Yung construction can share the same random coins. We find that this is possible, provided that the underlying PKE scheme meets an additional simple property. The motivation for re-using the same random coins is that this allows to design much more efficient NIZK proofs. We showcase such an improvement in the random oracle model, under standard complexity assumptions including Decisional Diffie-Hellman, Quadratic Residuosity, and Subset Sum. The length of the resulting ciphertexts is reduced by 50%, yielding truly efficient PKE schemes achieving CCA security under KDM and key-leakage attacks. As an additional contribution, we design the first PKE scheme whose CPA security under KDM attacks can be directly reduced to (low-density instances of) the Subset Sum assumption. The scheme supports keydependent messages computed via any affine function of the secret ke

Bounded-Collusion IBE from Key Homomorphism

In this work, we show how to construct IBE schemes that are secure against a bounded number of collusions, starting with underlying PKE schemes which possess linear homomorphisms over their keys. In particular, this enables us to exhibit a new (bounded-collusion) IBE construction based on the quadratic residuosity assumption, without any need to assume the existence of random oracles. The new IBE’s public parameters are of size O(tλlogI) where I is the total number of identities which can be supported by the system, t is the number of collusions which the system is secure against, and λ is a security parameter. While the number of collusions is bounded, we note that an exponential number of total identities can be supported. More generally, we give a transformation that takes any PKE satisfying Linear Key Homomorphism, Identity Map Compatibility, and the Linear Hash Proof Property and translates it into an IBE secure against bounded collusions. We demonstrate that these properties are more general than our quadratic residuosity-based scheme by showing how a simple PKE based on the DDH assumption also satisfies these properties.National Science Foundation (U.S.) (NSF CCF-0729011)National Science Foundation (U.S.) (NSF CCF-1018064)United States. Defense Advanced Research Projects Agency (DARPA FA8750-11-2-0225

Robustness of the Learning with Errors Assumption

Starting with the work of Ishai-Sahai-Wagner and Micali-Reyzin, a new goal has been set within the theory of cryptography community, to design cryptographic primitives that are secure against large classes of side-channel attacks. Recently, many works have focused on designing various cryptographic primitives that are robust (retain security) even when the secret key is “leaky”, under various intractability assumptions. In this work we propose to take a step back and ask a more basic question: which of our cryptographic assumptions (rather than cryptographic schemes) are robust in presence of leakage of their underlying secrets? Our main result is that the hardness of the learning with error (LWE) problem implies its hardness with leaky secrets. More generally, we show that the standard LWE assumption implies that LWE is secure even if the secret is taken from an arbitrary distribution with sufficient entropy, and even in the presence of hard-to-invert auxiliary inputs. We exhibit various applications of this result. 1. Under the standard LWE assumption, we construct a symmetric-key encryption scheme that is robust to secret key leakage, and more generally maintains security even if the secret key is taken from an arbitrary distribution with sufficient entropy (and even in the presence of hard-to-invert auxiliary inputs). 2. Under the standard LWE assumption, we construct a (weak) obfuscator for the class of point functions with multi-bit output. We note that in most schemes that are known to be robust to leakage, the parameters of the scheme depend on the maximum leakage the system can tolerate, and hence the efficiency degrades with the maximum anticipated leakage, even if no leakage occurs at all! In contrast, the fact that we rely on a robust assumption allows us to construct a single symmetric-key encryption scheme, with parameters that are independent of the anticipated leakage, that is robust to any leakage (as long as the secret key has sufficient entropy left over). Namely, for any k < n (where n is the size of the secret key), if the secret key has only entropy k, then the security relies on the LWE assumption with secret size roughly k

08491 Abstracts Collection -- Theoretical Foundations of Practical Information Security

From 30.11. to 05.12.2008, the Dagstuhl Seminar 08491 ``Theoretical Foundations of Practical Information Security \u27\u27 was held in Schloss Dagstuhl~--~Leibniz Center for Informatics. During the seminar, several participants presented their current research, and ongoing work and open problems were discussed. Abstracts of the presentations given during the seminar as well as abstracts of seminar results and ideas are put together in this paper. The first section describes the seminar topics and goals in general. Links to extended abstracts or full papers are provided, if available

Security Through Amnesia: A Software-Based Solution to the Cold Boot Attack on Disk Encryption

Disk encryption has become an important security measure for a multitude of clients, including governments, corporations, activists, security-conscious professionals, and privacy-conscious individuals. Unfortunately, recent research has discovered an effective side channel attack against any disk mounted by a running machine\cite{princetonattack}. This attack, known as the cold boot attack, is effective against any mounted volume using state-of-the-art disk encryption, is relatively simple to perform for an attacker with even rudimentary technical knowledge and training, and is applicable to exactly the scenario against which disk encryption is primarily supposed to defend: an adversary with physical access. To our knowledge, no effective software-based countermeasure to this attack supporting multiple encryption keys has yet been articulated in the literature. Moreover, since no proposed solution has been implemented in publicly available software, all general-purpose machines using disk encryption remain vulnerable. We present Loop-Amnesia, a kernel-based disk encryption mechanism implementing a novel technique to eliminate vulnerability to the cold boot attack. We offer theoretical justification of Loop-Amnesia's invulnerability to the attack, verify that our implementation is not vulnerable in practice, and present measurements showing our impact on I/O accesses to the encrypted disk is limited to a slowdown of approximately 2x. Loop-Amnesia is written for x86-64, but our technique is applicable to other register-based architectures. We base our work on loop-AES, a state-of-the-art open source disk encryption package for Linux.Comment: 13 pages, 4 figure

On the design of forgiving biometric security systems

This work aims to highlight the fundamental issue surrounding biometric security systems: it's all very nice until a biometric is forged, but what do we do after that? Granted, biometric systems are by physical nature supposedly much harder to forge than other factors of authentication since biometrics on a human body are by right unique to the particular human person. Yet it is also due to this physical nature that makes it much more catastrophic when a forgery does occur, because it implies that this uniqueness has been forged as well, threatening the human individuality; and since crime has by convention relied on identifying suspects by biometric characteristics, loss of this biometric uniqueness has devastating consequences on the freedom and basic human rights of the victimized individual. This uniqueness forgery implication also raises the motivation on the adversary to forge since a successful forgery leads to much more impersonation situations when biometric systems are used i.e. physical presence at crime scenes, identi cation and access to security systems and premises, access to nancial accounts and hence the ability to use the victim's nances. Depending on the gains, a desperate highly motivated adversary may even resort to directly obtaining the victim's biometric parts by force e.g. severing the parts from the victim's body; this poses a risk and threat not just to the individual's uniqueness claim but also to personal safety and well being. One may then wonder if it is worth putting one's assets, property and safety into the hands of biometrics based systems when the consequences of biometric forgery far outweigh the consequences of system compromises when no biometrics are used