## CryptoDB

### Venkata Koppula

#### Publications

Year
Venue
Title
2019
CRYPTO
We provide generic and black box transformations from any chosen plaintext secure Attribute-Based Encryption (ABE) or One-sided Predicate Encryption system into a chosen ciphertext secure system. Our transformation requires only the IND-CPA security of the original ABE scheme coupled with a pseudorandom generator (PRG) with a special security property.In particular, we consider a PRG with an n bit input $s \in \{0,1\}^n$ and $n \cdot \ell$ bit output $y_1, \ldots , y_n$ where each $y_i$ is an $\ell$ bit string. Then for a randomly chosen s the following two distributions should be computationally indistinguishable. In the first distribution $r_{s_i, i} = y_i$ and $r_{\bar{s}_i, i}$ is chosen randomly for $i \in [n]$. In the second distribution all $r_{b, i}$ are chosen randomly for $i \in [n], b \in \{0,1\}$.We show that such PRGs can be built from either the computational Diffie-Hellman assumption (in non-bilinear groups) or the Learning with Errors (LWE) assumption (and potentially other assumptions). Thus, one can transform any IND-CPA secure system into a chosen ciphertext secure one by adding either assumption. (Or by simply assuming an existing PRG is hinting secure.) In addition, our work provides a new approach and perspective for obtaining chosen ciphertext security in the basic case of public key encryption.
2018
CRYPTO
In this work we seek to construct collusion-resistant traitor tracing systems with small ciphertexts from standard assumptions that also move toward practical efficiency. In our approach we will hold steadfast to the principle of collusion resistance, but relax the requirement on catching a traitor from a successful decoding algorithm. We define a f-risky traitor tracing system as one where the probability of identifying a traitor is $f(\lambda ,n)$f(λ,n) times the probability a successful box is produced. We then go on to show how to build such systems from prime order bilinear groups with assumptions close to those used in prior works. Our core system achieves, for any $k > 0$k>0, $f(\lambda ,n) \approx \frac{k}{n + k - 1}$f(λ,n)≈kn+k-1 where ciphertexts consists of $(k + 4)$(k+4) group elements and decryption requires $(k + 3)$(k+3) pairing operations.At first glance the utility of such a system might seem questionable since the f we achieve for short ciphertexts is relatively small. Indeed an attacker in such a system can more likely than not get away with producing a decoding box. However, we believe this approach to be viable for four reasons:1.A risky traitor tracing system will provide deterrence against risk averse attackers. In some settings the consequences of being caught might bear a high cost and an attacker will have to weigh his utility of producing a decryption D box against the expected cost of being caught.2.Consider a broadcast system where we want to support low overhead broadcast encrypted communications, but will periodically allow for a more expensive key refresh operation. We refer to an adversary produced algorithm that maintains the ability to decrypt across key refreshes as a persistent decoder. We show how if we employ a risky traitor tracing systems in this setting, even for a small f, we can amplify the chances of catching such a “persistent decoder” to be negligibly close to 1.3.In certain resource constrained settings risky traitor tracing provides a best tracing effort where there are no other collusion-resistant alternatives. For instance, suppose we had to support 100 K users over a radio link that had just 10 KB of additional resources for extra ciphertext overhead. None of the existing $\sqrt{N}$N bilinear map systems can fit in these constraints. On the other hand a risky traitor tracing system provides a spectrum of tracing probability versus overhead tradeoffs and can be configured to at least give some deterrence in this setting.4.Finally, we can capture impossibility results for differential privacy from $\frac{1}{n}$1n-risky traitor tracing. Since our ciphertexts are short ($O(\lambda )$O(λ)), we get the negative result which matches what one would get plugging in the obfuscation based tracing system Boneh-Zhandry [9] solution into the prior impossibility result of Dwork et al. [14].
2018
TCC
In this work we study the feasibility of achieving simulation security in functional encryption (FE) in the random oracle model. Our main result is negative in that we give a functionality for which it is impossible to achieve simulation security even with the aid of random oracles.We begin by giving a formal definition of simulation security that explicitly incorporates the random oracles. Next, we show a particular functionality for which it is impossible to achieve simulation security. Here messages are interpreted as seeds to a (weak) pseudorandom function family F and private keys are ascribed to points in the domain of the function. On a message s and private key x one can learn F(s, x). We show that there exists an attacker that makes a polynomial number of private key queries followed by a single ciphertext query for which there exists no simulator.Our functionality and attacker access pattern closely matches the standard model impossibility result of Agrawal, Gorbunov, Vaikuntanathan and Wee (CRYPTO 2013). The crux of their argument is that no simulator can succinctly program in the outputs of an unbounded number of evaluations of a pseudorandom function family into a fixed size ciphertext. However, their argument does not apply in the random oracle setting since the oracle acts as an additional conduit of information which the simulator can program. We overcome this barrier by proposing an attacker who decrypts the challenge ciphertext with the secret keys issued earlier without using the random oracle, even though the decryption algorithm may require it. This involves collecting most of the useful random oracle queries in advance, without giving the simulator too many opportunities to program.On the flip side, we demonstrate the utility of the random oracle in simulation security. Given only public key encryption and low-depth PRGs we show how to build an FE system that is simulation secure for any poly-time attacker that makes an unbounded number of message queries, but an a-priori bounded number of key queries. This bests what is possible in the standard model where it is only feasible to achieve security for an attacker that is bounded both in the number of key and message queries it makes. We achieve this by creating a system that leverages the random oracle to get one-key security and then adapt previously known techniques to boost the system to resist up to q queries.Finally, we ask whether it is possible to achieve simulation security for an unbounded number of messages and keys, but where all key queries are made after the message queries. We show this too is impossible to achieve using a different twist on our first impossibility result.
2017
EUROCRYPT
2017
PKC
2017
PKC
2017
TCC
2016
EUROCRYPT
2016
CRYPTO
2016
TCC
2015
TCC
2015
TCC
2015
EUROCRYPT
2015
ASIACRYPT
2014
EPRINT