Lakshmi Kuppusamy

Stronger difficulty notions for client puzzles and denial-of-service-resistant protocols

Client puzzles are meant to act as a defense against denial of service (DoS) attacks by requiring a client to solve some moderately hard problem before being granted access to a resource. However, recent client puzzle difficulty definitions (Stebila and Ustaoglu, 2009; Chen et al., 2009) do not ensure that solving n puzzles is n times harder than solving one puzzle. Motivated by examples of puzzles where this is the case, we present stronger definitions of difficulty for client puzzles that are meaningful in the context of adversaries with more computational power than required to solve a single puzzle. A protocol using strong client puzzles may still not be secure against DoS attacks if the puzzles are not used in a secure manner. We describe a security model for analyzing the DoS resistance of any protocol in the context of client puzzles and give a generic technique for combining any protocol with a strong client puzzle to obtain a DoS-resistant protocol.

Efficient modular exponentiation-based puzzles for denial-of-service protection

Client puzzles are moderately-hard cryptographic problems -- neither easy nor impossible to solve -- that can be used as a countermeasure against denial of service attacks on network protocols. Puzzles based on modular exponentiation are attractive as they provide important properties such as non-parallelisability, deterministic solving time, and linear granularity. We propose an efficient client puzzle based on modular exponentiation. Our puzzle requires only a few modular multiplications for puzzle generation and verification. For a server under denial of service attack, this is a significant improvement as the best known non-parallelisable puzzle proposed by Karame and Capkun (ESORICS 2010) requires at least 2k-bit modular exponentiation, where k is a security parameter. We show that our puzzle satisfies the unforgeability and difficulty properties defined by Chen et al. (Asiacrypt 2009). We present experimental results which show that, for 1024-bit moduli, our proposed puzzle can be up to 30 × faster to verify than the Karame-Capkun puzzle and 99 × faster than the Rivest et al.s time-lock puzzle.

CRT-Based Outsourcing Algorithms for Modular Exponentiations

The problem of securely outsourcing cryptographic computations to the untrusted servers was formally addressed first by Hohenberger and Lysyanskaya in TCC 2005. They presented an algorithm which outsources computation of modular exponentiations securely to two non-interacting third-party servers but the checkability of third-party computations has probability 1 / 2. Chen et al. improved this algorithm for two non-colluding servers by increasing the checkability probability to 2/3. For real-world cryptographic applications it is desirable that the checkability probability is \(1-\epsilon \), where \(\epsilon \) becomes negligible for appropriate parameter choices. Towards a more practical use, we present an algorithm(s) for secure outsourcing of (simultaneous) modular exponentiation(s) which can be seen as another application of the Chinese remainder theorem (CRT). Interestingly the checkability probability of our algorithm is 1 in the presence of two non colluding servers. Our algorithm is superior in both efficiency and checkability compared to that of the previously known schemes of the same kind. Finally we discuss the potential practical applications for our outsourcing schemes, for example computing the final exponentiation in pairings.

Cryptographic Approaches to Denial-of-Service Resistance

Authentication is a promising way to treat denial-of-service (DoS) threats against nonpublic services because it allows servers to restrict connections only to authorised users. However, there is a catch with this argument since authentication itself is typically a computationally intensive rocess that is necessarily exposed to unauthenticated entities. This means that the authentication protocol can become a source of denial-of-service vulnerability itself, thereby causing the same problem it is aimed at solving.

Towards a provably secure dos-resilient key exchange protocol with perfect forward secrecy

Just Fast Keying (JFK) is a simple, efficient and secure key exchange protocol proposed by Aiello et al. (ACM TISSEC, 2004). JFK is well known for its novel design features, notably its resistance to denial-of-service (DoS) attacks. Using Meadows’ cost-based framework, we identify a new DoS vulnerability in JFK. The JFK protocol is claimed secure in the Canetti-Krawczyk model under the Decisional Diffie-Hellman (DDH) assumption. We show that security of the JFK protocol, when reusing ephemeral Diffie-Hellman keys, appears to require the Gap Diffie-Hellman (GDH) assumption in the random oracle model. We propose a new variant of JFK that avoids the identified DoS vulnerability and provides perfect forward secrecy even under the DDH assumption, achieving the full security promised by the JFK protocol.

Hide the Modulus: A Secure Non-Interactive Fully Verifiable Delegation Scheme for Modular Exponentiations via CRT

Security protocols using public-key cryptography often requires large number of costly modular exponentiations (MEs). With the proliferation of resource-constrained (mobile) devices and advancements in cloud computing, delegation of such expensive computations to powerful server providers has gained lots of attention. In this paper, we address the problem of verifiably secure delegation of MEs using two servers, where at most one of which is assumed to be malicious (the OMTUP-model). We first show verifiability issues of two recent schemes: We show that a scheme from IndoCrypt 2016 does not offer full verifiability, and that a scheme for n simultaneous MEs from AsiaCCS 2016 is verifiable only with a probability 0.5909 instead of the author’s claim with a probability 0.9955 for \(n=10\). Then, we propose the first non-interactive fully verifiable secure delegation scheme by hiding the modulus via Chinese Remainder Theorem (CRT). Our scheme improves also the computational efficiency of the previous schemes considerably. Hence, we provide a lightweight delegation enabling weak clients to securely and verifiably delegate MEs without any expensive local computation (neither online nor offline). The proposed scheme is highly useful for devices having (a) only ultra-lightweight memory, and (b) limited computational power (e.g. sensor nodes, RFID tags).

Evaluation of puzzle-enabled proxy-assisted denial-of-service protection for web services

Denial-of-service (DoS) attacks are a fast growing, severe menace to the availability of desired services. In this work, we investigate the efficacy of a cryptographic DoS countermeasure, namely, client puzzles which can be used to achieve a weak authentication as it forces the client to solve a somewhat-difficult computational problem in order to get serviced. We aim to make a web service more resilient to DoS attacks by using a reverse proxy between clients and the service provider. Unlike previous works, we integrate puzzles into reverse proxy and demonstrate that the proposed approach is indeed effective and advantageous in protecting the web servers from both flooding and semantic-type attacks.

Improved Cryptographic Puzzle Based on Modular Exponentiation

Cryptographic puzzles are moderately hard–neither easy nor hard to solve—computational problems. They have been identified to be useful in mitigating a type of resource exhaustion attacks on Internet protocols. Puzzles based on modular exponentiation are interesting as they possess some desirable properties such as deterministic solving time, sequential (non-parallelizable) solving process and linear granularity. We propose a cryptographic puzzle based on modular exponentiation. Our puzzle is as efficient as the state-of-art puzzle of its kind and also overcomes the major limitation of the previous schemes.