On Non-Parallelizable Deterministic Client Puzzle Scheme with Batch Verification Modes

A (computational) client puzzle scheme enables a client to prove to a server that a certain amount of computing resources (CPU cycles and/or Memory look-ups) has been dedicated to solve a puzzle. Researchers have identified a number of potential applications, such as constructing timed cryptography, fighting junk emails, and protecting critical infrastructure from DoS attacks. In this paper, we first revisit this concept and formally define two properties, namely deterministic computation and parallel computation resistance. Our analysis show that both properties are crucial for the effectiveness of client puzzle schemes in most application scenarios. We prove that the RSW client puzzle scheme, which is based on the repeated squaring technique, achieves both properties. Secondly, we introduce two batch verification modes for the RSW client puzzle scheme in order to improve the verification efficiency of the server, and investigate three methods for handling errors in batch verifications. Lastly, we show that client puzzle schemes can be integrated with reputation systems to further improve the effectiveness in practice

Public Key Encryption Supporting Plaintext Equality Test and User-Specified Authorization

In this paper we investigate a category of public key encryption schemes which supports plaintext equality test and user-specified authorization. With this new primitive, two users, who possess their own public/private key pairs, can issue token(s) to a proxy to authorize it to perform plaintext equality test from their ciphertexts. We provide a formal formulation for this primitive, and present a construction with provable security in our security model. To mitigate the risks against the semi-trusted proxies, we enhance the proposed cryptosystem by integrating the concept of computational client puzzles. As a showcase, we construct a secure personal health record application based on this primitive

A Novel WLAN Client Puzzle against DoS Attack Based on Pattern Matching

Despite the popularity of 802.11 based networks, they suffer several types of DoS attack, launched by an attacker whose aim is to make an access point (AP) unavailable to legitimate users. One of the most common DoS attacks on 802.11 based networks is to deplete the resources of the AP. A serious situation like this can occur when the AP receives a burst of connection requests. This paper addresses this common DoS attack and proposes a lightweight puzzle, based on pattern-matching. Using a pattern-matching technique, this model adequately resists resource-depletion attacks in terms of both puzzle generation and solution verification. Using a sensible series of contextual comparisons, the outcomes were modelled by a simulator, and the security definition and proofs are verified, among other results

Efficient trapdoor-based client puzzle system against DoS attacks

Denial of service (DoS) and distributed denial of service (DDoS) are serious threats to computer networks. DoS and DDoS attacks aim to shut down a target server by depleting its resources and rendering it incapable of offering stable and integrated service to legitimate clients. Preventing DoS and DDoS attacks is a difficult task. A promising countermeasure against DoS attacks is the Client Puzzle method, which nevertheless faces a number of challenges, such as the complexity of puzzle construction and solution verification. Our research focuses on exploring novel puzzle constructions to satisfy the high demands of DoS defence in practice. In this thesis, we first identify the underlying weaknesses of existing client puzzles. To mitigate these vulnerabilities, we recommend the necessary requirements for good client puzzles. Based on this, we propose a new model for puzzle distribution, called the Trapdoor-based Client Puzzle System (TCPS). Two specific schemes are presented to construct puzzles within TCPS. We depict these two schemes, where each trapdoor algorithm is applied respectively. Both schemes have two distinct features: the computational overheads are low, and the difficulty level of puzzles is measurable. Moreover, both puzzle schemes are provably secure under traditional hard problems in mathematics. Our contribution to client puzzle defence against DoS attacks can be summarised as follows: * Identify the shortcomings of existing client puzzles. * Recommend the requirements of good client puzzles. * Formally define the Trapdoor-based Client Puzzle System, along with strict security conditions. * Propose a client puzzle scheme whose security is based on the RSA Assumption. Effectiveness and security are analysed and proven. * Propose a second client puzzle scheme whose security is based on the Discrete Logarithm Problem (DLP). Similarly, effectiveness and security are also analysed. * Provide a possible configuration for system parameters. * Discuss further possible attacks and their solutions. As our research is carried out in DoS attack scenarios, we also introduce this technical background before our achievements are presented

KeyForge: Mitigating Email Breaches with Forward-Forgeable Signatures

Email breaches are commonplace, and they expose a wealth of personal, business, and political data that may have devastating consequences. The current email system allows any attacker who gains access to your email to prove the authenticity of the stolen messages to third parties -- a property arising from a necessary anti-spam / anti-spoofing protocol called DKIM. This exacerbates the problem of email breaches by greatly increasing the potential for attackers to damage the users' reputation, blackmail them, or sell the stolen information to third parties. In this paper, we introduce "non-attributable email", which guarantees that a wide class of adversaries are unable to convince any third party of the authenticity of stolen emails. We formally define non-attributability, and present two practical system proposals -- KeyForge and TimeForge -- that provably achieve non-attributability while maintaining the important protection against spam and spoofing that is currently provided by DKIM. Moreover, we implement KeyForge and demonstrate that that scheme is practical, achieving competitive verification and signing speed while also requiring 42% less bandwidth per email than RSA2048