TumbleBit: an untrusted Bitcoin-compatible anonymous payment hub

This paper presents TumbleBit, a new unidirectional unlinkable payment hub that is fully compatible with today s Bitcoin protocol. TumbleBit allows parties to make fast, anonymous, off-blockchain payments through an untrusted intermediary called the Tumbler. TumbleBits anonymity properties are similar to classic Chaumian eCash: no one, not even the Tumbler, can link a payment from its payer to its payee. Every payment made via TumbleBit is backed by bitcoins, and comes with a guarantee that Tumbler can neither violate anonymity, nor steal bitcoins, nor print money by issuing payments to itself. We prove the security of TumbleBit using the real/ideal world paradigm and the random oracle model. Security follows from the standard RSA assumption and ECDSA unforgeability. We implement TumbleBit, mix payments from 800 users and show that TumbleBits offblockchain payments can complete in seconds.https://eprint.iacr.org/2016/575.pdfPublished versio

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

Cryptographic Analysis of Secure Messaging Protocols

Instant messaging applications promise their users a secure and private way to communicate. The validity of these promises rests on the design of the underlying protocol, the cryptographic primitives used and the quality of the implementation. Though secure messaging designs exist in the literature, for various reasons developers of messaging applications often opt to design their own protocols, creating a gap between cryptography as understood by academic research and cryptography as implemented in practice. This thesis contributes to bridging this gap by approaching it from both sides: by looking for flaws in the protocols underlying real-world messaging applications, as well as by performing a rigorous analysis of their security guarantees in a provable security model.Secure messaging can provide a host of different, sometimes conflicting, security and privacy guarantees. It is thus important to judge applications based on the concrete security expectations of their users. This is particularly significant for higher-risk users such as activists or civil rights protesters. To position our work, we first studied the security practices of protesters in the context of the 2019 Anti-ELAB protests in Hong Kong using in-depth, semi-structured interviews with participants of these protests. We report how they organised on different chat platforms based on their perceived security, and how they developed tactics and strategies to enable pseudonymity and detect compromise.Then, we analysed two messaging applications relevant in the protest context: Bridgefy and Telegram. Bridgefy is a mobile mesh messaging application, allowing users in relative proximity to communicate without the Internet. It was being promoted as a secure communication tool for use in areas experiencing large-scale protests. We showed that Bridgefy permitted its users to be tracked, offered no authenticity, no effective confidentiality protections and lacked resilience against adversarially crafted messages. We verified these vulnerabilities by demonstrating a series of practical attacks.Telegram is a messaging platform with over 500 million users, yet prior to this work its bespoke protocol, MTProto, had received little attention from the cryptographic community. We provided the first comprehensive study of the MTProto symmetric channel as implemented in cloud chats. We gave both positive and negative results. First, we found two attacks on the existing protocol, and two attacks on its implementation in official clients which exploit timing side channels and uncover a vulnerability in the key exchange protocol. Second, we proved that a fixed version of the symmetric MTProto protocol achieves security in a suitable bidirectional secure channel model, albeit under unstudied assumptions. Our model itself advances the state-of-the-art for secure channels

TEEvil: Identity Lease via Trusted Execution Environments

We investigate identity lease, a new type of service in which users lease their identities to third parties by providing them with full or restricted access to their online accounts or credentials. We discuss how identity lease could be abused to subvert the digital society, facilitating the spread of fake news and subverting electronic voting by enabling the sale of votes. We show that the emergence of Trusted Execution Environments and anonymous cryptocurrencies, for the first time, allows the implementation of such a lease service while guaranteeing fairness, plausible deniability and anonymity, therefore shielding the users and account renters from prosecution. To show that such a service can be practically implemented, we build an example service that we call TEEvil leveraging Intel SGX and ZCash. Finally, we discuss defense mechanisms and challenges in the mitigation of identity lease services.Comment: 21 pages, 5 figure

Attacking and securing Network Time Protocol

Network Time Protocol (NTP) is used to synchronize time between computer systems communicating over unreliable, variable-latency, and untrusted network paths. Time is critical for many applications; in particular it is heavily utilized by cryptographic protocols. Despite its importance, the community still lacks visibility into the robustness of the NTP ecosystem itself, the integrity of the timing information transmitted by NTP, and the impact that any error in NTP might have upon the security of other protocols that rely on timing information. In this thesis, we seek to accomplish the following broad goals: 1. Demonstrate that the current design presents a security risk, by showing that network attackers can exploit NTP and then use it to attack other core Internet protocols that rely on time. 2. Improve NTP to make it more robust, and rigorously analyze the security of the improved protocol. 3. Establish formal and precise security requirements that should be satisfied by a network time-synchronization protocol, and prove that these are sufficient for the security of other protocols that rely on time. We take the following approach to achieve our goals incrementally. 1. We begin by (a) scrutinizing NTP's core protocol (RFC 5905) and (b) statically analyzing code of its reference implementation to identify vulnerabilities in protocol design, ambiguities in specifications, and flaws in reference implementations. We then leverage these observations to show several off- and on-path denial-of-service and time-shifting attacks on NTP clients. We then show cache-flushing and cache-sticking attacks on DNS(SEC) that leverage NTP. We quantify the attack surface using Internet measurements, and suggest simple countermeasures that can improve the security of NTP and DNS(SEC). 2. Next we move beyond identifying attacks and leverage ideas from Universal Composability (UC) security framework to develop a cryptographic model for attacks on NTP's datagram protocol. We use this model to prove the security of a new backwards-compatible protocol that correctly synchronizes time in the face of both off- and on-path network attackers. 3. Next, we propose general security notions for network time-synchronization protocols within the UC framework and formulate ideal functionalities that capture a number of prevalent forms of time measurement within existing systems. We show how they can be realized by real-world protocols (including but not limited to NTP), and how they can be used to assert security of time-reliant applications-specifically, cryptographic certificates with revocation and expiration times. Our security framework allows for a clear and modular treatment of the use of time in security-sensitive systems. Our work makes the core NTP protocol and its implementations more robust and secure, thus improving the security of applications and protocols that rely on time

Message Endorsement and Informant Confidentiality In Wireless Set of Connections

Privacy and security to the information is really given by verification. Verification includes the certain distinguishing proof of one gathering by another gathering or a procedure of affirming a personality .But now a days there are different routines for confirmation, for example, Message Authentication Code, Signcryption, Key Aggregate System are developed quickly for better security safety measure. This paper attempt to explore how to give validation in remote sensor systems. Different objectives are to give a prologue to general security in remote sensor systems. As Wireless sensor systems are the purpose of consideration of various scientists with respect to the security issue in the previous quite a while. Message validation is a standout amongst the best approach to figure out an interloper who can trade off with the hubs and can access to the information and degenerate the information in remote sensor system. There were different systems have been produced to tackle the issue, for example, symmetric key cryptography and open key cryptography. Each would have their own issues, for example, edge overhead and key administration and calculation overhead and versatility .keeping in mind the end goal to take care of such issue we built up another Confirmation plan utilizing the elliptic bend cryptography .In this plan any hub can transmit n number of message without limit issue. This paper is to do review before really actualizing it. The Internet Key-Exchange (IKE) conventions are the center cryptographic conventions to guarantee Internet security, which indicate key trade systems used to set up shared keys for utilization in the Internet Protocol Security (IPsec) guidelines. For key-trade over the Internet, both security and protection are coveted. Consequently, numerous message confirmation plans have been set up, made on both symmetric-key cryptosystems and open key cryptosystems. Be that as it may, it has the constraints of high computational and correspondence overhead notwithstanding absence of adaptability and strength to hub trade off spells. The proposed plan is Signature and ID era, which are utilized to give high security to message going in Internet. This proposed system is an effective key administration structure to guarantee separation of the traded off hubs. Every hub will have singular mark, and every message going between middle of the road hubs have one key to verify. Message going between every hubs have a validation utilizing signature and key. This successful system will give high secure to message passing other than existing strategies in Internet