Digital Signatures for Consensus

We present a pairing-based signature scheme for use in blockchains that achieves substantial savings in bandwidth and storage requirements while providing strong security guarantees. Our signature scheme supports aggregation on the same message, which allows us to compress multiple signatures on the same block during consensus, and achieves forward security, which prevents adaptive attacks on the blockchain. Our signature scheme can be applied to all blockchains that rely on multi-party consensus protocols to agree on blocks of transactions (such as proof-of-stake or permissioned blockchains)

Puncturable Encryption: A Generic Construction from Delegatable Fully Key-Homomorphic Encryption

Puncturable encryption (PE), proposed by Green and Miers at IEEE S&P 2015, is a kind of public key encryption that allows recipients to revoke individual messages by repeatedly updating decryption keys without communicating with senders. PE is an essential tool for constructing many interesting applications, such as asynchronous messaging systems, forward-secret zero round-trip time protocols, public-key watermarking schemes and forward-secret proxy re-encryptions. This paper revisits PEs from the observation that the puncturing property can be implemented as efficiently computable functions. From this view, we propose a generic PE construction from the fully key-homomorphic encryption, augmented with a key delegation mechanism (DFKHE) from Boneh et al. at Eurocrypt 2014. We show that our PE construction enjoys the selective security under chosen plaintext attacks (that can be converted into the adaptive security with some efficiency loss) from that of DFKHE in the standard model. Basing on the framework, we obtain the first post-quantum secure PE instantiation that is based on the learning with errors problem, selective secure under chosen plaintext attacks (CPA) in the standard model. We also discuss about the ability of modification our framework to support the unbounded number of ciphertext tags inspired from the work of Brakerski and Vaikuntanathan at CRYPTO 2016

Forward-Secure 0-RTT Goes Live: Implementation and Performance Analysis in QUIC

Modern cryptographic protocols, such as TLS 1.3 and QUIC, can send cryptographically protected data in zero round-trip times (0-RTT) , that is, without the need for a prior interactive handshake. Such protocols meet the demand for communication with minimal latency, but those currently deployed in practice achieve only rather weak security properties, as they may not achieve forward security for the first transmitted payload message and require additional countermeasures against replay attacks. Recently, 0-RTT protocols with full forward security and replay resilience have been proposed in the academic literature. These are based on puncturable encryption, which uses rather heavy building blocks, such as cryptographic pairings. Some constructions were claimed to have practical efficiency, but it is unclear how they compare concretely to protocols deployed in practice, and we currently do not have any benchmark results that new protocols can be compared with. We provide the first concrete performance analysis of a modern 0-RTT protocol with full forward security, by integrating the Bloom Filter Encryption scheme of Derler et al. (EUROCRYPT 2018) in the Chromium QUIC implementation and comparing it to Google\u27s original QUIC protocol. We find that for reasonable deployment parameters, the server CPU load increases approximately by a factor of eight and the memory consumption on the server increases significantly, but stays below 400 MB even for medium-scale deployments that handle up to 50K connections per day. The difference of the size of handshake messages is small enough that transmission time on the network is identical, and therefore not significant. We conclude that while current 0-RTT protocols with full forward security come with significant computational overhead, their use in practice is not infeasible, and may be used in applications where the increased CPU and memory load can be tolerated in exchange for full forward security and replay resilience on the cryptographic protocol level. Our results also serve as a first benchmark that can be used to assess the efficiency of 0-RTT protocols potentially developed in the future

SOFTWARE DEFINED NETWORKS: DIALECTING SECURITY

OpenFlow is the standard used in Software Defined Networks. It handles the communication between the network devices. However, there are some weaknesses linked to OpenFlow. With the use of TLS as a security solution, it inherits the vulnerabilities of TLS in downgrade attacks. Furthermore, TLS is optional. To enhance the security in OpenFlow, previous research work provided a solution that comes with the notion of protocol dialects. Protocol dialects are variations of an existing implementation of an open-source protocol, such as OpenFlow. They are implemented either by adding proxies or directly modifying the protocol to the core. The protocol dialect we analyze in this research follows the first approach by manipulating the protocol in such a way that the actual devices continue to function as before, but additional security measures are put in place with the use of proxies. Desired additional functionality, additional security measures, and changes in fields of the actual protocol are performed within the proxies. The devices “think” that they are communicating with each other exactly as before, but in reality a proxy is standing in front of each device, and the actual communication takes place with the proxies' mediation. In this research, we aim to show the enhanced security of the dialected OpenFlow protocol. We follow the computational analysis model to conduct a security proof for the dialect, and we also analyze some difficulties in conducting such a proof.The Office of Naval Research (ONR)Lohagos, Hellenic ArmyApproved for public release. Distribution is unlimited

0-RTT Key Exchange with Full Forward Secrecy

Reducing latency overhead while maintaining critical security guarantees like forward secrecy has become a major design goal for key exchange (KE) protocols, both in academia and industry. Of particular interest in this regard are 0-RTT protocols, a class of KE protocols which allow a client to send cryptographically protected payload in zero round-trip time (0-RTT) along with the very first KE protocol message, thereby minimizing latency. Prominent examples are Google\u27s QUIC protocol and the upcoming TLS protocol version 1.3. Intrinsically, the main challenge in a 0-RTT key exchange is to achieve forward secrecy and security against replay attacks for the very first payload message sent in the protocol. According to cryptographic folklore, it is impossible to achieve forward secrecy for this message, because the session key used to protect it must depend on a non-ephemeral secret of the receiver. If this secret is later leaked to an attacker, it should intuitively be possible for the attacker to compute the session key by performing the same computations as the receiver in the actual session. In this paper we show that this belief is actually false. We construct the first 0-RTT key exchange protocol which provides full forward secrecy for all transmitted payload messages and is automatically resilient to replay attacks. In our construction we leverage a puncturable key encapsulation scheme which permits each ciphertext to only be decrypted once. Fundamentally, this is achieved by evolving the secret key after each decryption operation, but without modifying the corresponding public key or relying on shared state. Our construction can be seen as an application of the puncturable encryption idea of Green and Miers (S&P 2015). We provide a new generic and standard-model construction of this tool that can be instantiated with any selectively secure hierarchical identity-based key encapsulation scheme

Pixel: Multi-signatures for Consensus

In Proof-of-Stake (PoS) and permissioned blockchains, a committee of verifiers agrees and sign every new block of transactions. These blocks are validated, propagated, and stored by all users in the network. However, posterior corruptions pose a common threat to these designs, because the adversary can corrupt committee verifiers after they certified a block and use their signing keys to certify a different block. Designing efficient and secure digital signatures for use in PoS blockchains can substantially reduce bandwidth, storage and computing requirements from nodes, thereby enabling more efficient applications. We present Pixel, a pairing-based forward-secure multi-signature scheme optimized for use in blockchains, that achieves substantial savings in bandwidth, storage requirements, and verification effort. Pixel signatures consist of two group elements, regardless of the number of signers, can be verified using three pairings and one exponentiation, and support non-interactive aggregation of individual signatures into a multi-signature. Pixel signatures are also forward-secure and let signers evolve their keys over time, such that new keys cannot be used to sign on old blocks, protecting against posterior corruptions attacks on blockchains. We show how to integrate Pixel into any PoS blockchain. Next, we evaluate Pixel in a real-world PoS blockchain implementation, showing that it yields notable savings in storage, bandwidth, and block verification time. In particular, Pixel signatures reduce the size of blocks with 1500 transactions by 35% and reduce block verification time by 38%

VISUAL MODELING AND SIMULATION OF CRYPTOGRAPHIC PROTOCOLS UNDER CONTESTED ENVIRONMENTS

With an ever-evolving battlefield in cyberspace, it is essential to stay abreast of current and developing security protocols that will maintain a state of authenticity, confidentiality, and integrity between communicating entities in information-contested environments. The Department of Defense is interested in transitioning its mission objective goals to establishing and maintaining a reliable security posture between communicating command-and-control platforms. However, the current security protocol visualizations need to cater more to military users and decision-makers to help decide which security protocols would best accommodate various operational environments. This research designed and developed a two-dimensional protocol visualization tool (ProVis) that simulates various security protocol interactions in non-contested and contested operational study environments that assist with understanding how security protocols work in the context of military-related usage. A user study was conducted to examine users’ understanding, accuracy, and overall benefit of ProVis concerning the visualization of the Transport Layer Security, Message Layer Security, and Pre-Shared Key protocols. The findings are highly satisfactory: the user subjects were able to easily interface with ProVis and complete the tasks given. The user subjects found ProVis to be a helpful tool in understanding security protocols quickly. This research provides an alternative to current visualization tools.NPS Naval Research ProgramThis project was funded in part by the NPS Naval Research Program.Outstanding ThesisLieutenant Commander, United States NavyApproved for public release. Distribution is unlimited

Formal Analysis of Session-Handling in Secure Messaging: Lifting Security from Sessions to Conversations

The building blocks for secure messaging apps, such as Signal’s X3DH and Double Ratchet (DR) protocols, have received a lot of attention from the research community. They have notably been proved to meet strong security properties even in the case of compromise such as Forward Secrecy (FS) and Post-Compromise Security (PCS). However, there is a lack of formal study of these properties at the application level. Whereas the research works have studied such properties in the context of a single ratcheting chain, a conversation between two persons in a messaging application can in fact be the result of merging multiple ratcheting chains. In this work, we initiate the formal analysis of secure mes- saging taking the session-handling layer into account, and apply our approach to Sesame, Signal’s session management. We first experimentally show practical scenarios in which PCS can be violated in Signal by a clone attacker, despite its use of the Double Ratchet. We identify how this is enabled by Signal’s session-handling layer. We then design a formal model of the session-handling layer of Signal that is tractable for automated verification with the Tamarin prover, and use this model to rediscover the PCS violation and propose two provably secure mechanisms to offer stronger guarantees

T0RTT: Non-Interactive Immediate Forward-Secret Single-Pass Circuit Construction

Maintaining privacy on the Internet with the presence of powerful adversaries such as nation-state attackers is a challenging topic, and the Tor project is currently the most important tool to protect against this threat. The circuit construction protocol (CCP) negotiates cryptographic keys for Tor circuits, which overlay TCP/IP by routing Tor cells over n onion routers. The current circuit construction protocol provides strong security guarantees such as forward secrecy by exchanging O(n^2) messages. For several years it has been an open question if the same strong security guarantees could be achieved with less message overhead, which is desirable because of the inherent latency in overlay networks. Several publications described CCPs which require only O(n) message exchanges, but significantly reduce the security of the resulting Tor circuit. It was even conjectured that it is impossible to achieve both message complexity O(n) and forward secrecy immediately after circuit construction (so-called immediate forward secrecy). Inspired by the latest advancements in zero round-trip time key exchange (0-RTT), we present a new CCP protocol Tor 0-RTT (T0RTT). Using modern cryptographic primitives such as puncturable encryption allow to achieve immediate forward secrecy using only O(n) messages. We implemented these new primitives to give a first indication of possible problems and how to overcome them in order to build practical CCPs with O(n) messages and immediate forward secrecy in the future