In a content delivery network (CDN), resources are strained during peak-time and underutilised in off-peak times when supplying digital content to users. Caching can help balance this. At the off-peak time some content is delivered to users’ local caches. During peak time, the use of cached data to serve users’ requests relieves strain on the network by reducing repeated transfer of popular content. In coded caching , the cache content placement is designed in conjunction with the delivery techniques to optimise network throughput. Since dissemination of information, as well as the delivery of entertainment, is reliant on CDNs, the security and privacy of cache placement, user demand, and content delivery, are paramount. In much of the literature in secure coded caching , security is built on top of solutions that have efficiency in mind, and most current proposals focus on the security of individual parts of the process. A lack of a unifying network model also makes it difficult to compare or combine solutions. In this survey we analyse the security and privacy requirements of secure coded caching, and evaluate existing schemes in terms of the security provided and the cost of this security provision. We also review the techniques used to achieve secure coded caching and analyse their limitations. In addition, we contextualise secure coded caching in the landscape of other secure content delivery primitives. As a result, we identify and prioritise open security and privacy challenges for the future.

We introduce incoercible digital signature schemes, a variant of a standard digital signature. Incoercible signatures enable signers, when coerced to produce a signature for a message chosen by an attacker, to generate fake signatures that are indistinguishable from real signatures, even if the signer is compelled to reveal their full history (including their secret signing keys and any randomness used to produce keys/signatures) to the attacker. Additionally, we introduce an authenticator that can detect fake signatures, which ensures that coercion is identified. We present a formal security model for incoercible signature schemes that comprises an established definition of unforgeability and captures new notions of weak receipt-freeness, strong receipt-freeness and coercion-resistance. We demonstrate that an incoercible signature scheme can be viewed as a transformation of any generic signature scheme. Indeed, we present two incoercible signature scheme constructions that are built from a standard signature scheme and a sender-deniable encryption scheme. We prove that our first construction satisfies coercion-resistance, and our second satisfies strong receipt-freeness. We conclude by presenting an extension to our security model: we show that our security model can be extended to the designated verifier signature scheme setting in an intuitive way as the designated verifier can assume the role of the authenticator and detect coercion during the verification process.

In this work, we provide a systematisation of knowledge of delay-based cryptography, in which we discuss and compare the existing primitives within cryptography that utilise a time-delay. We start by considering the role of time within cryptography, explaining broadly what a delay aimed to achieve at its inception and now, in the modern age. We then move on to describing the underlying assumptions used to achieve these goals, and analyse topics including trust, decentralisation and concrete methods to implement a delay. We then survey the existing primitives, discussing their security properties, instantiations and applications. We make explicit the relationships between these primitives, identifying a hierarchy and the theoretical gaps that exist. We end this systematisation of knowledge by highlighting relevant future research directions within the field of delay-based cryptography, from which this area would greatly benefit.

When peers rate each other, they may rate inaccurately to boost their own reputation or unfairly lower another’s. This could be mitigated by having a reputation server incentivise accurate ratings with a reward. However, assigning rewards becomes challenging when ratings are anonymous, since the reputation server cannot tell which peers to reward for rating accurately. To address this, we propose an anonymous peer rating system in which users can be rewarded for accurate ratings, and we formally define its model and security requirements. In our system ratings are rewarded in batches, so that users claiming their rewards only reveal they authored one in this batch of ratings. To ensure the anonymity set of rewarded users is not reduced, we also split the reputation server into two entities, the Rewarder, who knows which ratings are rewarded, and the Reputation Holder, who knows which users were rewarded. We give a provably secure construction satisfying all the security properties required. For our construction we use a modification of a Direct Anonymous Attestation scheme to ensure that peers can prove their own reputation when rating others, and that multiple feedback on the same subject can be detected. We then use Linkable Ring Signatures to enable peers to be rewarded for their accurate ratings, while still ensuring that ratings are anonymous. Our work results in a system which allows accurate ratings to be rewarded, whilst still providing anonymity of ratings with respect to the central entities managing the system.

Cryptocurrencies are digital assets which depend upon the use of distributed peer-to-peer networks. The method a new peer uses to initially join a peer-to-peer network is known as bootstrapping. The ability to bootstrap without the use of a centralized resource is an unresolved challenge. In this paper we survey the bootstrapping techniques used by 74 cryptocurrencies and find that censorship-prone methods such as DNS seeding and IP hard-coding are the most prevalent. In response to this finding, we test two other bootstrapping techniques less susceptible to censorship, Tor and ZMap, to determine if they are operationally feasible alternatives more resilient to censorship. We perform a global measurement study of DNS query responses for each the 92 DNS seeds discovered across 42 countries using the distributed RIPE Atlas network. This provides details of each cryptocurrencies' peer-to-peer network topology and also highlights instances of DNS outages and query manipulation impacting the bootstrapping process. Our study also reveals that the source code of the cryptocurrencies researched comes from only five main repositories; hence accounting for the inheritance of legacy bootstrapping methods. Finally, we discuss the implications of our findings and provide recommendations to mitigate the risks exposed.

End-to-end encryption seemingly signifies the death of caching, because current methods ensure that no two sessions are alike. In this paper, we show that servers can reuse encrypted content between sessions, thereby rejuvenating caching. The main idea of our technique is to allow interim nodes to cache content based on pseudo-identifiers instead of real file identities. This enables caching of reusable pseudo-identifiers, whilst maintaining content confidentiality, i.e., ensuring that only the client and the server know the actual identity of the requested file. Furthermore, we provide an extension that prevents client linkability, i.e., ensuring it is impossible to tell if two clients are viewing the same content. Finally, we formally analyse the balance between security and the hit probability performance of the cache.

The exchange of confidential messages is an inherent problem in wireless communication due to the broadcast nature of the radio channel. In this paper, we enhance standard cryptography with information-theoretic techniques by exploiting relays to increase the confidentiality of wireless communication in the presence of one or more eavesdroppers with low-noise receivers. To achieve this, we present a protocol which makes use of relays in two ways. First, the relays re-transmit disjoint encrypted chunks of a message. Second, the relays utilize cooperative jamming techniques to generate pseudo-random signals in order to increase the interference level in the propagation domain. Chunks and interference levels are allocated over relays in such a way that the message can only be decoded within a critical area around the intended receiver. Our simulation results show that this area can be minimized under realistic assumptions on propagation environment and channel knowledge.