In the wake of fraud scandals involving decentralized exchanges and the significant financial loss suffered by individuals, regulators are pressed to put mechanisms in place that enforce customer protections and capital requirements in decentralized ecosystems. Proof of liabilities (PoL) is such a mechanism: it allows a prover (e.g., an exchange) to prove its liability to a verifier (i.e., a customer).

Token payment systems were the first application of blockchain technology and are still the most widely used one. Early implementations of such systems, like Bitcoin or Ethereum, provide virtually no privacy beyond basic pseudonymity: all transactions are written in plain to the blockchain, which makes them linkable and traceable.

Fabric is a modular and extensible open-source system for deploying and operating permissioned blockchains and one of the Hyperledger projects hosted by the Linux Foundation (www.hyperledger.org).

Deniable encryption, first introduced by Canetti et al . 1997, allows equivocation of encrypted communication. In this work, the authors generalise its study to functional encryption (FE). The authors’ results are summarised as follows: They first put forward and motivate the concept of receiver‐deniable FE, for which they consider two models. In the first model, as previously considered by O'Neill et al . 2011 in the case of identity‐based encryption, a receiver gets assistance from the master authority to generate a fake secret key. In the second model, there are ‘normal’ and ‘deniable’ secret keys, and a receiver in possession of a deniable secret key can produce a fake but authentic‐looking normal key on its own. In the first model, they show a compiler from any FE scheme for circuits to a FE scheme having receiver deniability. In addition, they show an efficient receiver‐deniable FE scheme for Boolean formulae from bilinear maps. In the second (multi‐distributional) model, they present a specific FE scheme for circuits having receiver deniability. To the authors’ knowledge, a scheme in the multi‐distributional model was not previously known even for the special case of identity‐based encryption. Finally, they construct the first sender (non‐multi‐distributional) deniable FE scheme.

Inspired by the fact that many e-mail addresses correspond to groups of users, Abdalla introduced the notion of identity-based encryption with wildcards (WIBE), which allows a sender to simultaneously encrypt messages to a group of users matching a certain pattern, defined as a sequence of identity strings and wildcards. This notion was later generalized by Abdalla, Kiltz, and Neven, who considered more general delegation patterns during the key derivation process. Despite its many applications, current constructions have two significant limitations: 1) they are only known to be fully secure when the maximum hierarchy depth is a constant; and 2) they do not hide the pattern associated with the ciphertext. To overcome these, this paper offers two new constructions. First, we show how to convert a WIBE scheme of Abdalla into a (nonanonymous) WIBE scheme with generalized key delegation (WW-IBE) that is fully secure even for polynomially many levels. Then, to achieve anonymity, we initially consider hierarchical predicate encryption (HPE) schemes with more generalized forms of key delegation and use them to construct an anonymous WW-IBE scheme. Finally, to instantiate the former, we modify the HPE scheme of Lewko to allow for more general key delegation patterns. Our proofs are in the standard model and use existing complexity assumptions.

It has been recently discovered that some cyclic groups that could be used in Cryptography admit a special bilinear pairing map that introduces extra structure to the group. Bilinear pairing maps were first used to break cryptosystems (see, for example, ) and later it was realized that the extra structure could be exploited to build cryptosystems with extra properties. Boneh and Franklins identity-based encryption scheme is the most famous early example of what could be achieved using bilinear maps. After that, a plethora of cryptosystems have been designed using bilinear maps. No full and freely available implementation of pairing based cryptography was available until this work. Recent proposals fall short of this goal as either their source code is not available or because they support a limited range of elliptic curve. Moreover, neither one of implements preprocessing that is crucial to reduce the computation time. In this work, we present jPBC a Java port of the PBC library written in C. jPBC provides a full ecosystem of interfaces and classes to simplify the use of the bilinear maps even for a non-cryptographer. jPBC supports different types of elliptic curves, preprocessing which can speedup the computation significantly and it is ready for the mobile world. Moreover a benchmark comparison between jPBC and PBC has been performed to measure the gap between the two libraries. Furthermore jPBC has been benchmarked on different Android mobile platforms.