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We present "Ouroboros Praos", a proof-of-stake blockchain protocol that, for the first time, provides security against fully-adaptive corruption in the semi-synchronous setting: Specifically, the adversary can corrupt any participant of a dynamically evolving population of stakeholders at any moment as long the stakeholder distribution maintains an honest majority of stake; furthermore, the protocol tolerates an adversarially-controlled message delivery delay unknown to protocol participants. To achieve these guarantees we formalize and realize in the universal composition setting a suitable form of forward secure digital signatures and a new type of verifiable random function that maintains unpredictability under malicious key generation. Our security proof develops a general combinatorial framework for the analysis of semi-synchronous blockchains that may be of independent interest. We prove our protocol secure under standard cryptographic assumptions in the random oracle model. Tokyo Institute of Technology and IOHK, bdavid@c.titech.ac.jp. IOHK, peter.gazi@iohk.io.

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This paper presents efficient structure-preserving signature schemes based on assumptions as simple as Decision-Linear. We first give two general frameworks for constructing fully secure signature schemes from weaker building blocks such as variations of one-time signatures and random-message secure signatures. They can be seen as refinements of the EvenGoldreich-Micali framework, and preserve many desirable properties of the underlying schemes such as constant signature size and structure preservation. We then instantiate them based on simple (i.e., not q-type) assumptions over symmetric and asymmetric bilinear groups. The resulting schemes are structure-preserving and yield constant-size signatures consisting of 11 to 17 group elements, which compares favorably to existing schemes relying on q-type assumptions for their security.

We present an efficient structure-preserving tagged one-time signature scheme with tight security reductions to the decision-linear assumption. Our scheme features short tags consisting of a single group element and gives rise to the currently most efficient structure-preserving signature scheme based on the decision-liner assumption with constant-size signatures of only 14 group elements, where the record-so-far was 17 elements.To demonstrate the advantages of our scheme, we revisit the work by Hofheinz and Jager (CRYPTO 2012) and present the currently most efficient tightly secure public-key encryption scheme. We also obtain the first structure-preserving public-key encryption scheme featuring both tight security and public verifiability.