Brief Announcement: Asymmetric Distributed Trust

Quorum systems are a key abstraction in distributed fault-tolerant computing for capturing trust assumptions. They can be found at the core of many algorithms for implementing reliable broadcasts, shared memory, consensus and other problems. This paper introduces asymmetric Byzantine quorum systems that model subjective trust. Every process is free to choose which combinations of other processes it trusts and which ones it considers faulty. Asymmetric quorum systems strictly generalize standard Byzantine quorum systems, which have only one global trust assumption for all processes. This work also presents protocols that implement abstractions of shared memory and broadcast primitives with processes prone to Byzantine faults and asymmetric trust. The model and protocols pave the way for realizing more elaborate algorithms with asymmetric trust

Asymmetric Distributed Trust

Quorum systems are a key abstraction in distributed fault-tolerant computing for capturing trust assumptions. They can be found at the core of many algorithms for implementing reliable broadcasts, shared memory, consensus and other problems. This paper introduces asymmetric Byzantine quorum systems that model subjective trust. Every process is free to choose which combinations of other processes it trusts and which ones it considers faulty. Asymmetric quorum systems strictly generalize standard Byzantine quorum systems, which have only one global trust assumption for all processes. This work also presents protocols that implement abstractions of shared memory and broadcast primitives with processes prone to Byzantine faults and asymmetric trust. The model and protocols pave the way for realizing more elaborate algorithms with asymmetric trust

Federated Consensus Protocols

Αυτή η διπλωματική εργασία μελετά πρωτόκολλα συναίνεσης και συγκεκριμένα το πρωτόκολλο Raft και το πρωτόκολλο Stellar Consensus. Αρχικά ορίζουμε το μοντέλο εκτέλεσης υπό το οποίο μελετάμε τα πρωτόκολλα όπως και την έννοια του ισχυρού κατάστιχου συναλλαγών και τις ιδιότητές του. Στη συνέχεια παρουσιάζουμε το πρωτόκολλο Raft σε αλγοριθμική μορφή και δείχνουμε ότι πράγματι το Raft υλοποιεί ένα ισχυρό κατάστιχο συναλλαγών. Συνεχίζουμε με το πρωτόκολλο Stellar Consensus και αναλύουμε την ομόσπονδη ψηφοφορία, το βασικό μέσο που χρησιμοποιεί το Stellar Consensus για να πετύχει συνέναιση. Έπειτα παρουσιάζουμε τα δύο πρωτόκολλα που αποτελούν το Stellar Consensus, το πρωτόκολλο Nomination και το πρωτόκολλο Ballot, σε αλγοριθμική μορφή. Τέλος δείχνουμε ότι το πρωτόκολλο Ballot έχει και τις δύο απαραίτητες ιδιότητες που οφείλει να έχει ένα πρωτόκολλο για να διατηρεί ένα ισχυρό κατάστιχο συναλλαγών.This dissertation studies consensus protocols and specifically Raft and the Stellar Consensus protocol. We first define the execution model under which we study the protocols as well as the notion of a robust transaction ledger that we want the protocols to maintain and its properties. We proceed by presenting Raft as concrete algorithm and we prove that indeed Raft maintains a robust transaction ledger. We then move to the Stellar Consensus protocol and analyse federated voting, Stellar’s mean to reach consensus. Subsequently, we present the two protocols that constitute the Stellar Consensus protocol, the Nomination and Ballot protocol, as concrete algorithms and further explore their properties. Finally, we show that the Ballot protocol has both persistence and liveness, the two necessary properties a protocol need to have to maintain a robust transaction ledger

Stellar Consensus by Instantiation

Stellar introduced a new type of quorum system called a Federated Byzantine Agreement System. A major difference between this novel type of quorum system and a threshold quorum system is that each participant has its own, personal notion of a quorum. Thus, unlike in a traditional BFT system, designed for a uniform notion of quorum, even in a time of synchrony one well-behaved participant may observe a quorum of well-behaved participants, while others may not. To tackle this new problem in a more general setting, we abstract the Stellar Network as an instance of what we call Personal Byzantine Quorum Systems. Using this notion, we streamline the theory behind the Stellar Network, removing the clutter of unnecessary details, and refute the conjecture that Stellar\u27s notion of intact set is optimally fault-tolerant. Most importantly, we develop a new consensus algorithm for the new setting

Study of consensus protocols and improvement of the Federated Byzantine Agreement (FBA) algorithm

At a present time, it has been proven that blockchain technology has influenced to a great extent the way of human interaction in a digital world. The operation of the blockchain systems allows the peers to implement digital transactions in a Peer to Peer (P2P) network in a direct way without the need of third parties. Each blockchain determines different rules for the record of the transactions in the ledger. The transactions are inserted in blocks and each one, in turn, is appended to the chain (ledger) based on different consensus algorithms. Once blocks have been inserted in the chain, the consensus has been reached and the blocks with corresponding transactions are considered immutable. This thesis analyses the main features of the blockchain and how the consensus can be achieved through the different kinds of consensus algorithms. In addition, a detailed reference for Stellar and Federated Byzantine Agreement (FBA) consensus protocols is made in order to explain these algorithms, their limitations as well as their improvement. The development of a reputation mechanism is necessary to the improvement of above algorithms