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Ouroboros Protocol
Ouroboros Protocol
Highly scalable and sustainably secure fundamental protocol of Cardano
Initial release23 Jul 2020
Written inHaskel
LicenseApache-2.0 license

Ouroboros is a proof-of-stake (PoS) blockchain protocol that was developed by the blockchain research and development company IOHK (Input Output Hong Kong). It is designed to be a fast and secure protocol for decentralized applications and is based on the Ouroboros proof-of-stake protocol, which was first introduced in a research paper by IOHK in 2017.

One of the key features of Ouroboros Classic is its use of a decentralized and secure network of "validators" to validate transactions and secure the network. These validators are selected through a randomized selection process responsible for producing new blocks added to the blockchain.

Ouroboros processes transaction blocks by dividing chains into epochs, which are further divided into time slots. A slot leader is elected for each time slot and is responsible for adding a block to the chain. To protect against adversarial attempts to subvert the protocol, each new slot leader is required to consider the last few blocks of the received chain as transient: only the chain that precedes a prespecified number of transient blocks is considered settled. This is also referred to as the settlement delay and is the mechanism through which the ledger is securely passed between participants.

Ouroboros Classic

A Provably Secure Proof-of-Stake Blockchain Protocol

Ouroboros is the first provably secure proof-of-stake protocol, and the first blockchain protocol to be based on peer-reviewed research. Ouroboros combines unique technology and mathematically-verified mechanisms - which, in turn, combine behavioral psychology and economic philosophy - to ensure the security and sustainability of the blockchains that depend upon it. The result is a protocol with proven security guarantees able to facilitate the propagation of global, permissionless networks with minimal energy requirements - of which Cardano is the first.

We present “Ouroboros”, the first blockchain protocol based on proof of stake with rigorous security guarantees. We establish security properties for the protocol comparable to those achieved by the bitcoin blockchain protocol. As the protocol provides a “proof of stake” blockchain discipline, it offers qualitative efficiency advantages over blockchains based on proof of physical resources (e.g., proof of work). We also present a novel reward mechanism for incentivizing Proof of Stake protocols and we prove that, given this mechanism, honest behavior is an approximate Nash equilibrium, thus neutralizing attacks such as selfish mining. We also present initial evidence of the practicality of our protocol in real-world settings by providing experimental results on transaction confirmation and processing.


Ouroboros BFT

A Simple Byzantine Fault Tolerant Consensus Protocol

Ouroboros-BFT is a new BFT ledger consensus protocol inspired by the design of the Ouroboros protocol, a proof-of-stake blockchain protocol. Ouroboros-BFT is a deterministic protocol with simplicity as one of its prime design criteria.


Ouroboros Chronos

Permissionless Clock Synchronization via Proof-of-Stake

Clock synchronization allows parties to establish a common notion of global time by leveraging a weaker synchrony assumption, i.e., local clocks with approximately the same speed. The problem has long been a major goal for fault-tolerant distributed computing with a number of ingenious solutions in various settings. However, despite intensive investigation, the existing solutions do not apply to common blockchain protocols, which are designed to tolerate variable---and potentially adversarial---participation patterns, e.g., sleepiness and dynamic availability. Furthermore, because such blockchain protocols rely on freshly joining (or re-joining) parties to have a common notion of time, e.g., a global clock that allows knowledge of the current protocol round, it is not clear if or how they can operate without such a strong synchrony assumption.


Ouroboros Crypsinous

Privacy-Preserving Proof-of-Stake

the first formally analyzed privacy-preserving proof-of-stake (PoS) blockchain protocol. To model its security we give a thorough treatment of private ledgers in the universal composition (UC) setting that might be of independent interest. To prove our protocol secure against adaptive attacks, which are particularly critical in the PoS setting, we introduce a new coin evolution technique that relies on SNARKs and key-private forward secure encryption. The latter primitive–and the associated construction–can be of independent interest. We stress that existing approaches to private blockchains, such as the proof-of-work-based Zerocash are analyzed only against static corruptions.


Ouroboros Praos

An adaptively secure, semi-synchronous proof-of-stake protocol.

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.