A Blockchain based Witness Model for Trustworthy Cloud Service Level Agreement Enforcement

Traditional cloud Service Level Agreement (SLA)

suffers from lacking a trustworthy platform for automatic

enforcement. The emerging blockchain technique brings in an

immutable solution for tracking transactions among business

partners. However, it is still very challenging to prove the credibility

of possible violations in the SLA before recording them onto

the blockchain. To tackle this challenge, we propose a witness

model using game theory and the smart contract techniques.

The proposed model extends the existing service model with

a new role called “witness” for detecting and reporting service

violations. Witnesses gain revenue as an incentive for performing

these duties, and the payoff function is carefully designed in a way

that trustworthiness is guaranteed: in order to get the maximum

profit, the witness has to always tell the truth. This is analyzed

and proved through game theory using the Nash equilibrium

principle. In addition, an unbiased sortition algorithm is proposed

to ensure the randomness of the independent witnesses selection

from the decentralized witness pool, to avoid possible unfairness

or collusion. An auditing mechanism is also introduced in

the paper to detect potential irrational or malicious witnesses.

We have prototyped the system leveraging the smart contracts

of Ethereum blockchain. Experimental results demonstrate the

feasibility of the proposed model and indicate good performance

in accordance with the design expectations.