Credit networks model transitive IOweYou (IOU) credit between their users. With their flexible-yet-scalable design and robustness against intrusion, we are observing a rapid increase in their popularity as a backbone of real-world permission-less payment settlement networks (e.g., Ripple and Stellar) as well as several other weak-identity systems requiring Sybil-tolerant communication. In payment scenarios, due to their unique capability to unite emerging crypto-currencies and user-defined currencies with the traditional fiat currency and banking systems, several existing and new payment enterprises are entering in this space. Nevertheless, this enthusiasm in the market significantly exceeds our understanding of security, privacy, and reliability of these inherently distributed systems. Currently employed ad hoc strategies to fix apparent flaws have made those systems vulnerable to bigger problems once they become lucrative targets for malicious players. In this tutorial, we first define the concept of IOU credit networks, and describe some of the important credit network applications. We then describe and analyze recent and ongoing projects to improve the credit-network security, privacy and reliability. We end our discussion with interesting open problems and systems challenges in the field. This introductory tutorial is accessible to the standard CCS audience with graduate-level security knowledge.
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