Decentralized systems are distributed systems that disperse computation tasks to multiple parties without relying on a trusted central authority. Since any party can be attacked and compromised by malicious adversaries, ensuring security becomes a major concern in decentralized systems. Depending on the model of decentralized systems, different computation tasks leverage cryptography and secure protocols to protect their security and obtain dependable outputs. In this dissertation, we examine prior security solutions and study the inherent difficulties of securely performing computation tasks in decentralized systems by focusing on three complementary components.

• We evaluate the performance of cryptographic algorithms in decentralized systems where nodes may have different amounts of computing resources. We provide a benchmark of widely deployed cryptographic algorithms on devices with a different extent of resource constraints, and show what computing capabilities are required for a device to perform expensive cryptographic operations.
• We investigate the dependability issue in individual decentralized systems, where parties are not allowed to communicate with each other. We show that even if some parties are compromised or malicious, the entire decentralized system can still converge to a dependable result.
• We address the privacy concern in collaborative decentralized systems, where parties need to share information with each other. We show that parties can collaborate with each other and obtain a dependable result without revealing any useful information about their privacy.