The Internet of Things (IoT) is one of the popular domains in real-time analytics, ma-chine learning, ubiquitous computing, commodity sensors, and embedded systems where remote smart devices play notable roles in smart homes and industry. The information from emerging IoT environments like remotely-controlled objects, autonomous vehicles (AVs), and energy management can produce a huge amount of data. Moreover, under-standing the security in a scalable decentralized IoT environment is a significant issue.Decentralization has become popular again in the world since cryptocurrencies started to be a part of businesses. Therefore, researchers invested in upgrading resources to increase the reliability of these systems among people when most of the activities and human works are now managed by smart electronic devices remotely. Distributed ledgers, Distributed Hash Tables (DHTs), and blockchain technologies are proper decentralized technologies that improve system security, scalability, and trustworthiness. Blockchains contain a group of connected blocks that are digitally signed transactions stored in a decentralized fashion. The DHT technology is another decentralized solution that helps applications keep files and information immutable in a decentralized manner to mitigate the high cost of storage without memory limitations.In this thesis, we argue for a decentralized systems paradigm and, in conjunction with IoT and the blockchain. Our contributions are as follows. First, we introduce the term networks and service architectures and how it is possible to use blockchain in the real world. We consider different architectures in IoT systems and show the blockchain en-counter with the IoT and the resulting behavior. Second, we detect most of the frequent types of attacks in IoT related to using blockchain in the systems. We also describe how the blockchain works and illustrate a variety of security problems in systems. Fur-thermore, we discuss how the blockchain solves security problems by comparing different blockchains and explain how users handle their communication without third-party de-pendence. As our third contribution, we propose a novel architecture that consists of finding global identification in distributed applications and enable decentralized systems to be more secure with the help of blockchain technology. We also validate the proposed architecture and novel decentralized application development to evaluate high efficiency by combining blockchain, DHT, and biometric technologies.