HYBRID BLOCKCHAIN ARCHITECTURE FOR SMART CITY APPLICATIONS

Smart cities integrate thousands of heterogeneous IoT subsystems—traffic management, public utilities, emergency services, and environmental monitoring—across a shared urban data fabric. This integration creates a security surface that existing centralized architectures cannot adequately protect: a single compromised control node can propagate incorrect commands across an entire district, traffic signals can be manipulated to cause gridlock or emergency vehicle delays, and citizen data collected at scale is routinely stored without the access controls that GDPR mandates. This paper proposes a hybrid blockchain architecture for smart city applications that addresses three persistent gaps: absence of tamper-evident cross-domain data governance, lack of decentralized access control that scales across heterogeneous city subsystems, and insufficient auditability for citizen-facing data processing. The proposed architecture combines a public Ethereum layer for cross-city transparency and regulatory reporting with a permissioned Hyperledger Fabric layer for high-throughput intra-city operations. Smart-contract-driven attribute-based access control enforces data sharing policies across subsystems, while a cross-layer audit bridge synchronizes tamper-evident event logs between both chains. Experimental evaluation across a simulated 12-district smart city deployment with 84,000 active IoT endpoints demonstrates 91.4% reduction in unauthorized cross-domain data access, 2,340 TPS sustained throughput on the Fabric layer, and full GDPR Article 25 compliance enforcement at the data collection point. The framework was validated against NIST SP 800-183 IoT security guidelines and ISO 27001 information security management requirements.