  Wearable fitness devices are widely used to track an individual's health and
physical activities to improve the quality of health services. These devices
sense a considerable amount of sensitive data processed by a centralized third
party. While many researchers have thoroughly evaluated privacy issues
surrounding wearable fitness trackers, no study has addressed privacy issues in
trackers by giving control of the data to the user. Blockchain is an emerging
technology with outstanding advantages in resolving consent management privacy
concerns. As there are no fully transparent, legally compliant solutions for
sharing personal fitness data, this study introduces an architecture for a
human-centric, legally compliant, decentralized and dynamic consent system
based on blockchain and smart contracts. Algorithms and sequence diagrams of
the proposed system's activities show consent-related data flow among various
agents, which are used later to prove the system's trustworthiness by
formalizing the security requirements. The security properties of the proposed
system were evaluated using the formal security modeling framework SeMF, which
demonstrates the feasibility of the solution at an abstract level based on
formal language theory. As a result, we have empirically proven that blockchain
technology is suitable for mitigating the privacy issues of fitness providers
by recording individuals' consent using blockchain and smart contracts.
