Blockchain is a growing list of records, called blocks, that are securely linked together using cryptography. Blockchain is employed in different types of applications, such as money transfer, secure sharing of medical data, supply chain and logistics monitoring, voting mechanism, and original content creation. The defense applications have specific security requirements such as integrity, availability, and trust. STPA extended with STRIDE is a method that has been used to identify not only safety requirements but also security requirements. We investigate how blockchain can be used to meet the security requirements, identified by STPA with STRIDE, of a messaging system in a defense organization.