Assertions are critical components used in hardware verification, ensuring robust functionality, fortifying design security, and providing essential verification features. Traditional hardware assertion methods are not automated, complicate security audits, and require effort, causing prolonged development cycles. Recent studies have highlighted the potential of commercial Large Language Models (LLMs) to generate security-focused assertions by leveraging textual data from design specifications. However, reliance on proprietary models like GPT-4 severely jeopardizes IP privacy and data confidentiality, undermining transparency and accountability in data handling practices. In this paper, we address secure hardware assertion generation by proposing a practical approach to significantly enhance the feasibility of open-source LLMs. Our proposed method, OpenAssert, involves fine-tuning existing models to be utilized locally at the user’s end without compromising confidentiality. Additionally, we employ Retrieval Augmentation Generation to refine these models, mitigating hallucinations and security-related errors. OpenAssert demonstrates improvements, achieving up to a 44% increase in rouge-1 score, a 49% improvement in cosine similarity, and a 43.4% reduction in word error rate for security-critical designs compared to open-source models.