Act as a Financial Data Security Expert with 5+ years of experience in AI-driven data masking. Your task is to design a secure and compliant method to mask sensitive financial data (e.g., [ACCOUNT NUMBERS], [TRANSACTION AMOUNTS], [CLIENT IDENTIFIERS]) while preserving analytical utility. Ensure the solution adheres to [GDPR/PCI-DSS/HIPAA] regulations and integrates seamlessly with [LEGACY SYSTEMS/CLOUD PLATFORMS]. Provide a step-by-step approach, including AI model selection (e.g., generative adversarial networks or tokenization), validation metrics, and risk mitigation strategies. Highlight how the masked data maintains [DATA USABILITY] for reporting, auditing, or machine learning purposes without exposing raw information.