Coop, a security manager, practices decrypting secure documents. He has plain text of some of the files and needs to decrypt the rest. Which attack should he use?

Domain 1: Security and Risk Management Practice Questions
Questions from the following topics are included in this domain:
Basics of security and risk management
    Differing data roles and responsibilities
    Identifying administrative, physical, and technical controls
    Ethics of security professionals
    Administrative policies, procedures, and guidelines
    Object categorization and classification
    Importance of security training