What is the main protective measure used to ensure the authenticity of information?

The primary protective measure employed to ensure the authenticity of information is rooted in the principles of communications security, often referred to as COMSEC. This encompasses various techniques and practices designed to safeguard sensitive information from unauthorized access or modification while ensuring that the information remains trustworthy.

COMSEC not only involves the encryption of data to protect its confidentiality but also employs mechanisms to verify the authenticity of the information transmitted. This includes measures like digital signatures, which help confirm that the data originated from a legitimate source and has not been altered during transit. By ensuring authentication, COMSEC plays a crucial role in maintaining the integrity and reliability of communication, making it a fundamental aspect of information security protocols.

In comparing this to the other options, while encryption is essential for safeguarding the data itself, it does not inherently provide assurance of the data's origin or integrity without additional measures like those found in COMSEC. Access control focuses on who can access information but does not address the authenticity directly. Monitoring is vital for detecting and responding to security incidents but is more about response rather than ensuring authenticity upfront. Thus, the focus on COMSEC highlights its comprehensive role in safeguarding not just the data, but also its legitimacy and integrity throughout its lifecycle.