A merged entity is facing a challenge integrating its central authentication service, based on a well-established directory protocol, with a newly acquired company's applications that authorize users through a popular web-based authorization standard. To achieve a cohesive authentication process across both existing and newly included systems, what is the appropriate solution to adopt?
Establish an encrypted channel between the two systems to securely map user accounts from one to the other.
Enforce strict two-factor authentication for users of both systems to enhance overall security posture.
Amend password management policies to standardize user credentials across all systems and applications.
Employ a federation service to bridge the two distinct user authentication systems for seamless operation.