Who is responsible for applying encryption to a data set on a hard disk?

The responsibility for applying encryption to a data set on a hard disk primarily falls under the role of the data custodian. A data custodian is tasked with the physical and technical management of data, which includes implementing security measures like encryption to protect the data from unauthorized access and breaches.

The data custodian ensures that data is handled according to the policies set by the data owner and is tasked with maintaining the security and integrity of the data. In this context of encryption, the custodian would typically be the individual or the team responsible for the actual implementation of encryption technologies and ensuring that those technologies align with the organizational security policies.

While the other roles mentioned, such as the data owner and data controller, have critical responsibilities regarding data governance and oversight, they do not directly apply encryption. The data owner determines who has access to the data and how it should be protected, while the data controller oversees the processing of the data according to the owner's directives. The data processor, on the other hand, handles data on behalf of the data controller but does not necessarily focus on the application of security measures such as encryption.

Therefore, the proper alignment of roles indicates that the data custodian is the most responsible party for applying encryption to datasets.