Which of the following is essential for supporting non-repudiation in security?

Supporting non-repudiation in security primarily relies on the use of unique user identities. Non-repudiation refers to the assurance that someone cannot deny the validity of their actions. This is crucial in scenarios where accountability is needed, such as financial transactions, legal communications, or sensitive data exchanges.

By assigning unique user identities, a system can ensure that each action taken within that system can be traced back to an individual user. This makes it easier to hold users accountable for their actions, as their unique identifier is linked to their activities.

The other options, while important in various contexts, do not directly contribute to non-repudiation in the same way. For example, shared IDs could lead to ambiguity, making it difficult to ascertain who performed a particular action. Multi-factor authentication enhances security by requiring more than one form of verification but does not necessarily establish identity uniquely. Open network architecture pertains to the structure and design of networks rather than specific user accountability. Thus, unique user identities are essential for establishing a clear and traceable link between users and their actions, fundamental to achieving non-repudiation.