Protection-in-depth, also known as defense-in-depth, is a fundamental security strategy that involves creating a series of independent, redundant, and overlapping security measures. These measures are arranged in layers, similar to the concentric rings of an onion, around the asset being protected. The objective is that if an adversary bypasses one layer of protection, subsequent layers are in place to detect, delay, or prevent them from reaching their target. This layered approach ensures that the failure of a single security control does not result in a total security failure, thereby increasing the overall resilience of the security system.

B. This describes the functions of security measures (deter, detect, delay), which are implemented within the layers, but it does not define the overarching strategy of layering itself.

C. This is a general statement about applying security but lacks the essential concept of multiple, redundant layers that is central to the protection-in-depth strategy.

D. This describes the principle of risk management, where the level of security is proportional to an asset's value, not the specific strategy of how those security measures are structured.

1. ASIS International. (2021). Protection of Assets: Physical Security. Alexandria

VA: ASIS International. In the chapter on "Physical Security's Role in the Enterprise

" the concept of protection-in-depth is explained as a layered security approach

using concentric circles of protection starting from the property line and moving inward to the asset itself (Section 1.3.2

"Layered Security").

2. Fennelly

L. J.

& Perry

M. A. (Eds.). (2018). The Handbook for Security Practitioners and Managers: A Criminological and Managerial Approach. Butterworth-Heinemann. Chapter 4

"Physical Security and Crime Prevention

" discusses protection-in-depth as the "concept of establishing concentric rings of protection around assets" to provide multiple opportunities to deter

detect

and delay an intruder.

3. Garcia

M. L. (2008). The Design and Evaluation of Physical Protection Systems. Butterworth-Heinemann. Chapter 2

"System Design

" introduces the concept of defense-in-depth as a primary strategy for physical protection systems

emphasizing the use of successive layers of security to protect assets (pp. 21-22).