This section is intended to explain how security works in the IMS. It is intentionally thin in cryptography and, thus, will not discuss algorithms and key lengths in depth, nor will it perform any cryptanalysis on IMS security. There are many books specific-ally written for that purpose.
 See, for example, V. Niemi and K. Nyberg (2003) UMTS Security, John Wiley & Sons, Chichester, UK.
Instead, what this chapter will do is give a high-level view of the security architecture and explain the components of that architecture, including the models and protocols used to provide the required security features. After reading this chapter the reader should be familiar with the main concepts in the IMS security architecture and understand the underlying models, especially those related to trust and identity that shape IMS security as a whole.
The IMS security architecture consists of three building blocks, as illustrated in Figure 3.29. The first building block is Network Domain Security (NDS) [3GPP TS 33.210], which provides IP security between different domains and nodes within a domain. Layered alongside NDS is IMS access security [3GPP TS 33.203]. The access security for SIP-based services is a self-sustaining component in itself, with the exception that the security parameters for it are derived from the UMTS Authentication and Key Agreement (AKA) Protocol [3GPP TS 33.102]. AKA is also used for bootstrapping purposes ...