  The widening spectrum of applications and services provided by portable and
embedded devices bring a new dimension of concerns in security. Most of those
embedded systems (pay-TV, PDAs, mobile phones, etc...) make use of external
memory. As a result, the main problem is that data and instructions are
constantly exchanged between memory (RAM) and CPU in clear form on the bus.
This memory may contain confidential data like commercial software or private
contents, which either the end-user or the content provider is willing to
protect. The goal of this paper is to clearly describe the problem of
processor-memory bus communications in this regard and the existing techniques
applied to secure the communication channel through encryption - Performance
overheads implied by those solutions will be extensively discussed in this
paper.
