When there is a need to compare the characteristics of several similar global communications systems, security is often an important factor to consider in the comparison. However, in most situations it is difficult to deduce whether a system is more secure than other; and it is even more difficult to deduce how much more secure a system is compared to another. This paper presents a method based on the use of 'attack trees' for the analysis of the security level supported by communications systems. Specifically this paper presents the most relevant aspects of the application of this method to protocols for distributing digital contents with copyright protection; to compare the security level of different systems and to obtain quantitative information about the level of security supported by each protocol.