By Dirk Rijmenants
Abstract The Cuban Intelligence Service has a long record of broadcasting encrypted numbers messages by shortwave radio to communicate with its clandestine agents abroad. Although it is considered a secure way to communicate covertly, there have been espionage cases where the use of this type of communication provided hard physical evidence, resulting in criminal complaints and convictions.
Cuban Intelligence
The United States is the principal foreign target of the Cuban Intelligence Service (CuIS). Therefore, it is no surprise that CuIS officers and agents, recruited and controlled by CuIS in the United States, are important targets of the counter-intelligence efforts of the FBI. In recent years, the FBI has uncovered several important Cuban spy operations.
One common link between all recent spy cases is how these agents received their operational messages. Apparently, the clandestine communication methods, presented in this paper, are standard CuIS procedures. Despite CuIS using a cryptographic system, proven to be unbreakable, the FBI did succeeded in reading some of these operational messages and subsequently used them in court.
This paper is based on official FBI documents and the court papers on these espionage cases. It shows procedural and implementation flaws by the CuIS and its agents. These flaws resulted in incriminating evidence that contributed to arrest and the conviction of the clandestine agents.
Read the entire assessment here: http://users.telenet.be/d.rijmenants/papers/cuban_agent_communications.pdf
Cuba Confidential 
http://stasi-minint.blogspot.de/2012/07/colaboracion-servicios-de-inteligencia.html