There are a number of well-developed standards currently being used to deliver digital video over cable systems in the area of modulation, forward error correction, transport and compression. However, encryption and conditional access systems are typically, by their very nature, highly proprietary systems. This presents a roadblock to the support of multi-vendor set-tops in cable systems. One possible solution is the adoption of a common, standard, service encryptor at the lowest level combined with multiple conditional access systems that provide key distribution and control functions. This paper will describe the technical challenges in building multiple-conditional access systems and argue that, in practice, other standards are required to support cost effective deployment of multi-vendor set-tops.