In recent years in order to access the persistent block storage devices i.e. virtual disks Block-level cloud storage (BLCS) could be used. By using this users and applications access those type of devices and used those as a raw physical disks. This survey discusses ENIGMA architecture which is used as the back-end of BLCS systems. The ENIGMA able to provide adequate levels of access and transfer performance, availability, integrity, and confidentiality, for the data it stores. ENIGMA provides LT rate less codes to store fragments on storage nodes which should be organized in clusters. The various ENIGMA system parameters quantitatively affect the performance, availability, integrity, and confidentiality of virtual disks. The analytical modeling evaluation could be done by measuring availability, integrity and confidentiality and discrete event simulation could be done by measuring the performance and by considering a set of realistic operational scenarios. The ENIGMA system useful to simultaneously achieve all the objectives set for BLCS systems. The careful choice of the various system parameters is crucial to achieve a good compromise among them. The LT codingbased BLCS systems outperform traditional BLCS systems in all the aspects.