# Blockchain and Cloud to Overcome the Problems of Buyer and Seller Watermarking Protocols
## 2 Related Work
The watermarking protocols reported above can be considered as some of the most relevant examples of how the \"buyer and seller\" scheme has undergone a constant evolution over time. However, in the literature, there are no significant examples of watermarking protocols able to employ both the blockchain technology and cloud resources. On the contrary, a number of DRM systems exploit blockchain and digital watermarking technologies to implement copyright management services, such as to keep track of content modifications, copyright transfers or other transaction trails related to the managed digital contents [29; 30]. The project documented in [31] represents a copyright management system that only involves buyers and sellers, which can directly interact in the purchase transactions without the need of TTPs. The system employs an improved ElGamal encryption algorithm to efficiently protect the keys used in the interaction among buyers and sellers, and exploits a public blockchain to securely record copyright and transaction data. It also resorts to smart contracts and InterPlanetary File System (IPFS): the former are used to manage copyright information and the purchase transactions, whereas the latter stores watermarked contents. Even though the system cleverly implements all the phases needed to digitally protect contents, its authors do not provide any evidence that the system is secure and not affected by the classic \"customer's rights problem\" or the \"unbinding problem\". BMCProtector [32] is an application to protect music copyright based on a public blockchain and smart contracts. In particular, it uses smart contracts registered in the blockchain to share the copyright parameters of the music owners and to automatically pay royalties to copyright owners. Moreover, it uses IPFS as an external storage for watermarked audio files, and exploits the AES algorithm to encrypt these files. Music distribution and usage are managed by an off-chain, classic access control mechanism characterizing DRM systems. Preliminary tests show that BMCProtector can achieve a transaction rate equivalent to that one exhibited by commercial and proprietary DRM systems. However, BMCProtector employs a TTP in the form of a Key Protection Center needed to manage the keys that music owners use to decrypt the audio files without disclosing them to buyers. Furthermore, as with [31], the security of BMCProtector is not proved. DRMChain [33] is a blockchain based DRM system that implements trusted content protection and conditional traceability of protected content violations. It employs IPFS to provide a flexible storage of protected contents, and implements a number of services, such as an efficient privacy preserving user authentication, content watermarking and encryption, and a license and violation tracing system based on a conditional identity management. Although DRMChain does not represent a proof-of-concept and its performance has been evaluated, security has been proved only for the single implemented services. As a consequence, the proposed system lacks a global security analysis concerning the behavior of DRMChain towards the classic security problems of watermarking protocols. The examples of the DRM systems reported above show how they mainly attempt to combat the unauthorized use of protected digital contents without payment. On the contrary, most of them cannot prove to be able to counter the illegal sharing of contents legitimately bought by web users [34]. In fact, once contents are downloaded and tampered, the above DRM systems cannot legally prove the ownership of the contents as well as identify those responsible for copyright infringement.