# Blockchain-Based Multimedia Content Protection: Review and Open Challenges
## 4 Limitations, Open Challenges and Future Research Directions
### Limitations and Research Challenges of Content Protection Techniques  
The limitations and research challenges of the multimedia content protection techniques presented in Section 2.6 are discussed below:  
* Encrypted content is only as secure as the key used for encrypting it. Thus, cryptographic keys must be carefully managed (e.g., transmission, storage or updating) to ensure data remains protected, yet accessible when needed among multiple users of a system.
* Encryption techniques cannot prevent a user from unauthorized usage and illegal distribution of the content upon decryption of the received content.
* Most of the research work involving DRM is non-interoperable, which does not constitute an efficient option for clients. For this reason, consumers may seek alternative options to obtain the content, such as P2P file-sharing applications. However, to make DRM systems interoperable, the content providers or vendors of multimedia players would need to know the sensitive information related to the DRM protection scheme, thus increasing the risk of leakage. In such a case, a single leakage (or \"hack\") has the potential to compromise not only one of several distribution channels, but the distribution of all interoperably DRMed content.
* DRM systems may give rise to a number of legal issues if not used correctly, e.g., the use of monitoring tools, either intentionally or unintentionally, to report and collect data pertaining to their consumers' habits and preferences (such as the type of content they enjoy, when they enjoy it and even where, by accessing users location details, among others). This may result in serious privacy implications, such as using these data for purposes unrelated to the platform or selling it to third parties.  
At the embedder's end in a digital watermarking scheme, maintaining an appropriate balance between robustness, capacity and imperceptibility properties is a challenging task, since these properties contradict one another, i.e., if one is increased, the other decreases. Security and complexity features have been given less priority in the development of the watermarking schemes. The more complex the watermarking scheme is, the higher the costs associated with the embedding and detection processes. The costs associated with the watermark embedding and detection should be minimal in order to meet the real-time requirements. In digital fingerprinting schemes based on collusion-resistant codes, there exists a trade-off among the size of a user base \(N\), the collusion resilience \(c_{0}\), and the codeword length \(m\). With an increase in \(N\) or \(c_{0}\), the length of \(m\) increases and vice versa. This trade-off may make the traceable code impractical because many applications require a large user base and collusion resistance. However, these requirements will result in long traceable codes. Most of the research work involving anonymous fingerprinting protocols assume the presence of a trusted third party that is responsible for generating the fingerprint and tracing the copyright violators. This trust implies a belief by the user that the trusted entity will behave in an expected manner in order to ensure security and anonymity. In a few other schemes that have avoided the use of such a party, the computational and communicational overheads are quite high due to the use of at least one of the following highly demanding technologies: homomorphic encryption, bit-commitment or secure multi-party computation. All types of attacks--internal and external--should be taken into consideration to evaluate the security and robustness of novel or existing watermarking and fingerprinting schemes.