*2.5. Consensus Protocols*

A consensus mechanism comprises protocols and algorithms, which establish the rules that the nodes (devices on the blockchain that maintain the blockchain and sometimes process transactions) must follow to validate the blocks. This mechanism solves data synchronization between the nodes that do not trust each other in a distributed system. The consensus protocol is a fault-tolerant mechanism that is used to achieve the necessary agreement on a single data value or a single state of the network. It consists of the following objectives: reaching an agreement, collaboration, cooperation, equal rights to every node, and mandatory participation of each node. Most blockchains use one of the following commonly used consensus protocols:


#### *2.6. Multimedia Content Protection Techniques*

Generally, a content protection technique can be defined as a measure to protect multimedia data against the threats arising from an unauthorized access to a user or a group of users. The protected properties generally include copy protection, traceability, authentication of content source and receivers, usage control, digital rights associated with the content, and secure distribution of content and access keys. According to Arnold et al. [32], any end-to-end content protection method should address all these basic security properties. Thus, an end-to-end copyright protection system is expected to provide security before and after transmission of the content, i.e., it ensures an authorized access of content, and controls the usage of the content once it is in the user's possession. In the following section, a brief overview of the most effective approaches for multimedia content protection is presented.

#### 2.6.1. Multimedia Encryption

The process of encoding plaintext messages into ciphertext messages is called encryption, and the reverse process of transforming ciphertext back to plaintext is called decryption. This technique is expected to provide one or more of the following properties:


In the naïve approach, the entire multimedia content is encrypted using standard encryption methods such as symmetric (e.g., Advanced Encryption Standard (AES), Rivest Cipher (RC5), etc.) and asymmetric (Rivest–Shamir–Adleman (RSA), Digital Signature Algorithm (DSA), etc.) cryptographic algorithms. Since the multimedia content such as audio or video data is typically very large in size, the naïve approach becomes computationally demanding. Nowadays, many new encryption algorithms for audio and video have been proposed to avoid the naïve approach and gain better efficiency. These new algorithms can be divided into various categories: full encryption, selective encryption, joint compression and encryption, syntax-compliant encryption, scalable encryption and multiaccess encryption.

Another important form of public-key cryptography is the homomorphic cryptosystem, which allows certain types of operations (addition/multiplication (partially homomorphic), or both (fully homomorphic) to be carried out over encrypted data while yielding the same encrypted results as if the operations were run on plaintext. Homomorphic cryptosystems are useful for copyright protection applications as they allow content owners to make computations directly to the ciphertext without exposing the keys. This property prevents the buyers' privacy-sensitive information from being exposed.
