# Anomaly Detection in Blockchain Networks Using Unsupervised Learning: A Survey
## 3 Blockchain Overview
### Basic Characteristics of Blockchain  
Several characteristics and properties are attributed to a fully operational blockchain network. Figure 1 depicts a subset of those characteristics that are well documented in the existing literature and are briefly analyzed within the subsequent paragraphs.  
Blockchain operates within a decentralized paradigm, enabling a network of individuals or organizations to securely record transactions [19; 69]. An innovative facet introduced by blockchain is its capacity to facilitate secure agreements between multiple entities over public networks, without the need for third-party intermediaries [71]. This process, known as \"mining\", assures the validity and consistency of appended agreements [59; 72].  
The immutability feature ensures that no tampering with the data can take place [73]. Given that each transaction is verified and recorded in distributed blocks, breaching the system becomes practically insurmountable. This intrinsic security feature guarantees the integrity of the blockchain [74], allowing for the creation of exact copies of transactions by the users, resulting in a situation where the data cannot be changed without the consent of the users [75; 76].  
The feature of flexibility refers to the fact that blockchain technologies are based on open-source code, meaning that any user or institution can develop applications or even new blockchain networks to fit their needs [77]. This is supported by the existing availability of several flexible blockchain platforms.  
The concept of anonymity is related to the users' engagement with the blockchain using a generated address, shrouding their identity [78]. Moreover, users can create multiple addresses to evade identity exposure [79]. Unlike centralized systems, no central entity retains users' private information, preserving a degree of privacy in blockchain-recorded transactions [75; 80].  
Transparency facilitates the users to perform clear reviews of historical transactions, while no one wields the authority to alter or expunge them, preserving an immutable record of the group's activities. Thus, the transparency feature, coupled with the blockchain's distributed nature, ensures heightened traceability and thwarts unauthorized interventions [75; 76].  
The security and privacy features are related to the use of public key encryption to protect data security. Blockchain enables a user to create private and public keys. The private key is used to sign data, whereas the public key to confirm the originality and authenticity of the signed data. Keeping the private key safe from leaking is of the utmost importance. Users are anonymous, and each one of them can be assigned multiple  
Figure 1: Basic characteristics possessed by a blockchain network.  
addresses. Then, user privacy is protected by using only one address for identification purposes, while the anonymous address cannot be mapped to a user [74; 75; 76; 77].  
A task strongly related to the security feature is digital signing. A digital signature process encompasses the signing and verification, while it serves as a cryptographic proof system, affirming the blockchain's validity and cultivating trust among users [25]. Grounded in asymmetric cryptography algorithms, each user applies the private key for signing transactions accessible via public keys, permeating the network [58; 80]. In essence, digital signing enhances the trust within decentralized blockchain networks.