# Latency Analysis of Blockchain-Based SSI Applications +
## 1 Introduction  
Distributed ledger technology (DLT) is a data storage technology managed by multiple nodes in a decentralized way. Blockchain (BC) [1] is a special case of DLT, where blocks contain the data, and every block references the hash of the previous block, which makes it more temper-proof. This technology is utilized for enhancing trust in distributed computing applications and managing distributed ledgers (DL). Applications integrated with permissionless BCs benefit from high levels of security and trust, where BCs provide a fully immutable log of transaction (TX) history without the control of a central authority. Similarly, permissioned BCs utilize a distributed Trusted Third Party (TTP) for providing services typically requested from a central trusted third party. In applications that require trusted BC nodes, permissioned BCs, typically referred to as Miners, Miners, Verifiers, or Validaters, are used to maintain a consistent and trusted DL. Initially, those participants apply to become BC nodes to a committee that votes for acceptance. Permissionless BC solutions have been designed first for realizing digital cryptocurrencies [2], but nowadays they address a wide variety of environments, such as document management [3], smart applications for eHealth [4], or Internet of Vehicles [5]. In such access model, any party can join the BC network and collaborate in validating and confirming new pieces of data added to the DL, without a network size limit. For applications that require a distributed yet controlled trusted third party, permissioned BC solutions are suitable, where a limited number of verifiers vote for appending data into the DL and for accepting new verifiers. Examples of applications that require such a BC model include distributed voting [6], Self Sovereign Identity (SSI) [7], and credential management [8]. Specifically, classical internet applications lack an identity layer [9]. Web applications, for instance, use usernames and passwords to identify their clients, yet the use of those/such attributes is typically limited to the specific contexts of individual applications. Such a framework is considered inconvenient and implies several security issues (e.g., identity theft and fake users) [10]. Global identity [11] providers, such as Google or Meta, have attempted to develop their access framework to enhance usability. However, data leaks are still argued to remain an open issue [12]. Additionally, the loss of clients' control over their private data, which is typically saved on centralized servers, implies trust issues. Some solutions allow users to request the deletion of their data, in order to comply with the European General Data Protection Regulations (GDPR), yet users have to also trust the provider to delete them. SSI concepts enable users to anonymously identify themselves, and verify some private attributes about them, while maintaining full control and permissions over their private data. Although permissioned BCs have shown unprecedented abilities to maintain DLs with high security and trust measures, compared with non-BC-based DLs, they still suffer from high latency compared with centralized solutions. Furthermore, different permissioned BC solutions are designed differently according to their application and the different layering of the solution. This usually causes a fluctuation of latency measures among different permissioned BC solutions and even for differently parameterized solutions. Such issues mainly appear due to several system properties and limitations, such as the minimum time needed to reach a consensus on a piece of data (i.e., the finality time), the average transmission delay between network entities, the total network size, and the average number of neighbors per miner [13]. This being said, researchers and practitioners tend to model their applications and test several BC solutions for a decision to be made regarding the deployment of a specific BC framework.