International Research Journal of Engineering and Technology (IRJET) e-ISSN: 2395-0056 Volume: 05 Issue: 05 | May-2018 www.irjet.net p-ISSN: 2395-0072 © 2018, IRJET | Impact Factor value: 6.171 | ISO 9001:2008 Certified Journal | Page 3423 Blockchain Technology : Classification, Opportunities, and Challenges Nadir Abdelrahman Ahmed Farah Dept. of Information Systems, Arts and Science College, University of Bisha, Saudi Arabia ---------------------------------------------------------------------***--------------------------------------------------------------------- Abstract - Blockchain is a new technology that has emerged with the appearance of the Bitcoin, which has added a new way of dealing financially. Based on the success of this technique with the idea of Bitcoin, the technique has been relied upon and applied gradually in various activities, whether governmental or private and received the confidence and satisfaction of customers. The paper highlights the challenges ahead and opportunities in this Modern technology that is all set to develop our digital world. Key Words: Blockchain, Bitcoin, Block chain Structure, Classification 1. INTRODUCTION Blockchain technology is one of the approaches that has the possibility to enhance decentralization, transparency, equality, and responsibility on the internet[1]. Blockchain is a distributed database of records that can be either public ledger of digital issues or transactions that got achieved and have been shared among participating parties across a large network of untrusted participants. It stores data in blocks that can verify information which are very difficult to hack. It avoids the requirement of a third-party verification and thus deactivates any sector that leverages it traditionally. [2]. Using blockchain can provide higher security compared to storing all data in a central database. The use of these technologies in Bitcoin “mining” was ground-breaking in the data storage and management side, harm from attacks on a database can be prevented. Further, since the blockchain has an openness attribute, it can provide transparency in data when applied to an area requiring the disclosure of data[3]. 2. THE STRUCTURE OF BLOCKCHAIN In general, the block contains main data; the hash of the previous block, a hash of current one, timestamp and other information. Figure 1 shows the structure of block. Main data: Depending on the kind of service in which this blockchain is applicable, for example, transaction records, bank clearing records, contract records or IOT data record. Hash: When a transaction executed, it had been hash to a code and then transmitted to each node. Because it could contained thousands of transaction records in each node’s block, blockchain used Merkle tree function to produce a final hash value, and also Merkle tree root. Timestamp: Time of block produced. Other Information: Like signature of the block, Nonce value, or other data that user defines [7]. Hash of current block1 Hash of previous block2 Timesta mp Other Inform ation Hash of current block2 Hash of previous block3 Timest amp Other Information Main Data 1 …….. Main Data N Main Data 2 …….. Main Data N Fig -1: Structure of Blockchain 3. BLOCKCHAIN CLASIFICATION Four types of blockchains can be defined as shown in Table 1. Table -1: Blockchain Types Based on access to Blockchain Data Based on access to Blockchain Public: All who access can modify Permission-less: Any one can join Private: Only specific users can write / modify Permissioned: Approved users only Public and Permission-less are used interchangeably and so are Private and Permissions. Depending on the use case, one needs to select an appropriate architecture from those defined in Table 1. There are different blockchain based system configurations against multiple parameters such as performance, cost efficiency, and flexibility. Different dimensions of a blockchain system such as blockchain configuration, storage, computation, a degree of decentralization are considered in coming up with the classification [4] 3.1 Public / Permission-less blockchains Public blockchains are open for all. Anyone can join them to post transactions and to participate in the mining and consensus process of adding a new block of a transaction to them. These blockchains usually use Proof of Work (protocol requires all nodes on the network to solve cryptographic puzzles by brute force) or Proof of Stake (protocol of block verification does not rely on excessive computations) for consensus mechanism. Having more number of participants working well for this model, as it further reduces the possibility of a 51% attack[4].