One of the authors recently attended a Chinese university alma mater reunion event in Beijing, where blockchain became a hot discussion topic. A very well-regarded schoolmate and scholar, Professor Yang, who has authored books on cryptography and public data safeguards, used genealogy to describe a blockchain. This is a well-thought-out analogy since it explains blockchain intuitively and easily. The analogy is borrowed here to illustrate the basic ideas behind the technology. 

Back in the old days in China, it was a custom for each family of a clan (sharing the same last name) to keep a copy of the genealogical tree of the clan. When members of a family changed due to either marriage or the birth of an offspring, as well as adoption, the new member's name would appear in each copy. However, the new member had to be accepted by the clan before the name could be added in. There were cases when a marriage was not endorsed by a majority of the clan due to various reasons. In this case, the new member's name would not be entered into the genealogy. In other words, when a new member joined in a family, the news was broadcast to other families of the clan. If the clan reached a consensus on accepting the new member, each family would update their copy of the genealogical tree to reflect the change. On the other hand, if the clan decided not to accept the new member, the name would not be added in. The genealogy could be used for verification purposes. For example, if a stranger made a claim to be a member of the clan, or two people with the same last name were eager to find out whether they shared the same ancestor, with the genealogy, it was easy to verify this. The outcome would be accepted since the genealogy was considered reliable thanks to the aforementioned consensus and decentralized records, which were difficult to manipulate unless the majority of families agreed.

A blockchain shares many of the characteristics of a genealogy. They are summarized as follows:

• Like a clan consisting of many related families, a blockchain network consists of nodes. Each node is like a family.
• Like every family keeping a copy of the clan's genealogy, each node of a blockchain maintains a copy of all transactions that have occurred on the chain, starting from the very beginning. The collection of all transactions is a ledger. This makes a blockchain a decentralized data repository.
• A genealogy starts with a common ancestor of the clan and names with direct relationships, such as parents and children, that are connected by a line for linkage. Similarly, a ledger consists of blocks. Each block contains one or multiple transactions depending on the type of blockchain. (As you will see later, blocks on Bitcoin or Ethereum host multiple transactions, while R3's Corda uses a block with only one transaction). Transactions are like names, and a block is similar to the invisible box containing a couple's names. An equivalent of the root ancestor is called the genesis block, which is the first block of a blockchain. Similar to a line linking parents and children, a hash, which will later be explained in more detail, points from the current block to its ancestor block. 
• Like the consensus mechanism for adding new names to a genealogy, the Bitcoin blockchain uses a mechanism called Proof-of-Work to decide whether a block can be added to the chain. Like a genealogy, after a block is added to a chain, it is difficult to change (hack) unless one possesses the majority (which is called a 51% attack) of the computing power of the network. 
• Genealogy provides transparency in a clan's history. Similarly, a blockchain allows a user to query the whole ledger or just a part of the ledger and find out about coin movements.
• Since every family kept a copy of the genealogy, it was unlikely to lose the genealogy even if many copies were lost due to a natural disaster, a war, or other reasons. As long as at least one family survived, the genealogy survived. Similarly, a decentralized ledger will survive as long as at least one node survives.

While genealogy is a good analogy to explain some key concepts of a blockchain, they are not the same. Inevitably, there are features that are not shared by them. For example, the blockchain uses cryptography and hashes extensively for data protection and deterring hackers. A genealogy does not have such a need. Therefore, next we move away from the genealogy analogy and explain key blockchain concepts chronically.

Blockchain technology initially caught people's attention due to the Bitcoin blockchain, an idea outlined by a white paper authored by Satoshi Nakamoto and published in October 2008 on the cryptography mailing list at metzdowd.com. It describes the Bitcoin digital currency (BTC) and was titled Bitcoin: A Peer-to-Peer Electronic Cash System. In January 2009, Satoshi Nakamoto released the first Bitcoin software, which launched the network and the first units of the Bitcoin cryptocurrency: BTC coins.

The creation of Bitcoin was right after the 2008 financial crisis, the most severe economic crisis since the Great Depression. This is not coincidental. The inventor of the Bitcoin cryptocurrency aimed at addressing people's disillusionment with financial institutions, whose epic failures in risk controls resulted in the 2008 financial crisis.

A fundamental role played by financial institutions is to be an intermediary entity and bring untrusting parties together to facilitate transactions. For example, a retail bank attracts residual money from individuals and lends to individuals or companies that need the money. The difference in interest paid to the money suppliers and borrowers is the fee a bank charges for providing the intermediary service. Financial institutions are very successful in providing these services and play a pivotal role in powering economies worldwide. However, there are many deficiencies associated with this business model. Here are some examples:

• Slow: It often takes days to complete a financial transaction. For instance, it takes three days (after an order is initially entered) to complete and settle a cross-border money transfer. To make it happen, multiple departments and application systems within an institution and across institutions have to work together to facilitate the transaction. Another example is stock trading. An investor hires a broker to enter an order to be routed to a stock exchange. Here, the broker is either a member of the exchange or routes the order to another intermediary institution with membership. After a match is found between a buyer and a seller at the exchange, the transaction details are recorded by two parties who send it to their back offices respectively. The back-office teams work with a clearing house for clearance and settlement. It takes T + 3 for both parties to complete the action of exchanging ownership of the security (stock) and the cash.
• Expensive: Financial intermediaries often charge hefty fees when providing these services. For example, a US bank could charge $10 to $30 USD to serve an individual by sending money from the US to a receiver in another country. In the case of stock trading, a full-service broker often charges tens of USD or more for a transaction. Even with a discount broker, an investor needs to pay $7 to $10 USD per transaction.
• Prone to be hacked: Since details on a customer and the transactions are saved in a centralized area within an institution, it is prone to being hacked and causing severe financial loss or leakage of confidential personal information about customers. Recently, there have been high-profile personal data leakage incidents at reputable companies such as JP Morgan (83 million accounts hacked in 2014), Target (up to 70 million customers' information hacked in 2013), and Equifax (148 million US consumers' information hacked in 2017).
• Not transparent: Financial institutions keep both detailed and aggregated information on transactions. However, most of the information is not open to the individual customer and this results in information imparity. In the example of cross-border money transfers, both the sender and receiver have to wait for three days to know whether the transaction has been completed successfully or not. If a transaction fails, a lengthy investigation has to be triggered. Imagine if the receiver was in an emergency and needed the funding immediately. Such a service is unsatisfactory despite the client having to pay a high fee.

With blockchain technology, the preceding problems are resolved elegantly. In the case of the Bitcoin blockchain, the underlying asset to be transferred is the digital coin, BTC. A cross-border BTC transaction can complete in no more than 1 hour. No settlement is needed since transaction and settlement are in one action. The cost of this transaction is a tiny fraction of a transfer via a bank. For example, a recent report published by the Bank of America (BoA) claims a transfer via blockchain costs 1/6000 of what BoA charges. However, for some clients, waiting an hour is still too long. Ripple, a payment provider for sending money globally, completes in under 1 minute.

The word Bitcoin often causes confusion as people use the word interchangeably for three things: the cryptocurrency, the blockchain, and the protocol. To avoid this confusion, we use BTC to refer to the cryptocurrency, and Bitcoin to refer to the blockchain and the corresponding network that uses the distributed ledger. For the protocol, we will fully spell out Bitcoin protocol or simply protocol.

To explain how Bitcoin works, let's look at what steps are involved with the existing business model for completing a cross-border transaction:

• A customer enters an order either by visiting a bank branch or via the web. The sender provides detailed information of an order such as the amount, sending currency, receiver name, receiving currency, receiver's bank name, account and branch numbers, and a SWIFT number. Here, SWIFT stands for the Society for Worldwide Interbank Financial Telecommunications, a messaging network used by financial institutions to transmit information and instructions securely through a standardized system of codes. SWIFT assigns each financial organization a unique code called, interchangeably, the bank identifier code (BIC), SWIFT code, SWIFT ID, or ISO 9362 code.
• The sending bank takes the order and verifies that the sender has sufficient funds available.
• The bank charges a fee and converts the remaining amount from the sending currency to an amount in the receiving currency by executing an FX transaction.
• The sending bank enters a transferring message to SWIFT with all the needed information.
• Upon receiving the message, the receiving bank verifies the receiver's account information.
• Upon a successful verification and settling the funds between sending and receiving banks following the protocol, the receiving bank credits the amount to the receiver's account.

Since there are multiple steps, entities, and systems involved, the preceding activities take days to complete.

A Bitcoin network connects computers around the world. Each computer is a node with equal status, except for a subset of nodes called miners, which choose to play the role of verifying transactions, building blocks and linking to the chain. With Bitcoin, the business model for completing a money transfer involves the following steps:

1. A sender enters the number of BTCs, the addresses of Bitcoins to be taken from, and addresses of Bitcoins to be transferred to, using an e-wallet.
2. The transaction request is sent to the Bitcoin network by the e-wallet.
3. After miners have successfully verified the transaction and committed it to the network, the BTCs are now available for use by the receiver.

The Bitcoin transfer is a lot faster (in 1 hour, or minutes if using Ripple) for the following reasons:

• The transaction and settlement are one step. This avoids the need to go through a time-consuming and expensive reconciliation process.
• No FX trade is needed since BTC is borderless. It can move worldwide freely and rapidly.
• No fund settlement is needed between banks since the transaction requires no intermediary banks.

In a case where a sender or receiver prefers to use a fiat currency such as USD, GBP, CNY, or JPY, a cryptocurrency market can be used for a conversion between BTC and a fiat currency. A website, CoinMarketCap, lists these markets: https://coinmarketcap.com/rankings/exchanges/. As of September 21, 2018, there are 14,044 markets. In terms of market capitalization, the top three are Binance (https://www.binance.com/), OKEx (https://www.binance.com/), and Huopi (https://www.huobi.pro).

A peer-to-peer network can connect nodes worldwide. However, a merely physical connection is not enough to make two untrusting parties trade with each other. To allow them to trade, Bitcoin takes the following measures:

• Every node saves a complete copy of all transactions in a decentralized ledger. This makes any alteration to a transaction on the chain infeasible.
• The ledger transactions are grouped in blocks. A non-genesis block is linked to its previous block by saving the hash of all preceding blocks' transactions. Consequently, changing a transaction requires changing the current block of transactions and all subsequent blocks. This makes hacking the decentralized ledger extremely difficult.
• Bitcoin addresses the double-spending issue, that is the same BTC being spent twice, by using the Proof-of-Work consensus algorithm.
• Hashes are used extensively to protect the identities of parties and detect any changes occurring in a block.
• Public/private keys and addresses are used to mask the identities of trading parties and to sign a transaction digitally .

With these measures, untrusting parties feel comfortable to trade due to these reasons:

• The transaction is immutable and permanent. Neither party can nullify a transaction unilaterally.
• No double spending is possible.
• Transaction and settlement occur simultaneously; therefore, there is no settlement risk.
• Identities are protected.
• Transactions are signed by both parties, which will avoid any future legal disputes.

Cryptography or cryptology is research on techniques for securing communication in the presence of adversaries. In the old days, cryptography was synonymous with encryption. Modern cryptography relies heavily on mathematical theory and computer science. It also utilizes works from other disciplines such as electrical engineering, communications science, and physics.

Cryptographic algorithms are designed around the assumption that with foreseeable computational hardware advances, it will not be feasible for any adversary to decipher encrypted messages based on these algorithms. In other words, in theory, it is possible to decode the encrypted message, but it is infeasible to do so practically. These algorithms are therefore defined to be computationally secure. Theoretical research (for instance, parallel or integer factorization algorithms) and computational technology advancements (for instance, quantum computers) can make these algorithms practically insecure and, therefore, encryption algorithms need to be adapted continuously.

Encryption is the process of converting plaintext into unintelligible text, called ciphertext. Decryption is the reverse, in other words moving from the unintelligible ciphertext back to plaintext.

The encryption algorithms used by Bitcoin mining are hash functions. A hash function is a function that maps data of any size to data of a fixed size. The values returned by a hash function are called hash values or simply hashes. A cryptographic hash function allows one to verify easily  that some input data maps to a given hash value. However, the reverse – when the input data is unknown—it is practically infeasible to reconstruct the input plaintext from a hash value. In other words, hashing is a one-way operation. Another notable attribute of a hashing function is that a minor change in the input plaintext will result in a completely different hash value. This feature is desirable for safeguarding information as any tiny change to the original data by a hacker results in a visibly different hash.

Two common hash algorithms are MD5 (message-digest algorithm 5) and SHA-1 (secure hash algorithm):

• Developed by Ronald Rivest in 1991, MD5 maps input plaintext into a 128-bit resulting hash value. MD5 Message-Digest checksums are commonly used to validate data integrity when digital files are transferred or stored. MD5 has been found to suffer from extensive vulnerabilities.
• SHA-1 is a cryptographic hash function mapping input plaintext into a 160-bit (20-byte) hash known as a message digest – often displayed as a hexadecimal number, 40 digits long. SHA-1 was designed by the United States national security agency and is a US federal information processing standard.

SHA-256 is a successor hash function to SHA-1. It is one of the strongest hash functions available and has not yet been compromised in any way. SHA-256 generates an almost unique 256-bit (32-byte) signature for a text. For example, My test string maps to 5358c37942b0126084bb16f7d602788d00416e01bc3fd0132f4458d
d355d8e76. With a small change, the hash of My test strings is 98ff9f0555435
f792339d6b7bf5fbcca82f1a83fde2bb76f6aa95d66050887cc, a completely different value. SHA-256 produces 2^256 possible hashes. There is yet to be a case where two different inputs have produced the same SHA-256 hash, an issue called collision in cryptography. Even with the fastest supercomputer, it will take longer than the age of our universe to hit a collision. As a result, SHA-256 is used by Bitcoin for encryption.

At a financial institution, a ledger is a book for recording financial transactions. Similarly, Bitcoin maintains a ledger for bookkeeping BTC transactions and balances by address. One key difference is that a bank's ledger is centralized and Bitcoin's ledger is decentralized. Consequently, a bank's ledger is much easier to be cooked. On the other side, Bitcoin's ledger is very difficult to cook as one has to change the ledger at all nodes worldwide.

A user submits a transaction containing the following information:

• Sources of the BTCs to be transferred from
• The amount of BTCs to be transferred
• Destinations the BTCs should be transferred to

As per the Wiki site, a transaction has a general structure shown as follows:

Both source and destination addresses are 64-character hashes. Here is an example of an address: 979e6b063b436438105895939f4ff13d068428d2f71312cf5594c132905bfxy1.

The term address is a bit confusing. A programmer may think it to be an address related to a disk or memory location. However, it has nothing to do with a physical location. Instead, it is a logical label for grouping BTCs that have been transferred from/to it. In a way, one can think of it as a bank account number, yet there are fundamental differences between them. For example, a bank has a centralized place where metadata on an account, for instance, owner name, account open date, and account type, is saved. In addition, the account balance is precalculated and saved. In Bitcoin, there is no metadata on an address and one has to query the entire ledger to find the balance of an address by counting the net BTCs being transferred in and out of the address.  Addresses are referred to only in Bitcoin transactions. When the balance of an address falls to 0, any future request for taking BTCs from the address will fail the transaction validation due to insufficient funds.

Bitcoin utilizes the UTXO model to manage its BTC transfer. The term was introduced by cryptocurrency, where it refers to an unspent transaction output. This is an output of a blockchain transaction that has not been spent and can be used as an input for a future transaction. In a Bitcoin transaction, only unspent outputs can be used as an input, which helps to prevent double spending and fraud. As a result, a committed transaction results in deleting inputs on a blockchain and creating outputs in the form of UTXOs. The newly created unspent transaction outputs can be spent by the owner holding the corresponding private keys. In other words, UTXOs are processed continuously and a committed transaction leads to removing spent coins and creating new unspent coins in the UTXO database.

Like an address, a BTC is not associated with any physical object such as a digital token file or a physically minted coin. Instead, it only exists in transactions in the distributed ledger. For example, if one wants to know the total number of BTCs minted so far, one has to go through all nonzero balance addresses on the blockchain and add up all the BTCs. Since every node of Bitcoin keeps a copy of the ledger, it is only a matter of taking computing time to find an answer.

When a user enters a BTC transaction request at a node, Bitcoin software installed at the node broadcasts the transaction to all nodes. Nodes on the network will verify the validity of the transaction by retrieving all historical transactions containing the input addresses and ensuring that BTCs from these addresses are legitimate and sufficient. After that, the mining nodes start to construct a block by collecting the verified transactions. Normally, a Bitcoin block contains between 1,500 to 2,000 transactions. A miner who wins the race to resolve a difficult mathematical puzzle gets the role to build and link a new block to the chain. On the Bitcoin blockchain, a new block is created around every 10 minutes. As of September 21, 2018, approximately 542,290 blocks have been created on Bitcoin. The structure of a Bitcoin block is shown as follows:

Here, the block header contains the following fields:

The concept of a nonce will be explained in the subsection on mining. hashPrevBlock is the same value as hashMerkleRoot. The Merkle tree hash root is essentially the hash of all transaction hashes in the block via a binary tree aggregation structure. The following diagram explains the idea:

If someone buys a bottle of water for $1, that person cannot spend the same $1 to buy a can of coke. If a person is free to double-spend a dollar, money would be worthless since everyone would have unlimited amounts and the scarcity, which gives the currency its value, would disappear. This is called the double-spending problem. With BTC, double spending is the act of using the same Bitcoin more than once. If this problem is not resolved, BTC loses its scarcity and cannot be used to facilitate a trade between two untrusting parties. The Bitcoin Core network protects against double spends via a consensus mechanism. To explain how the Bitcoin consensus mechanism works, we first describe the concepts of PoW (Proof-of-Work) and mining.

As explained earlier, a miner needs to solve a difficult mathematical puzzle ahead of other miners in order to receive the role of being a builder of the current new block and receive a reward for doing the work. The work of resolving the math problem is called PoW.

Why is PoW needed? Think of this: in a network consisting of mutually untrusting parties, more honest parties are needed than dishonest attackers in order to make the network function. Imagine if upon collecting sufficient transactions for a new block, a miner is allowed to build the new block immediately. This simply becomes a race for whoever can put enough transactions together quickly. This leaves a door wide open for malicious attackers to hack the network by including invalid or fake transactions and always win the race. This would allow hackers to double-spend BTCs freely.

Therefore, to prevent attackers from introducing bad transactions, a sufficient window of time is needed for participating nodes to verify every transaction's validity by making sure a BTC has not been spent yet. Since every node maintains a copy of the ledger, an honest miner can trace the history and ensure the following to confirm the validity of a transaction:

• The requestor of a transaction does own the BTCs.
• The same BTCs have not been spent by any other transactions in the ledger.
• The same BTCs have not been spent by other transactions within the candidate block.

This window of time is currently set to be around 10 minutes. To enforce the 10-minute waiting time, Bitcoin asks a miner to solve a sufficiently difficult mathematical puzzle. The puzzle requires only a simple computation. Miners have to repeat the same computation many times in order to burn enough CPU time to reach the network's goal of building a new block every 10 minutes on average. The process of repeated guessing is called mining and the device (specially made) is called a mining rig.

Since, in order to win the mining race, a miner needs to invest heavily in hardware, these miners are dedicated to the work of mining and aim to receive sufficient BTCs to cover the cost of running the mining operation and make a profit. As of the first half of 2018, the reward given to a winning miner is 12.5 BTCs. One can find the price of BTC by visiting the CoinMarketCap website (https://coinmarketcap.com/). As of September 21, 2018, one BTC is traded at around $6,710. Therefore, 12.5 BTC is worth about $83,875 USD.

Per Bitcoin protocol, mining is the only way for a new BTC to be issued (minted). Having a miner be rewarded handsomely serves three purposes:

• Compensates a miner's investment on hardware.
• Covers mining operational costs such as utility bills, which can be significant due to the large mining rigs being deployed at a mining site, human salaries, and site rentals.
• Gives miners incentives to safeguard the network from being attacked by malicious hackers. Miners are motivated to maintain the Bitcoin network in order not to lose value in their BTCs and their mining infrastructure. If Bitcoin is breached by hackers, Bitcoin's reputation will suffer badly and BTC prices would freefall. This is exactly what the Bitcoin inventor hoped for: having more good miners than bad miners to address the double-spending issue.

The total number of BTC that can be issued is fixed to be 21 million. As of today (September 19, 2018), around 17 million BTCs have been issued. The Bitcoin protocol defines a rule for dynamically adjusting the payout rate and the remaining 4 million coins aren't expected to be mined completely for another 122 years. The following point explains how the block creation payout rate is dynamically adjusted:

• The rate changes at every 210,000 blocks. It is a function of block height on the chain with genesis=0, and is calculated using 64-bit integer operations such as: (50 * 100000000) >> (height / 210000). The rate initially started with 50 BTCs, and fell to 25 BTCs at block 210,000. It fell to 12.5 BTCs at block 420,000, and will eventually go down to 0 when the network reaches 6,930,000 blocks.

A Bitcoin blockchain can diverge into two potential paths since miners do not necessarily collect transactions and contract block candidates in the same way, nor at the same time. Other reasons such as hacking or software upgrades can also lead to path divergence. The splitting patches are called forks. There are temporary forks and permanent forks.

If a permanent fork occurs due to, for example, malicious attacks, a hard fork occurs. Similarly, there is the concept of soft fork. Both hard fork and soft fork refer to a radical change to the protocol. Hard fork makes previously invalid blocks/transactions valid and a soft fork makes previously valid blocks/transactions invalid. 

To remove a temporary fork, Bitcoin protocol dictates that the longest chain should be used. In other words, when facing two paths, a winning miner will choose the longer chain to link a new block. As a result, the longer path continues to grow and the blocks on the losing (shorter) path becomes orphaned. Bitcoin nodes will soon discard or not take the orphaned blocks. They only keep the blocks on the longest chain as being the valid blocks.

In the case of a permanent fork, nodes on the network have to choose which chain to follow. For example, Bitcoin Cash diverged from Bitcoin due to a disagreement within the Bitcoin community on how to handle the scalability problem. As a result, Bitcoin Cash became its own chain and shares the transaction history from the genesis block up to the forking point. As of September 21, Bitcoin Cash's market cap is around $8 billion, ranking fourth, versus Bitcoin's $215 billion.

There is one more issue that needs to be resolved: how to maintain the new block building rate of 10 minutes. If nothing is done, the mining rate will change due to the following factors:

• The number of miners on the network can vary in response to the BTC price
• Technology advancements make mining rigs progressively faster
• The total number of mining rigs varies

Bitcoin adjusts the difficulty level of the mathematical puzzle in order to keep the building rate at 10 minutes. The difficulty level is calculated from the rate at which the most recent blocks were added in. If the average rate of new blocks being added is less than 10 minutes, the difficulty level will be increased. If the average rate takes more than 10 minutes, it's decreased. The difficulty level is updated every 2,016 blocks. The following graph displays the historical trend in Bitcoin difficulty level.

We have yet to talk about the actual mining algorithm. Assume the current difficulty level is to find the first hash value with the leading character to be 0. In Bitcoin, the process of solving a puzzle, that is, mining, requires a miner to follow these steps:

• First, find the SHA-256 hash of the block in construction.
• If the resulting hash has a leading 0, the miner solves the puzzle. The miner links the block to the ledger on the node and claims the trophy, 12.5 BTCs. The miner's node broadcasts the news to all nodes. All other nodes and miners on the network validate the answer (by mapping the block information plus nonce to get the same hash) and validate the entire history of the ledger, making sure that the block contains valid transactions.
• If it passes the checks, all nodes on the network add the block to their copies of the ledger. Miners start to work on the next new block.
• If the winning miner is a malicious attacker and includes bad transactions in the block, the validation of these transactions will fail and other miners will not include the block in their ledger copies. They will continue to mine on the current block. As time passes, the path containing the bad block will no longer be the longest path and, therefore, the bad block will become an orphaned block. This is essentially how all nodes on the network reach consensus to add only good blocks to the network and prevent bad blocks from sneaking in, therefore resolving the double-spending issue.
• If the resulting hash does not start with 0, then the miner is allowed to append a sequence number, known to be a nonce, starting from 0 to the input text, and retry the hash.
• If the resulting hash still does not contain a leading 0, the miner will add another sequence number, 1, to the input text and obtain a new hash. The miner will keep trying in this way until it finds the first hash with a leading zero.

The following is an example of how the plaintext and nonce work together. The original plaintext is input string and the nonce varies from 0 to 1:

• input string: f23f4781d6814ebe349c6b230c1f700714f4f70f735022bd4b1fb69421859993
• input string0: 5db70bb3ae36e5b87415c1c9399100bc60f2068a2b0ec04536e92ad2598b6bbb
• input string1: 5d0a0f2c69b88343ba44d64168b350ef62ce4e0da73044557bff451fd5df6e96

In Bitcoin, adjusting difficult level largely refers to changing the required number of leading zeros. (The actual adjustment involves some other miner tuning to the requirement.) Each addition of a leading zero will increase the average number of tries significantly and therefore will increase the computing time. This is how Bitcoin manages to maintain the average rate of 10 minutes for new blocks being added in. The current Bitcoin difficulty level is 18 leading zeros.

Thanks to the rising price of BTC, the mining operation has become more attractive. Investments are rushing in and large mining pools involving thousands of rigs or more have joined the network in order to gain an advantage in the race to solve the puzzle first and get the reward. For players without large capital from investments, they have a choice to participate in a mining pool. When the pool wins a race, the award will be allocated to each participant based on the computational power contributed.

This ever-growing computational power of a pool poses a real threat due to the so-called 51% problem. This problem occurs when a miner manages to build up computational power to total at least 51% of the total computing power of the network. When this occurs, the miner will have a chance to outrun other miners. The miner can continue to grow the ledger with blocks containing bad transactions since this miner has more than a 50% chance of solving the puzzle first. Soon, the malicious miner's ledger will grow to be the longest path and all other nodes have to save this path based on Bitcoin's consensus protocol.

For a large and well-established network such as Bitcoin, the 51% problem is not as critical an issue, mainly due to the following reasons:

• A well-established network will attract a much larger number of participating parties and connect a very significant number of nodes. It will take an exorbitantly high initial investment for a hacker to purchase the necessary mining rigs. When such a network is attacked, the price of cryptography will drop quickly when the news becomes public and the hacker will have a low chance of recovering the investment.
• In the history of Bitcoin, there have been cases when a mining pool that accumulated dangerously high computing power approached this line. When the participating miners in the pool realized the problem, many of them chose to leave the pool. Soon, the computational power of the pool fell to a safe level.
• In the case of a small and immature network, it is not difficult for a miner to muster computing power of more than 51%. However, the cryptocurrency value of these networks is minimal and it gives hackers very little financial incentive to take advantage of the 51% problem.

As discussed earlier, BTCs do not physically exist. The only evidence of their existence is when they are associated with addresses, which are referred to in transactions. When an address is initially created, a pair of public and private keys are generated with it. The public key is made known to the public and the private key is kept only by the owner of the address. When the owner wants to spend all or a portion of their BTCs, the owner provides a digital signature signed with the private key and sends the BTC request to the Bitcoin network. In other words, one has to know both the address and its private key to spend the BTC. 

If an owner loses a private key, its associated BTCs will be lost permanently. Therefore, it is advised to keep this information in a safe place. It is generally good practice to keep the address and private keys in separate places. To prevent a digital copy getting lost, an owner should maintain physical copies of printouts. To make conversion easier, an owner can print a QR code and later scan the QR code whenever it is needed.

Bitcoin wallet applications are available to help a user manage keys and addresses. One can use a wallet to do the following:

• Generate addresses and corresponding public/private keys
• Save and organize a BTC's information
• Send a transaction request to the Bitcoin network

In Bitcoin, a private key is a 256-bit-long hash and a public key is 512 bits long. They can be converted into shorter lengths in hexadecimal representation. The following screenshot gives an example of a pair of public/private keys along with an address:

Bitcoin private keys can also be expressed in a string of 51 characters starting with a 5 and a public key in a string of 72 characters. A sample private key is 5Jd54v5mVLvy
RsjDGTFbTZFGvwLosYKayRosbLYMxZFBLfEpXnp and a sample public key is BFCDB2DCE28D959F2815B16F81798483ADA7726A3C4655DA4FBFC0E1108A8FD17B448A68.

One can install the following development tools for programming Bitcoin operations:

• NodeJS: This is an open source, cross-platform JavaScript runtime environment that executes JavaScript code outside of a browser. It allows a programmer to write and execute scripts quickly and easily. These scripts can be written to be run in a web browser or on a server.
• BitcoinJS: This is a JavaScript library for working with Bitcoin and its cryptographic functions. BitcoinJS can be used to generate public/private keys and addresses.
• Blockchain.info: This is a public API that can be used to query the blockchain to find out balances and broadcast transactions to the network. It can be used to implement a Bitcoin node and install and run a Bitcoin node.

After installing the preceding tools, one can execute the following operations:

• Generate a new private key and compute a public key
• Check the balance for a certain address
• Generate addresses
• Construct a new transaction
• Send a transaction, which involves three steps:
  • Build a transaction with a list of inputs and outputs
  • Sign the transaction with the required private keys
  • Broadcast the transaction to the network
• Build an escrow account
• Broadcasts the transaction

Thanks to Bitcoin, blockchain technology has attracted worldwide attention. Like any new technology, it has its limitations. Many variations of Bitcoin were created to address a particular limitation of Bitcoin. Here, we mention a few of them:

• Bitcoin Cash: This is a hard fork of the Bitcoin chain that was created because a group of Bitcoin core developers wanted to use a different way of addressing the scalability issue.
• Litecoin: This is almost identical to Bitcoin except that the time for adding a new block was reduced from 10 minutes to 2 minutes.
• Zcash: This is based on Bitcoin but offers total payment confidentiality.
• Monero and Zcash: Both altcoins address the privacy issue by making transaction history untraceable, but they implement two different solutions.
• Dash: This mainly improves user-friendliness. For example, transactions are made untraceable and a user does not have to wait for several additional new blocks to be added before considering a transaction to be committed to the chain.
• Namecoin: This extends the use case of Bitcoin, which is for trading BTCs only, to providing domain name services.
• Peercoin: This altcoin addresses the deficiencies of PoW, which is environmentally unfriendly and is low in throughput. Instead, it adopts proof of stake for achieving consensus. Based on this rule, a miner validates block transactions according to how many coins a miner holds. In other words, the mining power of a miner is in proportion to the number of peercoins owned.
• Primecoin: A primecoin miner competes to be the first to find the next biggest prime number.

Regardless of the efforts made from the steps-mentioned altcoins in addressing some part of the Bitcoin's limitations, there are several fundamental issues that are not being addressed yet:

• Bitcoin and these altcoins are specific to one purpose: trading either BTC or an altcoin.
• Although a programmer can use tools such as BitcoinJS to interact with the network, the resulting code sits outside of the blockchain and is not guaranteed to run. The chain itself does not have a Turing complete programming language for coding directly on a blockchain.
• These blockchains are stateless and one has to search through the entire ledger to find an answer such as the total number of BTC minted.

In response to these problems, Vitalik Buterin, a Canadian cryptocurrency researcher and programmer, proposed the idea of Ethereum in late 2013. Funded by an online crowdsale, the system went live on 30 July 2015, with 11.9 million coins premined for the crowdsale.

The core idea for Ethereum was to build a general-purpose blockchain so users could solve a wide range of business problems not just limited to cryptocurrency transfer. Ethereum introduced a few new and critical concepts:

• The concept of saving a smart contract on a blockchain
• The concept of implementing a smart contract with a Turing complete programming language such as Solidity and running the piece of code on the blockchain

Solidity was initially proposed in August 2014 by Gavin Wood. The Ethereum project's Solidity team led by Christian Reitwiessner later developed the language. It is one of the five languages, (Solidity, Serpent, LLL, Vyper, and Mutan) designed to target the Ethereum virtual machine (EVM).

Nick Szabo, a programmer and lawyer, initially proposed the term smart contract in 1996. In his blog, Nick Szabo described it as the granddaddy of all smart contracts, the vending machine.
A vending machine shares the exact same properties as a smart contract on a blockchain today. A vending machine is built with hardcoded rules that define what actions to execute when certain conditions are fulfilled, for example:

• If Susan inputs a dollar bill in the vending machine, then she will receive a bag of pretzels.
• If Tom puts in a five-dollar bill, Tom will receive a bag of pretzels and also change of four dollars.

In other words, rules are defined and enforced by a vending machine physically. Similarly, a smart contract contains rules in program code that are run on the blockchain and triggered when certain conditions are met.

The introduction of the smart contract concept is significant:

• A smart contract is a scripted legal document.
• The code built into the contract is stored on the Ethereum blockchain and cannot be tampered with or removed. This greatly increases the credibility of the legal document.
• This code cannot be stopped, meaning any party—regardless of how powerful the party is—cannot order or interfere with the running of the smart contract code. As long as certain conditions are met, the code will run and the legally defined actions will be fulfilled.
• Ethereum to blockchain is like an OS to a computer. In other words, the platform is generic, no longer serving only one specific purpose.
• It now has a Turing complete language: Solidity.

The arrival of Ethereum revolutionized blockchain technology. Applying technology to resolve business problems well beyond the financial industry has become feasible. However, there are many scenarios where Ethereum is not enough. Ethereum's issues include the following:

• Real enterprise applications, particularly in the financial industry, require a high throughput, which can mean billions of transactions a day. The current form of Ethereum has a maximum capacity of 1.4 million a day. Bitcoin is even worse: 300,000 transactions a day. During a stress test, Bitcoin Cash reached 2.2 million. Ethereum 2.0 under development aims at getting to a billion transactions a day while maintaining a decentralized and secure public blockchain.
• Many financial markets, for instance OTC Derivatives or FX, are permission-based. A public blockchain supported by Ethereum or Bitcoin does not meet such a need.

To satisfy their needs, well-established companies across industries form consortiums to work on enterprise blockchain projects, which are permission-based only. In other words, a node has to receive approval before it can join in the blockchain network. Examples of enterprise blockchains are Hyperledger and R3's Corda.

In December 2015, the Linux Foundation (LF) announced the creation of the Hyperledger Project. Its objective is to advance cross-industry collaboration by developing blockchains and distributed ledgers. On 12 July 2017, the project announced its production-ready Hyperledger Fabric (HF) 1.0.

Currently, Hyperledger includes five blockchain frameworks:

• Hyperledger Fabric (HF): A permissioned blockchain, initially contributed by IBM and Digital Asset, it is designed to be a foundation for developing applications or solutions with a modular architecture. It takes plugin components for providing functionalities such as consensus and membership services. Like Ethereum, HF can host and execute smart contracts, which are named chaincode. An HF network consists of peer nodes, which execute smart contracts (chaincode), query ledger data, validate transactions, and interact with applications. User-entered transactions are channeled to an ordering service component, which initially serves to be HF's consensus mechanism. Special nodes called Orderer nodes validate the transactions, ensure the consistency of the blockchain, and send the validated transactions to the peers of the network as well as to membership service provider (MSP) services that are implemented to be a certificate authority.
• Hyperledger Iroha: Based on HF, it is designed for mobile applications. Iroha was contributed by Soramitsu, Hitachi, NTT Data, and Colu. It features a modern and domain-driven C++ design. It implements a consensus algorithm called Sumeragi.
• Hyperledger Burrow: Contributed initially by Monax and Intel, Burrow is a modular blockchain that was client-built to follow EVM specifications.
• Hyperledger Sawtooth: Contributed by Intel, it implemented a consensus algorithm called Proof of Elapsed Time (PoET). PoET is designed to achieve distributed consensus as efficiently as possible. Sawtooth supports both permissioned and permissionless networks. Sawtooth is designed for versatility.
• Hyperledger Indy: Contributed initially by the Sovrin foundation, it is intended to support independent identity on distributed ledgers. Indy provides tools, libraries, and reusable components, which are implemented to provide digital identities.

Early members of the initiative include the following:

• Blockchain ISVs, (Blockchain, ConsenSys, Digital Asset, R3, Onchain)
• Technology platform companies such as Cisco, Fujitsu, Hitachi, IBM, Intel, NEC, NTT DATA, Red Hat, and VMware
• Financial institutions such as ABN AMRO, ANZ Bank, BNY Mellon, CLS Group, CME Group, the Depository Trust and Clearing Corporation (DTCC), Deutsche Börse Group, J.P. Morgan, State Street, SWIFT, and Wells Fargo
• Software companies such as SAP
• Academic institutions such as Cambridge Centre for Alternative Finance, blockchain at Columbia, and UCLA blockchain lab
• Systems integrators and other firms such as Accenture, Calastone, Wipro, Credits, Guardtime, IntellectEU, Nxt Foundation, and Symbiont

Blockchain technology is still at an early stage. It will take many years before it becomes mature and its potential is fully explored and harnessed. Currently, there is no universally agreed way to classify or define blockchain generation.

In her book on blockchain, Melanie Swan defined blockchain 1.0 to 3.0 based on the use scenarios that blockchain platforms are created to serve:

Some others divided blockchain evolution into four generations from blockchain 1.0 to 4.0:

• Blockchain 1.0: With Bitcoin being the most prominent example in this segment, use cases were based on the distributed ledger technology (DLT) where financial transactions could be executed. Cryptocurrency was used as cash for the Internet.
• Blockchain 2.0: With Ethereum being the most prominent example in this segment, the new key concept was Smart Contracts, which are stored and executed on a blockchain.
• Blockchain 3.0: The keyword is DApps, an abbreviation for decentralized applications, which avoided centralized infrastructure. They use decentralized storage and decentralized communication. Unlike a smart contract which only involves a backend or server-side code, a DApp can have frontend code and user interfaces, i.e. client-side code to interact with its backend code on a blockchain. Like the smart contract code, a DApp's frontend can be stored and executed on decentralized storage such as Ethereums Swarm. In summary, a DApp is frontend plus contracts running on Ethereum.
• Blockchain 4.0: Blockchain platforms in this segment are built to serve for Industry 4.0. Industry 4.0 refers, in a simple way, to automation, enterprise resource planning, and integration of different execution systems.

Regardless of how the blockchain technology is divided into versions, it is certain that the technology growth is far from being over. New ideas and implementations will be incorporated into the existing platforms to deal with challenges for real-life problems. In other words, blockchain technology will be nimble and is self-adjusted to be an enabler in resolving business problems.

Blockchain is an emerging technology. Thanks to its immutability, transparency, the consensus mechanism for avoiding double spending, along with other clever designs such as blocks chained with the hashes of the previous blocks, the technology allows untrusting parties to trade with each other. In this chapter, we explained the basic concepts of its important features. Most of the discussions were about Bitcoin, which is the mother of the technology. We briefly talked about Ethereum, which extended Bitcoin and introduced the concept of smart contracts. The introduction of smart contracts makes the Ethereum blockchain generic and allows us to develop applications beyond the borderless cash payment use case for which Bitcoin was invented. The concept of an enterprise chain, along with one of the examples, Hyperledger, was mentioned as well. Finally, we briefly touched on the evolution of blockchain to give readers an idea of the trend in the technology. In the next chapter, we will discuss the concepts of Ethereum in detail.