Bitcoin and other cryptocurrencies currently secure their blockchain by requiring new entries to include a proof of work. To prolong the blockchain, bitcoin uses Hashcash puzzles. While Hashcash was designed in 1997 by Adam Back, the original idea was first proposed by Cynthia Dwork and Moni Naor and Eli Ponyatovski in their 1992 paper "Pricing via Processing or Combatting Junk Mail".
Sidechains are an essential innovation in the blockchain field with some interesting long-term implications and effects on the broader interoperability and scalability of blockchain networks. They are effectively extensions of existing blockchains that increase their functionality and allow for validation of data from other blockchains and for assets to be seamlessly transferred between them.
The paper outlines some critical developments and associated problems that were both currently trending and forward-thinking at the time, many of them still very much relevant today. At the time, altcoins were quickly gaining prominence and the problems associated with their volatility, security, and lack of interoperability with Bitcoin raised concerns. The paper primarily addressed 6 issues that pegged sidechains aimed to provide a solution:
The Blockstream Satellite network broadcasts the Bitcoin blockchain to the entire planet. The satellite network provides an opportunity for nearly 4 billion people without Internet access to utilize bitcoin while simultaneously ensuring bitcoin use is not interrupted due to network interruption. Utilizing the latest open source Software Defined Radio (SDR) technologies, the Blockstream Satellite network offers a breakthrough in the cost effectiveness of satellite communications.
This type of blockchains can be considered a middle-ground for companies that are interested in the blockchain technology in general but are not comfortable with a level of control offered by public networks. Typically, they seek to incorporate blockchain into their accounting and record-keeping procedures without sacrificing autonomy and running the risk of exposing sensitive data to the public internet.

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Since 2008 when Satoshi Nakamoto published a white paper considering Bitcoin and blockchain technology, the latter gained fame as a tool for combating trust issues and bringing transparency to transactions between independent participants. Even though a decade passed, for a lay public, blockchain is still not the easiest concept to deal with. As a rule, people generalize things they don’t understand deeply in detail. Thus, when they hear “blockchain,” they tend to think there’s just one transcendental blockchain that hosts thousands of projects. But it’s a wrong perception as there are numerous blockchains and they differ.
Always there is a balance in nature, even in blockchains. If you want to have extra features, you need to make a sacrifice from your current features. For example to have high speed and volume; you need to give some from your security & immutability by doing consensus with smaller groups or you need to use different methods in consensus like POS / PBFT. (Proof of Stake / Practical Byzantine Fault Tolerance)
That might sound like a problem, but it isn’t because the box can only be opened infrequently (two or three times a year), and a super-majority of miners must leave a note on the box in advance. This note states exactly where the miners intend to transfer the money. The “correct” note is automatically generated by sidechain software, and is easy to check.
The first question to answer is “What is public blockchain?” The very name of this type of networks implies that they are open and permissionless. It means that anyone in the world can join the network, add blocks and view the information stored there. Indeed, public blockchains are totally transparent as any of their members can audit them. For this reason, independent participants can easily agree on transactions without middlemen and the fear of deception.
For example, Banks A and B often settle thousands of transactions per day. It would be extremely expensive for all of those transactions to be committed to the main blockchain, so A and B set up a side-chain. At the end of each day, at most one transaction is committed to the main blockchain (the only possible outcomes are A and B's balances remain the same, or one of their balances decreases and the other's increases).
Our Proof of Work tutorial talks about it in depth, but the best explanation might come from Satoshi Nakamoto himself. If the camps above start receiving messages that don’t agree, they rely on executing a Proof of Work. The Proof of Work is sufficiently complicated and requires significant computing power. Once one camp solves the Proof of Work, it broadcasts the results to the other camps. This message is now accepted in a chain of messages and the competing messages are dropped by the other camps.
A blockchain is a distributed computing architecture where every node runs in a peer-to-peer topology, where each node executes and records the same transactions. These transactions are grouped into blocks. Each block contains a one-way hash value. Each new block is verified independently by peer nodes and added to the chain when a consensus is reached. These blocks are linked to their predecessor blocks by the unique hash values, forming a chain. In this way, the blockchain’s distributed dataset (a.k.a. distributed ledger) is kept in consensus across all nodes in the network. Individual user interactions (transactions) with the ledger are append-only, immutable, and secured by strong cryptography. Nodes in the network, in particular the public network, that maintain and verify the transactions (a.k.a. mining) are incentivized by mathematically enforced economic incentives coded into the protocol. All mining nodes will eventually have the same dataset throughout.
A public blockchain is ideal when the network must be truly decentralized, which means that no central entity controls the entry of the members on the network and the consensus mechanism is democratic. A democratic mechanism of consensus means that all members can become a minor and that these miners are in competition to add the blocks to the blockchain (at least when the mechanism of the evidence of the work is used).
Instead, what if the game was played in its own “channel”? Each time a player made a move, the state of the game is signed by each player. After an epic battle where the Protoss player takes out the remaining Zerg forces and forces a gg, the final state of the game (Protoss wins) is sent to a smart contract on the main chain. This neutral smart contract, known as a Judge, waits a while to see if the Zerg player disputes the outcome. If the Zerg player doesn’t, the Protoss player is paid the 1 ETH. </injects>