The sidechains vision of the future is of a vast globe-spanning decentralized network of many blockchains, an intertwined cable rather than a single strand, each with its own protocol, rules, and features — but all of them backed by Bitcoin, and protected by the Bitcoin mining network, as the US dollar was once backed by gold. Sidechains can also be used to prototype changes to the fundamental Bitcoin blockchain. One catch, though: this will require a small tweak to the existing Bitcoin protocol.
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Jump up ^ Redrup, Yolanda (29 June 2016). "ANZ backs private blockchain, but won't go public". Australia Financial Review. Archived from the original on 3 July 2016. Retrieved 7 July 2016. Blockchain networks can be either public or private. Public blockchains have many users and there are no controls over who can read, upload or delete the data and there are an unknown number of pseudonymous participants. In comparison, private blockchains also have multiple data sets, but there are controls in place over who can edit data and there are a known number of participants.
For example, let’s say we have side chain 1 (SC1) and side chain 2 (SC2). A transaction occurs on SC1. A node in SC1 broadcasts the transaction to nodes in the main chain to record this transaction. The same node of SC1 calls a function from SC2 with a proof. The function in the nodes of SC2 verifies the proof on the main chain. The function gets executed.
The great thing about Bitcoin, for a tech columnist like me, is that it’s simultaneously over-the-top cinematic and technically dense. Richard Branson recently hosted a “Blockchain Summit” at his private Caribbean island. There’s a Bitcoin Jet. At the same time, 2015 has seen the release of a whole slew of technically gnarly–and technically fascinating–proposals built atop the Bitcoin blockchain.
Step back from the details for moment and consider what’s been described.  We now have a way to move coins from Bitcoin onto another platform (a sidechain) and move them back again.   That’s pretty much what we do when we move them to a wallet platform or an exchange.  The difference is that the “platform” they’ve been moved to is also a blockchain… so it has the possibility of decentralised security, visibility and to gain from other innovation in this space.
– A cost per transactions which can be high: Miners only participate in the process of mining because they hope to get the reward (coinbase and fees) allocated to minors who have added a block to the blockchain. For them it is a business, this reward will finance the costs they have incurred in the process of mining (electricity, computer equipment, internet connection). Tokens that are distributed to them are directly issued by the Protocol, but the fees are supported by the users. In the case of the bitcoin, for example, minors receive 12.5 bitcoins for each block added, to which are added fees paid by the users to add their transactions to the blocks. These fees are variable and the higher the demand to add transactions, the higher the fees.
Plasma, a project by Ethereum, uses this side chain concept. It encourages transactions to happen on side chains (or child chains). An authority governs each of the child chains. If the authority starts acting maliciously, anyone on the child chain can quit the child chain and take back their pegged assets on the main chain. It’s in its early stages of development but shows a lot of promise in handling some of Ethereum’s scalability issues.
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:

Public blockchains are also expensive, and not just in terms of money. The time and energy required to process transactions on public chains is more intensive than that of non-public chains. This is because every single node on the chain must authorize each new transaction before it is added to the chain, which requires a large amount of electricity and time (not to mention money).
Public blockchains are also expensive, and not just in terms of money. The time and energy required to process transactions on public chains is more intensive than that of non-public chains. This is because every single node on the chain must authorize each new transaction before it is added to the chain, which requires a large amount of electricity and time (not to mention money).

3) the argument ‘let’s harden internal IT as if it worked outside the firewall’ makes a ton of sense to me. We need to construct a lot of hoops for hackers to jump through, as permitter defense is not holding up anymore. And we need to make our systems anti-fragile. The blockchain data structure is a good tool, other P2P tools can be used too. Also, the blockchain has initiated a renaissance of crypto tech, like multisig, payment channels., HD wallets, hot-cold storage, and other innovations in key management.
Consider a proof-of-existence application, where you want to authenticate your document in the Ethereum (for example) network, but you do not need your document to be online. So, you will store the hash generated from your document in the blockchain, but the document itself will be in your local machine, out of any blockchain-related structured, being off-chain.

In general, so far there has been little emphasis on the distinction between consortium blockchains and fully private blockchains, although it is important: the former provides a hybrid between the “low-trust” provided by public blockchains and the “single highly-trusted entity” model of private blockchains, whereas the latter can be more accurately described as a traditional centralized system with a degree of cryptographic auditability attached. However, to some degree there is good reason for the focus on consortium over private: the fundamental value of blockchains in a fully private context, aside from the replicated state machine functionality, is cryptographic authentication, and there is no reason to believe that the optimal format of such authentication provision should consist of a series of hash-linked data packets containing Merkle tree roots; generalized zero knowledge proof technology provides a much broader array of exciting possibilities about the kinds of cryptographic assurances that applications can provide their users. In general, I would even argue that generalized zero-knowledge-proofs are, in the corporate financial world, greatly underhyped compared to private blockchains.
^ Jump up to: a b c d e f g h i j k l "Blockchains: The great chain of being sure about things". The Economist. 31 October 2015. Archived from the original on 3 July 2016. Retrieved 18 June 2016. The technology behind bitcoin lets people who do not know or trust each other build a dependable ledger. This has implications far beyond the crypto currency.
A blockchain is a continuously growing list of records called blocks, these blocks are linked and secured using cryptographic algorithms. Each block typically contains a hash (a link to a previous block), a timestamp as well as transaction data. Full nodes validate all the transactions, but are unable to settle the disagreements in regards to the order in which they were received. To prevent double-spending, the entire network needs to reach global consensus on the transaction order. It achieves this by using centralised parties or a decentralised proof of work or proof of stake algorithm (and its derivatives).
Similarly, a side chain is a separate blockchain that runs in parallel to the main chain. The term is usually used in relation to another currency that’s pegged to the currency of the main chain. For example, staying with the Starcraft motif, say we had an in-game currency called Minerals (oh wait, we do!). We could allow players to peg their Ether (or ETH) to purchase more Minerals in-game. So we reserve some ETH on the main chain, and peg, say 500 Minerals to 1 ETH.
Many people believe this is the future of the blockchain. It maintains network security and allows for scalability. The biggest criticism is that it heavily favors those with more funds as smaller holders have no chance of becoming witnesses. But the reality is, smaller players have no hope of participating in Proof of Work either, as mining from your own laptop at home is no longer a reality. Smaller players get outcompeted by bigger players who have massive mining rigs. STEEM and EOS are examples of DPOS blockchains. Even Ethereum is moving to POS with its Casper project.
In private blockchains, only specific, pre-chosen entities have the ability to create new transactions on the chain (this is known as “write permissions”). Thus, a private blockchain is a closed network that offers constituents the benefits of the technology, but is not necessarily decentralized or distributed, even among its members. The extent to which each constituent can view (“read”) and create and validate transactions (“write”) is up to the developers of the chain.
The Bitcoin Blockchain is a game changer, because it is public and permissionless. Anyone in the world can download the open source code, and can start verifying transaction, being rewarded with bitcoin, through a concept called mining. All stakeholders in the bitcoin network, who do not know and trust each other, are coordinated through an economical incentive framework pre-defined in the protocol and auto enforced by machine consensus of the P2P Network. The smart contract in the blockchain protocol therefore  provides an coordination framework for all network participants, without the use of traditional legal contracts. In private and permissioned blockchain, all network participants validating transactions are known. Bilateral or multilateral legal agreements provide a framework for trust, not the code.
The two-way peg is the mechanism for transferring assets between sidechains and is set at a fixed or predefined rate. Bitcoin’s Dynamic Membership Multi-Party Signature (DMMS) plays a vital role in the functionality of the two-way peg. The DMMS is one of Bitcoin’s lesser known but incredibly important components. It is a group digital signature — composed of the block headers in Bitcoin — that has no fixed size due to the computationally powered PoW nature of its blockchain. The Pegged Sidechain paper further describes it as:
The witnesses who put more funds in escrow have a greater chance of mining (or minting) the next block. The incentives line up nicely here. There are only a few witnesses and they get paid to be witnesses, so they are incentivized to not cheat. If they do cheat and get caught, they not only get voted out in favor of the next eagerly awaiting witness, they lose all the funds they had in escrow.

Sidechains have been a concept for a relatively long time in the cryptocurrency space. The idea took flight in 2014 when several eminent figures in cryptography and early digital currency innovations published an academic paper introducing Pegged Sidechains. Several of the authors are central figures at Blockstream, who is at the forefront of innovation in sidechains and other Bitcoin developments.
“Given all of this, it may seem like private blockchains are unquestionably a better choice for institutions. However, even in an institutional context, public blockchains still have a lot of value, and, in fact, this value lies to a substantial degree in the philosophical virtues that advocates of public blockchains have been promoting all along, among the chief of which are freedom, neutrality and openness.” 
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