The Loom Network recently released their SDK which supports what they call “Dappchains,” an Ethereum layer-2 sidechain solution with each sidechain comprised of their own DPoS consensus mechanism. This enables highly scalable dapps, specifically games built using their tools. Loom emphasizes the earlier comment about sidechains enabling innovation in scalability, rather than providing it directly. Loom’s sidechains have their own set of rules and are used to offload computation from the primary Ethereum chain. Their sidechains are application-specific, meaning that they enable highly scalable dapps through an efficient consensus mechanism and can periodically be settled on the main Ethereum chain depending on their security needs. You can find more information on their model here.
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.
Blockstream recently released a whitepaper on “strong federations,” which is essentially their vision of a federated two-way peg system. Liquid is a sidechain created by Blockstream that uses the strong federations model. The sidechain is used to transfer bitcoins between centralized bitcoin institutions, such as exchanges, at a faster pace than the public Bitcoin blockchain.
“A private blockchain is hardly different from a traditional database. The term is synonymous with glorified databases. But the advantage is that if they are to ever start adding public nodes to it then it becomes so much more. An open blockchain is the best method for having a trustless ledger. The broader the range of decentralized adoption the better. The Bitcoin blockchain hits all those points. 
Counterfeiting items is a $1.2 trillion global problem, according to Research and Markets 2018 Global Brand Counterfeiting Report. The rise of online commerce and third-party marketplace sellers have made the crime more prevalent in recent years. Blockchain technology can help consumers verify what they ordered online and what they receive in the mail is what they intended to purchase.
Things get a bit more interesting when you replace the single custodian with a federation of notaries by way of a multisignature address. In this model, a federation of entities must sign-off on movements to and from the sidechain, so more parties must be compromised for a failure situation to unfold where the bitcoins frozen on the main chain are stolen.
@mowliv I think a good way to think about it is by looking at our economy. The Federal Reserve prints US dollars for the US Government (the main blockchain) to boost the US economy. However, US dollars can be exported to other countries (a side chain) that could have a completely independent economy but still use a currency backed by the US government. – Olshansk May 30 '17 at 0:56
These in-channel payments would be instant, unlike current Bitcoin payments, which require an hour to be fully verified on the blockchain. What’s more, payments would be routable across multi-hop paths, like packets across the Internet — so instead of having to create a channel to every new counterparty, you could maintain a few channels to a small number of well-connected secure intermediaries and send/receive money through them.
Frankly, secure implementation of Bitcoin is already a pain in the ass .. adding more complexity just seems like the wrong move at this point. It’s already trying to be a currency, a networking protocol and a client in the same codebase. Adding turing complete (or not) scripts with arbitrary outcomes, multiple versions of the official client cooperating, multiple clients, and now multiple blockchains is basically the nail in the coffin in terms of widespread implementation.
What if we could run heavy computations in a more centralized fashion, say on a single server, and then periodically integrate the results onto the main blockchain for posterity. We temporarily expose some vulnerability while the parallel server runs the heavy computation, but we get a massive benefit in that we don’t have to run the computation on chain, and simply need to store the results for future verification. This is the general premise behind Truebit. We won’t get into all the details of Truebit but there is a concept of challengers, who check to see the computations that were made have high fidelity.
Write permissions are kept centralized to one organization. Read permissions may be public or restricted to an arbitrary extent. Example applications include database management, auditing, etc. which are internal to a single company, and so public readability may in many cases not be necessary at all. In other cases public audit ability is desired. Private blockchains are a way of taking advantage of blockchain technology by setting up groups and participants who can verify transactions internally. This puts you at the risk of security breaches just like in a centralized system, as opposed to public blockchain secured by game theoretic incentive mechanisms. However, private blockchains have their use case, especially when it comes to scalability and state compliance of data privacy rules and other regulatory issues. They have certain security advantages, and other security disadvantages (as stated before).
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.
I said above that you can build sophisticated rules into Bitcoin transactions to specify how ownership is proved. However, the Bitcoin scripting language is deliberately limited and many ideas in the Smart Contracts space are difficult or impossible to implement. So projects such as Ethereum are building an entirely new infrastructure to explore these ideas

My chief concern is not with the concept of side chains per se (yet). I have still much to learn about how they are being considered. I am only concerned with the way the concept is being presented here. However, I am sure that much of this was due to space restrictions as much as anything. The concept of side chains is an intriguing one. It is also clearly attempting to address a major problem with the whole Bitcoin scheme- namely the verification latency it introduces for transactions. This is only one of the hurdles facing Bitcoins acceptance into the world of commerce, but it is a considerable one.
Imagine there is a Bitcoin-like system out there that you’d like to use. Perhaps it’s litecoin or ethereum or perhaps it’s something brand new.   Maybe it has a faster block confirmation interval and a richer scripting language. It doesn’t matter.   The point is: you’d like to use it but would rather not have to go through the risk and effort of buying the native tokens for that platform. You have Bitcoins already. Why can’t you use them?
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You cannot be a crypto investor or entrepreneur without having a real understanding of the differences between these types of blockchains as well as their implications. Even if they are based on similar principles, their operation is, in fact, different to all levels. So the tokens issued by these blockchains will not be assessed in the same manner.
The main point of a side-chain is to allow cryptocurrency networks to scale and interact with one-another. For example alt-coins and Bitcoin run on separate chains, however side chains allow for these separate currencies to be transferred through these two-way 'portal's or interfaces via a fixed conversion amount. Added benefits of side-chains are different asset classes like,stocks, bonds etc being integrated through a converted price onto the main chain, along with additional functionality like smart contracts,unique D-Apps, micro-payments and security updates that can be later incorporated into the primary network from these side-chains.
– A consensus much faster: the fact that the consensus mechanism is centralized makes it much quicker. In fact, the term “consensus” is no longer adapted since it is rather a recording of transactions on the blockchain. Note that the entity responsible for managing the blockchain can decide to change the parameters of the blockchain and in particular to increase the size of the blocks to be able to add more transactions.

This approach isn’t fool-proof, but it’s not by mistake that the system looks the way it does today (that’s my history degree talking). Despite best technical efforts, human problems remain within the realm of probability. From “…blame cannot be easily assigned: not even the most sophisticated economists of the era could accurately predict disaster, let alone guard against it. The effects of a public herd mentality at the time of the [insert catastrophe here] are depicted, all too recognizably, as unstoppable.”

The words block and chain were used separately in Satoshi Nakamoto's original paper, but were eventually popularized as a single word, blockchain, by 2016. The term blockchain 2.0 refers to new applications of the distributed blockchain database, first emerging in 2014.[13] The Economist described one implementation of this second-generation programmable blockchain as coming with "a programming language that allows users to write more sophisticated smart contracts, thus creating invoices that pay themselves when a shipment arrives or share certificates which automatically send their owners dividends if profits reach a certain level."[1]

Open blockchains are more user-friendly than some traditional ownership records, which, while open to the public, still require physical access to view. Because all early blockchains were permissionless, controversy has arisen over the blockchain definition. An issue in this ongoing debate is whether a private system with verifiers tasked and authorized (permissioned) by a central authority should be considered a blockchain.[36][37][38][39][40] Proponents of permissioned or private chains argue that the term "blockchain" may be applied to any data structure that batches data into time-stamped blocks. These blockchains serve as a distributed version of multiversion concurrency control (MVCC) in databases.[41] Just as MVCC prevents two transactions from concurrently modifying a single object in a database, blockchains prevent two transactions from spending the same single output in a blockchain.[42]:30–31 Opponents say that permissioned systems resemble traditional corporate databases, not supporting decentralized data verification, and that such systems are not hardened against operator tampering and revision.[36][38] Nikolai Hampton of Computerworld said that "many in-house blockchain solutions will be nothing more than cumbersome databases," and "without a clear security model, proprietary blockchains should be eyed with suspicion."[9][43]
Public blockchains are open, and therefore are likely to be used by very many entities and gain some network effects. To give a particular example, consider the case of domain name escrow. Currently, if A wants to sell a domain to B, there is the standard counterparty risk problem that needs to be resolved: if A sends first, B may not send the money, and if B sends first then A might not send the domain. To solve this problem, we have centralized escrow intermediaries, but these charge fees of three to six percent. However, if we have a domain name system on a blockchain, and a currency on the same blockchain, then we can cut costs to near-zero with a smart contract: A can send the domain to a program which immediately sends it to the first person to send the program money, and the program is trusted because it runs on a public blockchain. Note that in order for this to work efficiently, two completely heterogeneous asset classes from completely different industries must be on the same database - not a situation which can easily happen with private ledgers. Another similar example in this category is land registries and title insurance, although it is important to note that another route to interoperability is to have a private chain that the public chain can verify, btcrelay-style, and perform transactions cross-chain.
There is a whole other issue of identity theft that needs to be addressed. Just a short note here as this is a big subject: If the private key to identity object is stolen, the true owner of the identity needs to have a way to change the key. One approach to that would be to use the private key of the bitcoin transaction that created the first version of the identity object. Another way could be to prove the ownership of other public keys on the identity object, like the one used for encryption (PGP key management suggests a separate key for each purpose, signing, encryption, etc.). Other non-automatic ways could include a trusted third-party, social proof, etc.
Sidechains, just like any other Blockchain, need their own miners to help protect them from nefarious actors and attacks which people would like to leverage against the network. However, since wealth isn't actually created on the Sidechain there is far less incentive for miners to actually work on it and help protect it. Because of this, transaction fees are the basic reward that is offered to miners. However, these often equate to mere pennies.
Implemented by	The initial design was published by Blockstream in 2014, but the implementation is blocked by the lack of native support for SPV proofs in Bitcoin (which may not be added at all). Rootstock workaround this by sacrificing decentralization (still work in progress).	The Ardor platform created by Jelurida is the first to propose and implement the concept of Child Chains. Already running on testnet, the production Ardor launch is scheduled for Q4 2017.

Security issues. Like the blockchain, the sidechain needs the work of miners to stay safe from attacks. Without sufficient power, the sidechain is vulnerable for assault. If hacked, only the sidechain will be damaged, while the main chain remains untouched and ready to continue work. If the main chain comes under the attack, the sidechain still operates, but without the value of the peg.
By design, a blockchain is resistant to modification of the data. It is "an open, distributed ledger that can record transactions between two parties efficiently and in a verifiable and permanent way".[7] For use as a distributed ledger, a blockchain is typically managed by a peer-to-peer network collectively adhering to a protocol for inter-node communication and validating new blocks. Once recorded, the data in any given block cannot be altered retroactively without alteration of all subsequent blocks, which requires consensus of the network majority. Although blockchain records are not unalterable, blockchains may be considered secure by design and exemplify a distributed computing system with high Byzantine fault tolerance. Decentralized consensus has therefore been claimed with a blockchain.[8]
That is however not all. Sidechains also have some specific use cases, unique to a certain blockchain. One example is the usage of sidechains in EOS. EOS is currently facing a RAM problem. RAM is too expensive and developers are complaining. Sidechains could compete with the EOS mainchain by having lower RAM prices, this would lead to competition, incentivizing both the EOS mainchain block producers and sidechain block producers (mainchain and sidechains of EOS are maintained by the same group of block producers) to keep the RAM price as low as possible. This also means there is more RAM available, so the RAM price will go down as a result.
“Blockchain could significantly reduce time delays and human mistakes, and monitor cost, labor, waste and emissions at every point in the supply chain. In the food sector, a manufacturer could automatically identify contaminated products in a matter of seconds and wouldn’t need to pull an entire product line from store shelves in the case of contamination.”
The first work on a cryptographically secured chain of blocks was described in 1991 by Stuart Haber and W. Scott Stornetta.[10][6] They wanted to implement a system where documents' timestamps could not be tampered with or backdated. In 1992, Bayer, Haber and Stornetta incorporated Merkle trees to the design, which improved its efficiency by allowing several documents to be collected into one block.[6][11]
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.
Things get a bit more interesting when you replace the single custodian with a federation of notaries by way of a multisignature address. In this model, a federation of entities must sign-off on movements to and from the sidechain, so more parties must be compromised for a failure situation to unfold where the bitcoins frozen on the main chain are stolen.

– A consensus much faster: the fact that the consensus mechanism is centralized makes it much quicker. In fact, the term “consensus” is no longer adapted since it is rather a recording of transactions on the blockchain. Note that the entity responsible for managing the blockchain can decide to change the parameters of the blockchain and in particular to increase the size of the blocks to be able to add more transactions.

Sidechain is a blockchain that runs parallel to the main blockchain. It extends the functionality of interplorable blockchain networks. Interpolable blockchain networks signifies the ability to share data between different computer systems on different machines. It means that data can be sent and received between interconnected networks eliminating the possibility of negative impact to the networks. Sidechain enables this to be done in a decentralised manner to transfer and synchronise tokens between two chains.

This comparison might make you think that private blockchains are more reasonable to use as they are faster, cheaper, and protect the privacy of their members. However, in certain cases, transparency is more crucial than the speed of transaction approval. So, every company interested in moving their processes to a blockchain evaluates the needs and goals and only then selects a particular type of distributed ledger.
Further, despite sidechains being independent of each other, they are responsible for their individual security and need the requisite mining power to remain secure. Bitcoin’s blockchain has sufficient PoW mining power to remain secure even from the most coordinated of attacks, but many more nascent sidechains lack the necessary network effects and mining power to guarantee security to users.
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.
Confidential Transactions — At present, all Bitcoin transactions are completely public, albeit pseudonymous. Confidential Transactions, as the name implies, conceal the amount being transferred to all except the sender, the recipient, and others they designate. The resulting transaction size is significantly larger, but includes a sizable “memo” field that can be used to store transaction or other metadata, and is still smaller than eg Zerocoin.(Note that this isn’t as confidential as Zerocash, which conceals both the amount and the participants involved in any transaction, through the mighty near-magic of zk-Snarks. Mind you, Zerocash would require an esoteric invocation ritual to initiate its network. No, really. But that’s a subject for a separate post.)
Sidechain transactions using a two-way peg effectively only allow for intra-chain transactions. A transfer from Bitcoin (parent chain) to Ethereum (sidechain) would allow a user to use the functionality of Ethereum (i.e., fully expressive smart contracts), but the underlying original asset would remain precisely that, Bitcoin. So, a Bitcoin on an Ethereum sidechain technically remains a Bitcoin.
The original Litecoin we started out with are now Rootstock Litecoin, which I can use for creating smart contracts and as previously mentioned Sidechains can exist for all types of digital assets with propositions of not only smart contracts but the ability to provide more freedom for experimentation with Beta releases of core software and Altcoins, as well as the taking over of traditional banking instruments such as the issuing and tracking of shares, bonds and other assets.

2) Yes – I had to keep things short/simple in this intro article in order to get across the key ideas. But you’re right: the sidechains need to be secured. But how that happens is a matter for the sidechain. If somebody can produce a false “proof” that the locked Bitcoins should be released on the Bitcoin side then that’s a problem for the sidechain, of course (somebody presumably just had their coins stolen!) but it’s irrelevant (at a macro level) on the Bitcoin side.

The sole distinction between public and private blockchain is related to who is allowed to participate in the network, execute the consensus protocol and maintain the shared ledger. A public blockchain network is completely open and anyone can join and participate in the network. The network typically has an incentivizing mechanism to encourage more participants to join the network. Bitcoin is one of the largest public blockchain networks in production today.