There has been tremendous interest in blockchain, the technology on which Bitcoin functions. Nakamoto developed the blockchain as an acceptable solution to the game theory puzzle – Byzantine General’s Problem. This lead to a number of firms adopting the technology in different ways to solve real world issues, wherever there was an element of trust involved. Majority of them could be relating to the ability to provide proof of ownership – for documents, software modules/licenses, voting etc.
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).
Note: Some would argue that such a system cannot be defined as a blockchain. Also, Blockchain is still in it’s early stages. It is unclear how the technology will pan out and will be adopted. Many argue that private or federated Blockchains might suffer the fate of Intranets in the 1990’s, when private companies built their own private LANs or WANs instead of using the public Internet and all the services, but has more or less become obsolete especially with the advent of SAAS in the Web2.
The Bitcoin White Paper was published by Satoshi Nakamoto in 2008; the first Bitcoin block got mined in 2009. Since the Bitcoin protocol is open source, anyone could take the protocol, fork it (modify the code), and start their own version of P2P money. Many so-called altcoins emerged and tried to be a better, faster or more anonymous than Bitcoin. Soon the code was not only altered to create better cryptocurrencies, but some projects also tried to alter the idea of blockchain beyond the use case of P2P money.
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@gendal, good question. Think of the identity hash as a bitcoin address, it is indeed public. So to assert anything with this identity you need to sign the object you are creating or changing with the identity’s private key. Specifically it is a private key that corresponds to a public key that you published in your identity’s object (json). The signature is not placed on the bitcoin transaction, as OP_RETURN has only 40 bytes. The signature is added to a [json] object that is modified with this identity. If you see any fault with this, please let me know.
This construction is achieved by composing smart contracts on the main blockchain using fraud proofs whereby state transitions can be enforced on a parent blockchain. We compose blockchains into a tree hierarchy, and treat each as an individual branch blockchain with enforced blockchain history and MapReducable computation committed into merkle proofs. By framing one’s ledger entry into a child blockchain which is enforced by the parent chain, one can enable incredible scale with minimized trust (presuming root blockchain availability and correctness).

I have a hard time swallowing that Bitcoin “isn’t a ledger”. That’s like saying “Bitcoin isn’t the blockchain”, and if you take the blockchain away from Bitcoin, you aren’t really left with much (including, sidechains). Perhaps Bitcoin isn’t a ledger *from the perspective* of individual transactions, but by the same logic, nothing that isn’t transaction data is.

New distribution methods are available for the insurance industry such as peer-to-peer insurance, parametric insurance and microinsurance following the adoption of blockchain.[71][72] The sharing economy and IoT are also set to benefit from blockchains because they involve many collaborating peers.[73] Online voting is another application of the blockchain.[74][75]
People believe that permissioned means that only a select group of people can access the data and that’s the security feature. But it’s not. Since there is no real user data on the blockchain, (you) as a member of the public, can’t verify the actual content of it. This means that data resides in a location where corruption can stay undetected and data can be easily modified. So why does it even exist? Mainly because of the phenomena known as “hype surfing”; essentially reusing old technology and strapping a blockchain sticker on it gets IBM salesmen a foot in the door to institutions who can’t evaluate the technology accurately in the first place. Unfortunately, even some teams doing public token offerings started to sell this deeply flawed approach to the public.

“RSK directly “plugs in” to achieve a perfect merged-mining and to ensure that cryptographic work, that will be discarded in Bitcoin mining, is reused in the first smart contract open-source platform secured by the Bitcoin network. RSK has an agreement with Bitcoin miners: we share with them 80% of the fees arising from transactions made within the smart contract network.”

Unfortunately our second option cannot be done yet, because to use these sidechains, main chain (here it is bitcoin) needs to do some upgrade (soft fork). By the way, upgrades in public blockchains are very painful yet. There will be a user activated soft fork (UASF) on August 1. All bitcoin forms’ trend topic is this soft fork which is about a code change for Segregated Witness Adoption.
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.
The consensus mechanism is centralized in the hands of a single entity which mission is to verify and add all transactions to the blockchain. A network based on a private blockchain, therefore does not need to use a mechanism such as “Proof of Work” or “Proof of Stake” which are complicated to implement and expensive. The problems of security being much more simple in the case of private blockchains, it is possible to apply the mechanisms of consensus lighter, more effective and therefore easy to deploy such that the BFT.
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Bitcoin se acerca a los 10,000 millones de capitalización, con una infraestructura y usuarios que requieren que todas las ideas e innovación que se desarrolla a su alrededor cumpla con un nivel de seguridad y testeo tan elevados como el propio Bitcoin. Es por esto que, al menos hasta no ser algo totalmente definnido y fiable en la blockchain test de Bitcoin, no se podrá empezar a presionar para una posible implementación en la blockchain live.
And now for the second clever part. The logic above is symmetric. So, at any point, whoever is holding these coins on the sidechain can send them back to the Bitcoin network by creating a special transaction on the sidechain that immobilises the bitcoins on the sidechain. They’ll disappear from the sidechain and become available again on the Bitcoin network, under the control of whoever last owned them on the sidechain.
In simple terms, public blockchains can receive and send transactions from anybody in the world. They can also be audited by anybody, and every node has as much transmission power as any other. Before a transaction is considered valid, it must be authorized by each of its constituent nodes via the chain’s consensus process. As long as each node abides by the specific stipulations of the protocol, their transactions can be validated, and thus add to the chain
A side-chain is a secondary blockchain layer designed to facilitate lower-cost and/or higher-speed transactions between two or more parties. One case in which they're often deployed is between parties who make many transactions amongst each other. Committing all of those transactions to the public blockchain would may undesirable for cost or other reasons, so the side-chain's job in this example would be to aggregate the activity into the least transactional activity necessary to reflect the final state of the side-chain's ledger.
Cuando esta transacción recibe las suficientes confirmaciones, se manda una notificación a la otra cadena de bloques (la que tú quieres utilizar) en el que se adjunta la prueba de que las monedas han sido enviadas por ti a esa dirección especial de la red. Tras ello, en la sidechain se creará, de forma automática, el mismo número exacto de activos que bitcoins se mandaron, dándote a ti el control de los mismos. Es decir, replica en el nuevo activo la cuantía que has enviado de la cadena principal a la sidechain. ¡Muy importante! Recordar que no se han creado o destruido nuevos bitcoins. Simplemente se han movido hasta que no estén usándose en la sidechain.
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 user on the parent chain first has to send their coins to an output address, where the coins become locked so the user is unable to spend them elsewhere. Once the transaction has been completed, a confirmation is communicated across the chains followed by a waiting period for extra security. After the waiting period, the equivalent number of coins is released on the sidechain, allowing the user to access and spend them there. The reverse happens when moving back from a sidechain to the main chain.
Forbes reports that blockchain and biometric eyeball scanning technologies underpin the systems that support food distribution in the Syrian refugee crisis. While there are many further uses of blockchain, at the core of its business functionality is the creation of transparent, stacking “ledgers” of information. This is where private blockchain can prove extremely useful.
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.

@quinn – thanks for the comment. I probably didn’t write clearly enough… I was trying to point out that none of the higher-level concepts we’re familiar with (addresses, bitcoins, the “ledger”, etc) actually exist at the protocol level…. it’s just transactions, transaction outputs, unspent transaction outputs, etc… they combine to create the illusion we’re all familiar with.
Side-chain is another blockchain for one blockchain. To use side-chain of Bitcoin, for instance, you need to move BTC from the original chain to the side-chain. Then, BTC on the original chain is locked and the same amount of BTC on the side-chain appears. This is how BTC can be used/tested on another chain where we use some features different from the original ones.

Security: RSK´s blockchain is secured by merge-mining, which means that they can achieve the same security as Bitcoin in terms of double-spend prevention and settlement finality. The 2way peg security will first rely in a federation holding custody of bitcoins, and later switch to an automatic peg, when the community accepts the security trade-offs of the automatic peg.

The cheapest and most simple option is doing calculations on your local network (off-chain) and integrating with main blockchain by sending the results. It has flaws; you cannot live full advantage of blockchain as we do in bitcoin, because you will still have existing constraints of your current system. Despite all this, it is still a valid option; perhaps you won't need all the features of blockchain technology. Perhaps it is just enough to use blockchain only for your pain points. Factom can be considered under that kind of option. They used bitcoin wisely in their design. They hold the actual mass data in their network and utilize stability of bitcoin in their solution. This project is so successful that at coindesk magazine, it is saying that Factom can be used for the land titles in Honduras.
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".
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Transactions are cheaper, since they only need to be verified by a few nodes that can be trusted to have very high processing power, and do not need to be verified by ten thousand laptops. This is a hugely important concern right now, as public blockchains tend to have transaction fees exceeding $0.01 per tx, but it is important to note that it may change in the long term with scalable blockchain technology that promises to bring public-blockchain costs down to within one or two orders of magnitude of an optimally efficient private blockchain system
This is justified by observing that, in our pre-sidechain world, miners always want things to be correct. In theory, the incentives of miners and investors are very strongly aligned: both are compensated most when the exchange rate is highest. And, in practice, we do not see large reorganizations (where miners can “steal”, by first depositing BTC to major exchanges, then selling that BTC for fiat (which they withdraw), and finally rewriting the last 3 or 4 days of chain history, to un-confirm the original deposits). These reorgs would devastate the exchange rate, as they would cast doubt on the entire Bitcoin experiment. The thesis of Drivechain is that sidechain-theft would also devastate the exchange rate, as it would cast doubt on the entire sidechain experiment (which would itself cast doubt on the Bitcoin experiment, given the anti-competitive power of sidechains).
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.
The immense promise and accelerated development of permissioned blockchain technology, combined with intense business interest from a wide range of industries, is acting as a perfect stimulant for more and more enterprises to start rolling out blockchain networks into production. I envision these permissioned networks will soon directly or indirectly influence every facet of human enterprise.