@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.
First of all, one should not confuse private and public blockchains. They have one obvious similarity – they are blockchains, decentralized networks. Every participant of the network keeps a copy of this shared ledger, and all these copies are kept sync with the help of a certain consensus protocol. It means that all the participants of the network have access to identical information. Also, all the networks are immutable, and the information they contain can’t be altered.
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.

Nikolai Hampton pointed out in Computerworld that "There is also no need for a '51 percent' attack on a private blockchain, as the private blockchain (most likely) already controls 100 percent of all block creation resources. If you could attack or damage the blockchain creation tools on a private corporate server, you could effectively control 100 percent of their network and alter transactions however you wished."[9] This has a set of particularly profound adverse implications during a financial crisis or debt crisis like the financial crisis of 2007–08, where politically powerful actors may make decisions that favor some groups at the expense of others,[51][52] and "the bitcoin blockchain is protected by the massive group mining effort. It's unlikely that any private blockchain will try to protect records using gigawatts of computing power—it's time consuming and expensive."[9] He also said, "Within a private blockchain there is also no 'race'; there's no incentive to use more power or discover blocks faster than competitors. This means that many in-house blockchain solutions will be nothing more than cumbersome databases."[9]
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?
RSK is the first open-source smart contract platform with a 2-way peg to Bitcoin that also rewards the Bitcoin miners via merge-mining, allowing them to actively participate in the Smart Contract revolution. RSK goal is to add value and functionality to the Bitcoin ecosystem by enabling smart-contracts, near instant payments and higher-scalability.

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.

As you know, we at LTP have been doing a lot of research to understand other use cases of blockchain apart from Bitcoin-based payments. Recently we had released a comprehensive analysis of 50+ startups and 20 use-cases of blockchain. Though there have been news of large companies accepting bitcoin (Ex.: Amazon, Microsoft, Dell) and the overall acceptance reaching a 100,000+ merchants figure, upon deeper examination we realize that large corporations do not store the Bitcoin payments. They generally partner with a Bitcoin payment processor who converts the Bitcoins to cash as and when they receive a payment and this converted amount is what the corporates take into their account. What a bummer!
¡Por supuesto! para todo ello existen muchas propuestas con soluciones muy interesantes, pero hacer cambios experimentales sobre el código de Bitcoin es arriesgado y, que la mayoría de nodos se adapten, lleva tiempo. Bitcoin es grande y esto hace que la toma de decisiones sea lenta al reflexionarse los cambios de manera muy profunda. Esta toma de decisiones lenta e incapacidad del protocolo de ampliar con modulos las capacidades de Bitcoin es el principal motivo por el que empezaron a salir otras criptomendas centradas en nichos y casos de usos concretos. Era más sencillo clonarse el código abierto de Bitcoin y adaptartlo que esperar a que en Bitcoin se decidiese aceptar su funcionalidad. Este es, principalmente, el motivo por el cual hay cientos de criptomonedas y se necesita un wallet por cada una de ellas, siendo un absoluto caos a veces, ya que todas están desconectadas entre ellas.
Recordemos, como hemos mencionado anteriormente, que actualmente son cientos los proyectos y monedas alternativas que trabajan con su propia cadena de bloques, totalmente desconectadas de la de Bitcoin. Todas con su cotización volatil. El problema de estas monedas es que ninguna de ellas dispone del efecto red ni de la seguridad que sí tiene Bitcoin. De hecho muchas, pese a haber implementado propuestas interesantes, se quedan en nada, con miles de horas y esfuerzo “tirado a la basura”. Incluso algunas de ellas han replicado el codigo de Bitcoin, pero también los fallos que en ese momento pudiera tener y mientras que en Bitcoin si se han solucionado, en esa Altcoin no.
“Blockchain offers a possible solution to these challenges with its decentralized ledger that can store a history of transactions across a shared database,” Cohen said in the report. “By making the record accessible and verifiable from anywhere in the world, blockchain can enable the authentication of goods and eradicate the criminal element of counterfeit goods in the retail supply chain. By pairing hardware chips with blockchain technology, a product can take on a digital history, going as far back as the raw materials that were used to make the product. This allows retailers and consumers to verify their purchased products are genuine.”

The consortium or company running a private blockchain can easily, if desired, change the rules of a blockchain, revert transactions, modify balances, etc. In some cases, eg. national land registries, this functionality is necessary; there is no way a system would be allowed to exist where Dread Pirate Roberts can have legal ownership rights over a plainly visible piece of land, and so an attempt to create a government-uncontrollable land registry would in practice quickly devolve into one that is not recognized by the government itself. Of course, one can argue that one can do this on a public blockchain by giving the government a backdoor key to a contract; the counter-argument to that is that such an approach is essentially a Rube Goldbergian alternative to the more efficient route of having a private blockchain, although there is in turn a partial counter-argument to that that I will describe later.

Bitdeal is a bitcoin cryptocurrency exchange software & Blockchain development company. The main focus of the firm is to reduce the risks in bitcoin trading and to encourage new bitcoin exchange startups by providing a well-developed bitcoin exchange script or a cryptocurrency exchange software.  Being a cryptocurrency exchange software solution, bitdeal has covered around 50+ countries around the world, and have collected more than 200+ ... Read more
Hasta la fecha (Agosto del 2016), las sidechains sobre Bitcoin no son más que algo teórico. Una implementación de este tipo requeriría de un cambio en el código Bitcoin (hay miembros de la comunidad Bitcoin con gran prestigio, como es el caso de Peter Todd, que argumentan que una sidechain, tal y como la describe Blockstream en su paper, no podrían llevarse a la práctica en Bitcoin sin hacer un gran cambio, hard fork, en Bitcoin). En el mismo paper de blockstream se reconoce que una implementación de este tipo, la cual su teoría es simple pero su implementación compleja, se enfrenta a problemas que no está del todo claro que puedan solventarse (y no todos son de tipo técnico).
The term “sidechains” was first described in the paper “Enabling Blockchain Innovations with Pegged Sidechains”, circa 2014 by Adam Back et al. The paper describes “two-way pegged sidechains”, a mechanism where by proving that you had “locked” some coins that were previously in your posession, you were allowed to move some other coins within a sidechain.
The idea emerged that the Bitcoin blockchain could be in fact used for any kind of value transaction or any kind of agreement such as P2P insurance, P2P energy trading, P2P ride sharing, etc. Colored Coins and Mastercoin tried to solve that problem based on the Bitcoin Blockchain Protocol. The Ethereum project decided to create their own blockchain, with very different properties than Bitcoin, decoupling the smart contract layer from the core blockchain protocol, offering a radical new way to create online markets and programmable transactions known as Smart Contracts.

Bitdeal is a bitcoin cryptocurrency exchange software & Blockchain development company. The main focus of the firm is to reduce the risks in bitcoin trading and to encourage new bitcoin exchange startups by providing a well-developed bitcoin exchange script or a cryptocurrency exchange software.  Being a cryptocurrency exchange software solution, bitdeal has covered around 50+ countries around the world, and have collected more than 200+ ... Read more
@tetsu – not sure what you mean. My reading of the sidechains paper is that the worst case scenario is that an attacker manages to “reanimate” Bitcoins on the main blockchain that had been sent to the sidechain… but that would be the attacker stealing the coins from the rightful owner on the sidechain. From Bitcoin’s perspective, the coins were always going to be reanimated…. so the risk is entirely borne by the holder(s) on the sidechain. Am I missing something?
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.