Changes to the bitcoin protocol have to be conservative and slow so as not to destabilize the network, and thereby diminish its value. This in turn prevents rapid innovation that is prevalent in software development. The original solution to innovate upon Satoshi Nakamoto’s invention was simply to fork the Bitcoin software, tweak it as desired and release a new system. In this article, we will examine not how the future of the bitcoin network lies in forking the software, but building it out via an economy of interconnected blockchains, a development initiative which is known as sidechain technology among the cryptocurrency community.

Namecoin was the first fork of the bitcoin protocol. It introduced features that were not present in bitcoin, such as an alias system, a .bit top level domain registration and DNS service. Not too long after that developers started releasing Bitcoin-based clones on a regular basis. In late 2013, early 2014 there were upwards of a 100 such clones introduced daily. Most of them modified only a few parameters,  (e.g. block time, hashing algorithm, number of total coins, reward schedule, etc.) and did not offer any significant improvements.

While such experimentation should be encouraged, there are a number of problems that arose as a result of the proliferation of these alternative blockchains (altchains). Each of the altchains had its own native token (coin), in order to experiment with the altchain a user had to either purchase the tokens, or acquire the appropriate mining hardware. The ability to sell newly created coins on these altchains attracted dubious characters trying to make a quick profit.

Security is another problem that plagued some altchains, including low network hashrate allowing an easy 51% attack. Clearly some kind of solution was required. Innovation must be allowed and encouraged, but not at a cost so great that it might damage or even destroy the cryptocurrency industry as a whole through massive fragmentation. To that end a group of researchers formed the concept of pegged sidechains.

Blockstream Two-Way Pegged Sidechains

As the name suggests, bitcoin sidechains would exist alongside some main chain. Again, currently the main chain would be Bitcoin (but that could change in the future). At its core pegged sidechains would be a technology that allows movement of assets (coins, tokens, etc) from one blockchain to another. The authors of the sidechains white paper identify five properties that are desired for pegged sidechains.

  1. Assets can be returned to the ‘parent’ chain only by their current holder.
  2. There should be no ability for a malicious party to prevent a transfer from occurring.
  3. Transfers should be atomic (completed in its entirety, or not at all).
  4. Bugs in one sidechain must not affect any other chains.
  5. Sidechains should be fully independent, with any external data required supplied by users.
  6. Reorganizations should be handled cleanly at all times, and disruption should remain on the affected chain only.

    Practical Aspects of Bitcoin Sidechains

    The above properties deal mainly with security issues. But what about the more practical matters? What do pegged sidechains offer over and above the status quo?

    A sidechain could be seeded through the bitcoin network with already-established value. For example, new coins on a bitcoin sidechain could only be created by transferring bitcoins at a predetermined exchange rate. Of course it would be still possible for a sidechain to have its own token that would be traded just like an altcoin.

    Currently existing solutions for exchange involve sending bitcoin to a publicly verifiable, unspendable address. This amounts to the destruction of some funds, however, it can be thought of as a transfer rather than a destruction. The authors of the sidechains whitepaper call this a one-way peg.

    The ability to create sidechains allows for permissionless experimentation with the protocol. For example a side chain could be created with much bigger block size allowing, as a result, more transactions per time frame. Bigger block size is something that Bitcoin could use, but experimentation on the Bitcoin chain may be costly if the change proves undesirable. Similarly, other features can be experimented with on sidechains and moved onto the main chain only when proven to work in practice. It is also conceivable, although not very likely, that a sidechain could overtake a main chain in popularity and therefore itself become a main chain.

    The two-way peg

    The main innovation of sidechains is the proposed two-way peg. Assets can be transferred from a parent chain to a sidechain (for example, from Bitcoin to a newly created sidechain), the original assets will be now locked on the parent chain. Because of security concerns there must be a confirmation period, once it passes, the coins can be moved onto the sidechain where they will be subject to a further delay in availability called a contest period. Both waiting periods are necessary to ensure integrity of funds on both the parent, and the sidechain. As mentioned above such a transfer is currently possible under the proof of burn concept.

    The new feature of sidechains is the ability to transfer the asset back from a sidechain to a parent chain, thereby completing the two-way portion of the peg. If a sidechain were ever to consider shutting down the assets can be transferred back, at the same exchange rate (assets can be transferred back at anytime). This way a user wanting to try out a new sidechain because of some new features does not stand to lose all of her initial investment.

    Bitcoin Sidechain Security

    Altcoins suffer from low network hash rates, making themselves vulnerable to a 51% attack. Merged mining is an existing solution to that problem. In merged mining a pair (or more) of blockchains are mined at the same time. Likewise a sidechain creator could take advantage of Bitcoin’s security and utilize its hashing power via merged mining. Another improvement in security for a sidechain would be to also keep track of the parent chain. This simplifies the transfers from the parent to the side chain, but at an additional cost of keeping track of the main chain. This is called an asymmetric two-way peg, but the authors do not explore it in detail.

    Drawbacks of Sidechains

    Sidechains of course are not free of drawbacks. The main one is the added complexity to the network, to keeping track of assets, and to user-end software. Another is the need to deal with and prevent fraudulent transfers. Centralization of mining is also an issue, as more and more resources would be needed to keep track of multiple chains, smaller miners might elect not to participate. Finally, risks of a soft-fork is somewhat increased in a sidechain as they have a different level of security when it comes to transfers. The last risk can be alleviated, but at a risk of losing isolation which was one of the main motivating features behind sidechains.


    Pegged bitcoin sidechains are an interesting technology still in its concept stage. The technology will make it possible to transfer assets from one chain to another and then back again. Hence it is called a two-way peg. The main contribution of sidechains would be the ability to experiment with the Bitcoin protocol in a permission-less way, but with the added legitimacy of assets being backed by an already-established network.

    There are some drawbacks associated with sidechains but they are not unsolvable. The benefits of sidechain innovation will be insurmountable, and will enable an age of ‘trustless computing‘.


Albert Szmigielski

Thinker. Computing Scientist, Software Engineer. Cryptocurrency researcher. Crypto 2.0 enthusiast. Startup advocate. Algorithms are Albert's forte. Probability theory is his nemesis. He has a fondness for graph theory, elegant algorithms, and respect for blockchain technology.

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  • Reply

    Sean Daley

    30 03 2015

    A good review and critique of the white paper on pegged sidechains was published by a few months back. Our tech lead Justin Turrell commented :
    “It appears that this white paper on pegged sidechains represents a
    primarily theoretical proposal at this point. It cannot be implemented
    at this time without a mod to the Bitcoin protocol. After such a mod,
    it would still require widespread adoption by miners and wallet clients
    in order to get very far. So, compared with what we have available
    today already, I think it’s brilliant without currently functioning as a
    The full article can be read here: .

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