About

This project attempts to create a comprehensive list of resources related to sidechain design and development.

Terminology

According to the original sidechains whitepaper by Back et al, a sidechain is:

a blockchain that validates data from other blockchains (p. 8)

An example of a sidechain under this definition would be Ethereum, a blockchain that can validate data from the bitcoin blockchain in the form of SPV proofs.

Back et al go on to define a pegged sidechain as:

a sidechain whose assets can be imported from and returned to other chains; that is, a sidechain that supports two-way pegged assets. (p.8)

The blockchain that an asset is imported from is the "mainchain" or "parent chain", and the blockchain that the asset is imported to is the sidechain. The mutual portability between a mainchain and a sidechain is referred to as a "two-way peg" due to the way that the value of an asset is "pegged" 1:1 when minted on a sidechain i.e. a pegged asset minted on a sidechain should always be redeemable 1:1 (minus any fees) for the asset it represents on the mainchain.

Thus, crucial to the definition of a pegged sidechain, in the author's humble opinion, is that the portability of assets between a mainchain and sidechain should be permissionless; that is, no permission from any third part(y)(ies) should be required to import an asset into and export an asset from a sidechain. Back et al say as much as well:

Assets which are moved between sidechains should be able to be moved back by whomever their current holder is, and nobody else (including previous holders).

Assets should be moved without counterparty risk; that is, there should be no ability for a dishonest party to prevent the transfer occurring. (p. 5)

Taken together, we can say that a pegged sidechain is:

a blockchain that validates data from other blockchains and supports permissionless, counterparty-free two-way pegged assets.

Based on observations by the author, this is the definition that most people probably think of when they use the term "sidechains", as opposed to the older and simpler definition given by Back et al. And so, while technically what is being referred to here is a "pegged sidechain", the same concept is now colloquially referred to simply as "sidechain" and this is how the term is used in this project as well.

Papers

2014-10-22: Enabling Blockchain Innovations with Pegged Sidechains
2016-04-01: Drivechains, Sidechains and Hybrid 2-way peg Designs
2017-08-21: Merged Mining: Curse of Cure?
2017-09-05: Driveproof: Sidechain Headers On Mainchain (SHM)
2017-09-23: Proof-of-mainstake
2017-12-02: Sidechain Weaknesses
2018-06-28: Research of Merged Mining Technology in Bitcoin Blockchain Scaling
2018-07-03: CountAcks Drivechain
2018-10-17: Sidechains: Decoupled Consensus Between Chains
2018-10-30: Proof-of-Work Sidechains
2018-12-18: Proof-of-Stake Sidechains
2019-01-18: Proof-of-Stake Bitcoin Sidechains v0.0.1
2020-01-01: Zendoo: a zk-SNARK Verifiable Cross-Chain Transfer Protocol Enabling Decoupled and Decentralized Sidechains
2020-11-12: Horizon: A Gas-Efficient Trustless Bridge for Cross-Chain Transactions
2020-12-31: Softchains: Sidechains as a Soft Fork via Proof-of-Work Fraud Proofs

Blog posts

2014-10-26: A simple explanation of Bitcoin "Sidechains"
2015-04-10: Pegged Sidechains Whitepaper Notes
2015-11-24: Drivechain - The Simple Two Way Peg
2017-01-30: Blind Merged Mining
2017-03-08: OP_WITHDRAWPROOFVERIFY — The op code that powers SPV sidechains
2018-08-30: Ion Stage 2: Toward a General Interoperability Protocol (Part 1)
2019-04-17: Why Drivechain Is Harder to Understand Than Previous Soft Forks
2019-04-17: Is Liquid A "Real" Sidechain? Greg Maxwell Weighs In
2019-12-10: Blind Merged Mining with covenants ( sighash_anyprevout / op_ctv )
2020-02-09: Ideas on how to construct sidechains for chains that have covenant capabilities
2020-04-30: The difference between XCAT and Drivechain
2020-08-02: Comparing two-way bitcoin pegs
2020-11-08: Scaling bitcoin with sidechains

Discussion threads

2014-04-12: Why do people think that side-chains are going to be secure?
2014-10-23: Sidechains whitepaper AMA
2014-12-20: The relationship between Proof-of-Publication and Anti-Replay Oracles
2016-10-02: Drivechain proposal using OP_COUNT_ACKS
2017-02-24: How an anchored Proof of Stake Sidechain can help the Bitcoin main chain
2017-06-10: Drivechain -- Request for discussion
2017-07-10: Updating the Scaling Roadmap
2017-09-05: Sidechain headers on mainchain (unification of drivechains and spv proofs)
2017-09-23: Sidechains: Mainstake
2017-12-01: Two Drivechain BIPs
2018-01-17: NiPoPoW criticism
2019-07-04: SIDECHAINS ARE NOT LAYER 2
2020-12-31: Softchains: Sidechains as a Soft Fork via Proof-of-Work Fraud Proofs

Presentations

2015-current: Drivechain presentations
2015-06-28: SF Bitcoin Devs Seminar: Sidechains: Bringing New Elements to Bitcoin
2018-10-17: Horizen Sidechains: Decoupled Consensus Between Chains
2019-04-18: Proof-of-Stake Sidechains for Cardano

Interviews

2015-08-17: Sidechains and Lightning Networks with Rusty Russell
2016-04-19: Interview with Greg Maxwell: Lightning Network Better Than Sidechains for Scaling Bitcoin
2017-11-15: Daniel Krawisz and Paul Sztorc talk Drivechain
2018-09-30: Let's Talk Bitcoin! #377 Sidechains, Drivechains and the Apple Store
2020-08-27: Cosmos Unchained: Bitcoin Peggy Showdown
2020-09-17: How Bitcoin Sidechains are the Altcoin Killer! An Interview with Paul Sztorc on Drivechains

News

2014-04-16: Side Chains: The How, The Challenges and the Potential
2014-10-23: Sidechains White Paper Imagines New Future for Digital Currency Development
2014-10-25: Bitcoin 2.0: Sidechains And Ethereum And Zerocash, Oh My!
2017-06-05: Are Sidechains a Better Solution for Bitcoin’s Scaling Debate?
2018-01-17: The Sidechains Breakthrough Almost Everyone in Bitcoin Missed
2018-02-19: Sidechains: Why These Researchers Think They Solved a Key Piece of the Puzzle
2018-09-25: Bitcoin Takes a Step Toward Becoming a ‘Multi-Network’ Cryptocurrency
2019-02-27: It's 2019, Where Are Bitcoin's Sidechains?
2020-12-08: RSK Is Evolving; Powpeg Leverages Hash Power To Switch From Bitcoin To Sidechain And Back

Patents

2018-03-27: Transferring ledger assets between blockchains via pegged sidechains

Semi-related

While not directly related to sidechains per se, the following resources may be useful to understand the design space and tradeoffs around sidechains, and may describe complementary technologies as well.