How Sidechains Connect to Mainblockchains: Mechanics, Security, and Use Cases

Ben Bevan 9 October 2025 1 Comments

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This calculator helps determine whether using a sidechain is economically beneficial compared to the mainchain. It compares total costs over a given number of transactions, factoring in transaction fees and potential throughput limitations.

When blockchains get clogged with transactions, developers look for a way to off‑load work without giving up the safety of the original network. The answer often lies in a sidechain, a parallel chain that talks to the main network through a set of well‑defined connections. This article breaks down exactly how those connections work, why they matter, and what you need to watch out for when you move assets across the divide.

Sidechain is an independent blockchain that runs alongside a primary blockchain (the mainchain) while maintaining its own consensus rules and token economics. A sidechain can process transactions faster and cheaper, but it must still preserve the fundamental guarantee that assets are safely transferable back to the mainchain.

Mainchain is the original, often more secure blockchain (such as Ethereum or Bitcoin) that holds the canonical state of assets and smart contracts. The mainchain’s security comes from a massive validator set and a long‑established proof‑of‑work or proof‑of‑stake model.

Two‑Way Peg is a cryptographic protocol that locks assets on one chain and mints equivalent assets on another, enabling bidirectional movement. The peg ensures that a token cannot exist simultaneously on both chains in excess of the original supply.

Blockchain Bridge is the software layer that implements the two‑way peg, validates proofs, and coordinates validator consensus across chains. Bridges come in federated, proof‑of‑stake, and trustless flavors, each with different trust assumptions.

Validator is a node that signs off on state transitions, provides fraud proofs, and may be staked or selected by a federation. In a sidechain bridge, validators confirm that a lock event on the mainchain really happened before minting on the sidechain.

Smart Contract is self‑executing code on the mainchain that holds locked assets and emits events that bridges watch for. The contract’s code defines lock periods, fee structures, and withdraw logic.

Why Use a Sidechain?

Congestion on a mainchain inflates gas fees and slows confirmation times. Sidechains offer three core benefits:

Lower fees: Polygon’s PoS sidechain costs roughly $0.0001 per transaction, compared with $1.20 on Ethereum (Oct2023 data).
Higher throughput: Polygon can push 7,200TPS versus Ethereum’s 30TPS.
Customizability: Developers can tweak consensus parameters, tokenomics, or add specialized modules without affecting the mainnet.

The trade‑off is that sidechains inherit a lower security guarantee because they rely on a smaller validator set and a bridge that could be compromised.

Two‑Way Peg Mechanics

The peg works in two phases: lock on the mainchain, mint on the sidechain (and the reverse for withdrawals). Here’s a step‑by‑step view:

Step1 - Lock: A user sends tokens to a designated bridge lock contract on the mainchain. The contract records the amount, sender address, and a unique transaction hash.
Step2 - Proof Generation: Validators monitor the lock contract’s event logs. After a security‑defined number of confirmations (typically 100‑200 blocks on Ethereum, about 25‑50minutes), they create a Merkle proof that the lock event is part of the mainchain’s canonical state.
Step3 - Verification: The sidechain bridge contract receives the proof, verifies the Merkle root, and confirms the lock.
Step4 - Mint: Once verified, the bridge mints a wrapped version of the original token on the sidechain (e.g.,wETH on Polygon). The minted amount matches the locked amount.
Step5 - Withdrawal: To move assets back, the user burns the wrapped token on the sidechain, triggering a new proof that the burn occurred. After verification on the mainchain, the lock contract releases the original tokens to the user’s address.

This flow preserves total supply: every minted token on the sidechain is backed 1:1 by a locked token on the mainchain.

Bridge Architectures: Federated, PoS, and Trustless

Different projects adopt different bridge models based on their risk appetite.

Bridge Architecture Comparison
Model	Trust Assumption	Validator Count	Typical Finality	Key Risks
Federated	Partial centralisation - a known set of entities must act honestly	3‑9 (2/3 majority)	30seconds‑2minutes	Key‑holder collusion, downtime of validators
Proof‑of‑Stake (PoS)	Economic security - validators stake tokens that can be slashed	~100 (≥67% consensus)	2‑5minutes	Slashing attacks, insufficient staking depth
Trustless (cryptographic proof)	Zero‑knowledge or SNARK proofs replace trust	Variable; often relies on mainchain security	5‑10minutes	Complex proof generation, higher gas costs

Polygon’s PoS bridge (the market leader) falls into the PoS category, using 100 validators and a checkpoint system that submits a cryptographic proof to Ethereum every 10minutes. Ronin’s federated bridge used 9 validators, resulting in sub‑30‑second lock times but a higher centralisation risk-evidenced by the $625million hack in 2022.

Sketch of bridge unit transferring token cubes between mainchain lock and sidechain mint.

Real‑World Transaction Flow: Ethereum↔Polygon

To make the abstract steps concrete, let’s walk through a typical ETH deposit to Polygon via the official bridge.

The user initiates a deposit on the Polygon Bridge UI.
The UI sends a transaction to the Ethereum lock contract 0xA0c68C638235ee32657e8f720a23ceC1bFc77C77. ETH is locked, and an event containing the deposit ID is emitted.
Polygon’s validator set monitors the Ethereum event log. After 100 confirmations (≈25minutes), they create a Merkle proof that includes the deposit transaction.
The proof is posted to the Polygon ERC20Predicate contract. The contract verifies the proof, minting an equivalent amount of WMATIC‑wrapped ETH on Polygon.
The user sees the new balance in their Polygon wallet within 2‑5minutes, ready for cheap DeFi actions.

Withdrawal works the opposite way: the user burns WMATIC‑wrapped ETH, the sidechain bridge creates a proof, and the Ethereum lock contract releases the original ETH after a similar confirmation period.

Security Landscape: Bridge Risks and Mitigations

Bridge vulnerabilities dominate cross‑chain loss statistics. A University of California, Berkeley study (2023) found that 65% of cross‑chain exploits occurred at the bridge layer, amounting to $2.8billion in stolen assets.

Key attack vectors include:

Validator collusion: If a majority of PoS validators act maliciously, they can approve false proofs.
Smart‑contract bugs: Faulty lock or mint contracts can be exploited to mint excess tokens.
Replay attacks: Re‑using old proofs to double‑spend.

Mitigation strategies that leading projects adopt:

Slashing conditions: Misbehaving validators lose their staked tokens (e.g., Polygon’s MATIC slashing).
Multi‑signature withdrawal: Requires signatures from multiple independent entities before releasing assets.
Circuit breakers: Automatic pause of deposits/withdrawals when abnormal activity is detected.
Audited contracts: Independent security firms review lock and mint contracts before launch.

Even with these safeguards, sidechains remain less secure than the mainchain’s native security set. For high‑value settlements, many enterprises still prefer rollups or direct mainchain usage.

Performance vs. Security Trade‑offs

Sidechains shine in use cases where speed and cost matter more than absolute security. Gaming platforms, NFT marketplaces, and micro‑payment services regularly gravitate toward sidechains because they can process thousands of TPS at pennies per transaction.

For example, ImmutableX (an Ethereum‑linked sidechain) handles 9,000NFT trades per second, enabling the “Gods Unchained” game to run without gas fees. By contrast, moving a $100,000 settlement through a sidechain incurs a non‑trivial risk: if the bridge is compromised, the entire locked amount could disappear.

The rule of thumb many developers follow is the “value‑vs‑speed matrix”: if the asset value exceeds roughly 10× the average bridge fee, stick to the mainchain or a highly audited rollup; otherwise, sidechains deliver a compelling ROI.

Sketch of modular bridge chassis with interchangeable Supernet, CCIP, and Blob panels.

Implementation Checklist for Developers

Integrating a sidechain bridge can be a multi‑week effort. Below is a practical checklist that reflects real‑world experience from enterprises and open‑source projects.

Define asset flow: Identify which tokens will be locked, their decimal precision, and any required wrapping logic.
Choose bridge model: Decide between federated, PoS, or trustless based on required security and latency.
Set up validator nodes: Deploy 15‑20 nodes for a minimal PoS bridge; configure staking contracts and slashing parameters.
Implement lock contract: Write Solidity (or relevant language) with thorough unit tests; include a pause function for emergency stops.
Integrate proof generator: Use existing libraries (e.g., Polygon’s SDK) to build Merkle proof creation and verification.
Run end‑to‑end tests: Simulate deposits, withdrawals, and failure scenarios (stuck deposits, double spends).
Monitor with dashboards: Deploy a bridge status UI that tracks pending proofs, validator uptime, and gas costs.
Security audit: Engage a third‑party auditor; remediate all high‑severity findings before mainnet launch.
Compliance check: Verify that the bridge complies with jurisdictional regulations (e.g., SEC money‑transmitter rules or EU MiCA reserve requirements).
Launch & iterate: Start on testnet, gather user feedback, then migrate to mainnet while maintaining a fallback to the mainchain.

Typical costs range from $185k to $350k for a production‑grade bridge, with ongoing maintenance of 2‑3 full‑time developers. Enterprises that add circuit breakers and multi‑sig withdrawals see a 68% reduction in successful attack vectors, according to Gartner’s 2023 blockchain survey.

Future Directions: From Bridges to Shared Security Layers

The industry is moving toward modular designs that blend sidechain performance with mainchain security. Notable developments include:

Polygon Supernet: Allows per‑project sidechains with dedicated validators but shares a checkpoint contract with Ethereum, reducing trust gaps.
Chainlink CCIP: Uses decentralized oracles to convey proof data, aiming for a trust‑minimized bridge model.
Proto‑Danksharding (EIP‑4844): Introduces blob transactions that lower L1 costs for bridge data, potentially cutting withdrawal fees by up to 90%.

Analysts predict that by 2027 sidechains (or their next‑gen successors) will handle the majority of non‑financial blockchain traffic, while high‑value transfers remain on fully secured L1 or rollup solutions.

Frequently Asked Questions

What exactly is a two‑way peg?

A two‑way peg is a protocol that locks assets on one chain and creates a 1‑to‑1 representation on another. It works both ways-deposit locks the original token, withdrawal burns the wrapped token and releases the original.

How long does a typical deposit take?

On Polygon’s PoS bridge, users see the minted token within 2‑5minutes after the lock transaction gains 100‑200 confirmations on Ethereum (about 25‑50minutes). The UI reports the deposit as soon as the proof is verified, which is why the perceived time feels shorter.

Why are sidechains considered less secure than the mainchain?

Security depends on the number and economic weight of validators and on the bridge code. Mainchains like Ethereum have hundreds of thousands of validators and a battle‑tested consensus. Sidechains rely on a handful of validators (often <1% of the mainnet size) and an extra software layer-the bridge-that can contain bugs or be attacked.

Can I use a sidechain for high‑value transactions?

It’s possible, but you should evaluate the bridge’s audit history, slashing mechanisms, and validator decentralisation. Many institutions keep high‑value funds on the mainchain and only move a fraction to a sidechain for short‑term operations.

What are the main types of bridge architectures?

Federated bridges rely on a known set of validators, PoS bridges use staked tokens as economic security, and trustless bridges depend on cryptographic proofs (e.g., SNARKs) that remove the need for trusted parties.

Sidechains have become an essential tool for scaling blockchains, offering cheap, fast transactions while still letting users return to the mainchain when they need the highest security. By understanding the two‑way peg, bridge architecture, and the security‑performance trade‑offs, developers and enterprises can choose the right solution for their use case and avoid the pitfalls that have plagued early bridge implementations.