Sidechain

A sidechain is a separate blockchain with its own validators and consensus rules that connects to a parent chain through a bridge, allowing assets to move between the two. Unlike layer 2 rollups that derive security from the parent chain, sidechains are responsible for their own security — an important distinction that affects trust assumptions.

The most prominent sidechain examples include Polygon PoS (an Ethereum sidechain with its own validator set of ~100 nodes), Liquid Network (a Bitcoin sidechain for faster, confidential transactions), and Ronin (the Axie Infinity sidechain). Polygon PoS processes transactions for a fraction of Ethereum's gas costs and confirms blocks in ~2 seconds, but its security depends on its own validators, not Ethereum's.

The key tradeoff between sidechains and rollups is security inheritance. Rollups (Optimistic and ZK) post transaction data to the parent chain and can be verified against it — users can exit to L1 even if the rollup operators misbehave. Sidechains lack this guarantee: if sidechain validators collude or are compromised, user funds on the sidechain are at risk. The March 2022 Ronin bridge hack demonstrated this risk when attackers compromised 5 of 9 validator keys and stole $625 million — possible because the sidechain's security was independent of Ethereum.

As of 2025, the industry trend is away from sidechains toward rollups for high-security applications. Polygon itself pivoted from a pure sidechain to a zkEVM rollup architecture (Polygon zkEVM) to inherit Ethereum's security. However, sidechains remain useful for applications where speed and cost matter more than maximum security guarantees.