Deciphr Academy
Track 1 · Foundations · Module 6

Layer 2s and bridges

Cheaper networks on top of Ethereum, and the bridges that link chains.

Lessons
6
Reading
36 min
Quiz
15 questions
Pass mark
80%
  1. 1Why Ethereum is slow and costlyWhy is Ethereum slow and expensive to use?5 min
  2. 2Rollups: optimistic and zero-knowledgeWhat is a rollup, and how do optimistic and ZK rollups differ?7 min
  3. 3Sidechains vs rollupsWhat is the difference between a sidechain and a rollup?5 min
  4. 4How bridges workHow does a crypto bridge move tokens between chains?6 min
  5. 5Bridge hacks and their lessonsWhat can we learn from the biggest crypto bridge hacks?7 min
  6. 6Picking a network safelyHow can you check whether a layer 2 or a bridge is safe enough to use?6 min

The case used in this module

Each module tells its own made-up Brindlecove story, so details can differ from one module to the next.

Key words

Layer 2
A separate blockchain that extends Ethereum and settles its transactions on Ethereum.1
Rollup
A layer 2 that runs transactions off Ethereum, then posts their data back to Ethereum in batches.21
Optimistic rollup
A rollup that assumes batches are valid unless someone proves fraud during a challenge period.3
Fraud proof
Evidence, checked on Ethereum, that a rollup batch was worked out wrongly.3
Challenge period
The time, roughly seven days, when anyone can challenge an optimistic rollup batch.3
ZK rollup
A zero-knowledge rollup: it posts a cryptographic proof that each batch is correct.4
Validity proof
A cryptographic proof that a batch's changes really come from running its transactions.4
Sequencer
An operator that orders a rollup's transactions and posts its batches. Some rollups have a single one.34
Sidechain
A separate blockchain with its own rules and validators, linked to Ethereum by a two-way bridge.5
Bridge
A link between blockchains that lets tokens and messages pass from one to another.6
Wrapped token
A token on one chain that stands for an asset held on another, such as WBTC for bitcoin.67
Blob
A cheaper, short-lived type of data that rollups post to Ethereum, added by EIP-4844.84
Sources
  1. What is layer 2?. ethereum.org, updated 23 July 2026.
  2. Scaling. ethereum.org developer docs, updated 13 April 2026.
  3. Optimistic rollups. ethereum.org developer docs, updated 23 June 2026.
  4. Zero-knowledge rollups. ethereum.org developer docs, checked 22 October 2026.
  5. Sidechains. ethereum.org developer docs, updated 23 February 2026.
  6. Introduction to blockchain bridges. ethereum.org, updated 24 July 2026.
  7. Another crypto bridge attack: Nomad loses $190 million in 'chaotic' hack. CNN Business, 3 August 2022.
  8. Timeline of all Ethereum forks (Cancun-Deneb, "Dencun"). ethereum.org, checked 22 October 2026.