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Bridging Between Ethereum and Aztec

Aztec is a Layer 2 rollup on Ethereum. This means assets like tokens need to be "bridged" between the two networks. This page explains how bridging works at a high level.

How Bridging Works​

Moving assets between Ethereum (L1) and Aztec (L2) involves a few key concepts:

Portal Contracts​

A portal is a smart contract on Ethereum that represents the L1 side of a bridge. Each bridged asset has a portal contract that:

  • Holds locked assets on L1
  • Sends messages to L2 when deposits are made
  • Releases assets when withdrawals are processed

Message Passing​

Unlike direct function calls, L1 and L2 communicate through messages. This asynchronous design is necessary because:

  • L2 transactions are private and can't be triggered directly from L1
  • Privacy requires that L2 "pulls" messages rather than having L1 "push" them
  • The rollup batches operations for efficiency

The Bridging Flow​

Depositing to Aztec (L1 → L2):

  1. You send tokens to the portal contract on Ethereum
  2. The portal creates a message for Aztec
  3. The message is included in the next rollup
  4. You (or anyone) can claim the tokens on Aztec

Withdrawing from Aztec (L2 → L1):

  1. You initiate a withdrawal on Aztec
  2. A message is created for the portal
  3. After the epoch is proven and verified on L1
  4. You can claim your tokens from the portal contract

Why "Pull" Instead of "Push"?​

Other rollups often have L1 contracts directly call L2 functions (push model). Aztec uses a pull model where L2 retrieves messages from L1. Here's why:

Privacy: If L1 pushed data to L2, all the call data would be visible on Ethereum. With the pull model:

  • The deposit amount and original sender are visible on L1
  • The recipient on L2 can remain private
  • L2 reveals only what's necessary

Timing Considerations​

Deposits (L1 → L2)​

  • The L1 transaction sends tokens to the portal and creates a message
  • The message becomes available on L2 once the L1 block is processed by the rollup
  • The recipient can then claim the tokens on L2

Withdrawals (L2 → L1)​

Withdrawals take longer:

  • Must wait for the epoch to end
  • Must wait for the proof to be generated
  • Must wait for proof verification on Ethereum
  • Only then can tokens be claimed on L1

This delay is inherent to ZK rollups since the proof must be generated and verified before L1 state can be updated.

What Can Be Bridged​

Aztec uses a portal contract pattern where each L2 token contract is paired with a portal contract on L1. Any token can be bridged if a corresponding portal contract is deployed. There is no fixed list of "supported" tokens - developers can create portals for any L1 asset.

The fee token used on Aztec has its own portal for bridging in and out of the network.

Security Considerations​

Bridging involves trust assumptions:

  • Smart contract security - Portal contracts must be correct and secure
  • Rollup validity - The Aztec proof system must be sound
  • Liveness - The network must continue operating for withdrawals

Aztec's ZK proofs provide strong guarantees - if a proof verifies on L1, the state transition was valid. There is no challenge period; finality on L1 is immediate once the proof is verified.

Using Bridges​

Bridging typically involves interacting with the portal contract on L1 (for deposits) or initiating a withdrawal on L2 (for withdrawals). The exact interface depends on the specific bridge implementation and any frontend tooling built around it.

For developers

Learn how to implement bridging in your applications in the Portal documentation.