When a message is sent between chains, it goes through a verification process to ensure its authenticity. This page explains how that process works.

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Overview

The verification process involves three key components:
  1. The Mailbox contract - The main entry point for message processing
  2. The ISM (Interchain Security Module) - The security verification contract
  3. The Recipient contract - The application that receives the message

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Sequence Diagram

  • If the recipient does not implement ISpecifiesInterchainSecurityModule or recipient.interchainSecurityModule() returns address(0), the default ISM configured on the Mailbox will be used to verify the message.
  • This is omitted from the sequence diagram for clarity.

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How It Works

  1. Type Identification
    • The Relayer calls moduleType() on the ISM to determine its category
    • Based on this type (e.g., MULTISIG, ROUTING, AGGREGATION), the Relayer knows what verification proof to collect
    • For Multisig ISMs, this means gathering cryptographic signatures from validators
  2. Message Submission
    • The Relayer calls Mailbox.process() providing both:
      • The original message
      • The collected verification metadata
  3. Verification Process
    • Mailbox queries the recipient contract via interchainSecurityModule()
    • If no ISM is specified, Mailbox uses the default ISM
    • Mailbox then forwards the message and metadata to the ISM’s verify() function
    • The ISM executes its security logic (e.g., signature verification, threshold checking)
  4. Message Delivery
    • Upon successful verification (when verify() returns true), Mailbox calls recipient.handle()
    • If verification fails, the transaction reverts and the message is rejected