Ensure Exit Message are compliant with EIP-7044

[boulder225]

🎯 Problem to be solved

EIP-7044 mandates exit messages to be signed on Capella's domain. In Charon, exit messages aren't signed, but their validity is verified, typically using the last available domain. It's crucial to ensure the domain from Capella is used for this purpose.

🛠️ Proposed solution

• Implement a mechanism to identify and use Capella's domain for signing exit messages in compliance with EIP-7044.
• Update the validity verification process in Charon to prioritize Capella's domain when checking exit messages.

[boulder225]

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