iMessage Contact Key Verification is a feature that adds an extra layer of security to iMessage conversations. With end-to-end encryption, only you and the person you’re messaging can read the messages. Contact Key Verification goes a step further by ensuring that you’re messaging the intended person automatically. If there’s an error in the verification process, alerts will notify you. You can also manually verify contacts by comparing verification codes. It’s important to have the necessary software versions and settings enabled to use Contact Key Verification. You can turn it on in the settings and verify contacts either through on-device comparison or by sharing your Public Verification Code. That code can be used by others to verify your identity. If the codes match, you can mark the contact as verified. The Conversation Details page provides an overview of the verification status for your contacts. If there are any issues with Contact Key Verification, you’ll receive an alert.
https://support.apple.com/en-gb/HT213465