An identity key is a long-term cryptographic key that represents a user or device in a secure messaging system. It helps other participants verify that a chat contact is still the same trusted party they previously communicated with, instead of a new device or an impersonator.
This matters because encryption alone does not prove identity. A message can be protected in transit and still be delivered to the wrong person if an attacker swaps keys or tricks a user into trusting a fake contact. Messaging apps often compare identity keys through safety numbers, fingerprints, or automatic verification checks. If the key changes unexpectedly, the app can warn the user that the chat may have been intercepted or re-established on a different device. In practice, identity keys are a core defense against man-in-the-middle attacks and account takeover attempts in encrypted chat.