Apple Strengthens iMessage Security with Contact Key Verification

Example of an alert from checking an iMessage contact key

Apple enhances iMessage security with 'Contact Key' Verification for privacy users and identity verification against evolving threats using cryptographic techniques and key transparency.

Pin Key Verification is a security feature that helps detect sophisticated attacks on iMessage servers and allows users to verify the identity of their communication partners. This feature specifically addresses vulnerabilities related to key directory services.

Such services typically map user IDs to public keys, which can become a single point of failure if they are compromised. Contact key verification aims to solve this problem by providing a more secure way to verify the identity of communication partners.

To solve this problem, Apple introduced a mechanism known as Key Transparency (KT). Similar to certificate transparency, KT uses a verifiable, journaled card data structure that allows for cryptographic proofs and consistency checks over time, ensuring scalability and user privacy. Apple's implementation of Key Transparency goes beyond the existing key directory system.

It represents an account-level ECDSA signing key that is generated and stored on the user's device in iCloud Keychain. Devices use this synchronized account key to sign iMessage public keys, and this data is stored in the Identity Directory Service (IDS) database and synchronized with the Key Transparency service.

When users enable pin key verification, their devices automatically and cryptographically verify the data presented by the IDS against the KT card. When discrepancies are found, users are notified. In addition, users' own devices periodically check the data to ensure consistency across all their devices.

Apple has also included a feature to manually verify contacts using shortcodes, providing an additional layer of security for users who require it. This verification process applies to all users' devices, ensuring consistency when signing in on new devices.

In addition, Apple has introduced public verification codes for users with public identities, ensuring that their contact key is accurately verified.

Apple's implementation includes on-device verification of critical append-only logs for user control and security. The Messages app verifies the integrity of logs, eliminating the potential threat of compromise of the KT service.

Apple will launch Key Transparency with internal audits of all production trees and more details on public audits. iMessage Contact Key Verification is available in developer previews of iOS 17.2, macOS 14.2, and watchOS 10.2, promising improved security for Apple's messaging platform.

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