An encryption key ID is a unique identifier assigned to a specific encryption key. In secure messaging apps and cryptographic systems, each device or user may have its own encryption key, and the key ID helps manage, reference, and select the correct key for encrypting or decrypting messages. This prevents confusion between multiple keys and ensures secure communication. Key IDs are especially important in systems where users have multiple devices or where keys are frequently rotated or updated. By using key IDs, applications can efficiently handle key management, improve security, and streamline the process of verifying which key was used for a particular message or session.