What is a session key in cryptography?

A session key in cryptography is designed to be a temporary key that is utilized for encrypting messages during a specific session of communication between users. The primary purpose of using a session key is to ensure that the communication remains secure while minimizing the amount of time the key is in use, which mitigates the risk of it being compromised.

Session keys are typically generated for single-use or for use over a limited time period, so that even if a key were to be exposed during that session, it would only affect that particular exchange and not future communications. This aspect of using session keys enhances security by limiting the potential impact of key exposure.

In contrast, other key types mentioned do not accurately capture the essence of a session key. For example, a long-term key is utilized continuously over time, which increases vulnerabilities. A key that cannot be used more than once refers more to one-time pads, which have specific usage rules that differ from the typical use of session keys. A key stored on a user's device can be vulnerable if the device is compromised and does not correspond to the dynamic, ephemeral nature of session keys.