12.2 TLS secrets
We saw in Chapter 3, A Secret to Share, that a good cryptographic system has multiple keys so that every key is used for a single purpose only. TLS is no exception, and in this chapter, we are going to discuss in detail what cryptographic keys client Bob and server Alice need to establish a secure TLS channel.
However, before discussing the cryptographic keys, we first need to understand what TLS secrets are and how they are derived. TLS uses a three-step approach for generation of cryptographic keys, in which the keys are generated from the secrets:
Alice and Bob first establish a shared master secret.
They derive context-specific secrets from the master secret.
Finally, they derive context-specific keys from these derived secrets.
Note that there is no conceptual (or cryptographic) reason to differentiate between secrets and keys. But because the TLS 1.3 specification uses this terminology and we want to provide a trustworthy guide to this specification...
