12.3 KDFs in TLS
TLS uses four different functions to derive secrets: HKDF-Extract, HKDF-Expand, HKDF-Expand-Label, and Derive-Secret. All these functions are based on the Hashed Message Authentication Code (HMAC)-based Extract-and-Expand Key Derivation Function (HKDF) defined in RFC 5869 [104].
We will have much more to say on hash functions, message authentication codes, and key derivation function in Chapter 11, Hash Functions and Message Authentication Codes. For now, it is sufficient to treat HKDF as an abstract function, as shown in Figure 12.2. It takes keying material as input and returns one or more secret keys as output:
Figure 12.2: High-level view of the HKDF function
HKDF follows an extract-then-expand approach consisting of two logical stages. The rationale for this two-step approach is explained nicely in the introduction of [104]: ”In many applications, the input keying material is not necessarily distributed uniformly, and the attacker...
