How deeply does Real-World Cryptography cover key derivation function?

Explains KDFs through HKDF-Extract, HKDF-Expand, salt, info, output length, and combined implementation, without implementation detail.

Key derivation function

As explained in Real-World Cryptography

A key derivation function transforms input keying material into one or more cryptographic keys or related pseudorandom values.

A key derivation function (KDF) turns one secret into one or more cryptographic keys or pseudorandom values. Unlike a PRNG, it can accept a biased secret with enough entropy, such as key-exchange output, and produce uniformly random secrets. KDFs are deterministic, helping participants rederive the same limited set of keys. section 8.6 section 8.9

HKDF, the most widely used KDF, separates derivation into two steps:

HKDF-Extract removes bias from an input secret and produces a uniformly random secret. An optional salt is used as the HMAC key, and different salts produce different outputs. section 8.6
HKDF-Expand uses a uniformly random secret to produce output of a requested length. An optional info value separates outputs: different info values with the same secret produce different results. section 8.6
Implementations commonly expose Extract and Expand together as one HKDF call. HKDF is built on HMAC and is defined in RFC 5869; SHA-2 is commonly used. section 8.6

How Real-World Cryptography covers it

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Related concepts

X3DHused by · X3DH is a non-interactive key-exchange protocol that combines multiple Diffie-Hellman exchanges to establish shared keying material for asynchronous messaging.
HKDFincludes · HKDF is a key derivation function that derives cryptographic keying material from input keying material using HMAC.
extendable output functionalternative to · An extendable-output function is a cryptographic function that maps an input to an output of arbitrary caller-specified length.
API keyuses · An API key is a credential used to identify or authorize a client accessing an application programming interface.

This explainer was generated by Manning's knowledge graph from the text of Real-World Cryptography. The section links point to the parts of the book it draws on.