HKDF-SHA-256

Overview & Cryptographic Context

HKDF (HMAC-based Extract-and-Expand Key Derivation Function), defined in RFC 5869, is the foundational key schedule primitive in TLS 1.3, Noise, Signal, and HPKE (RFC 9180). It separates key derivation into two formal stages:

1. HKDF-Extract: PRK = HMAC-Hash(salt, IKM) concentrates potentially non-uniform entropy (such as a raw Diffie-Hellman shared secret) into a fixed-length pseudorandom key (PRK).
2. HKDF-Expand: Iteratively computes T(i) = HMAC-Hash(PRK, T(i-1) ‖ info ‖ byte(i)) to stretch PRK into L bytes of domain-separated Output Keying Material (OKM).

This workflow reproduces RFC 5869 Appendix A.1 (Test Case 1) with SHA-256 and output length L = 42 bytes, exercising two HMAC-SHA-256 expansion blocks (⌈ 42 / 32 ⌉ = 2) truncated to 42 bytes.

Pipeline Architecture & Wiring Flow

1. Keying Material & Context (Workflow Input): Supplies the 22-byte IKM, 13-byte non-secret Salt, and 10-byte application context Info.
2. Extract & Expand (HKDF Key Derivation): Configured with hashAlgorithm = "SHA-256" and keyLength = 42.
3. Derived Key Output (Workflow Output): Collects the 42-byte OKM byte stream.

Nodes & Configuration

Node reference →

Inputs, Outputs & Test Vector Verification

Verification Source: RFC 5869 §A.1 (Test Case 1, SHA-256, L=42)

Inputs

Expected Outputs