Why it matters

HKDF is standard KDF. Understanding shapes crypto.

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The architecture

Extract: PRK = HMAC(salt, IKM).

Expand: OKM = HMAC(PRK, info + counter).

HKDF flowExtractPRK from IKM + saltExpandOKM per info + counterMultiple keysfrom one IKMKrawczyk 2010 RFC 5869; extract-then-expand paradigm
HKDF.
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How it works end to end

Krawczyk 2010 RFC 5869.

SHA-256 based common.

info parameter for context.

TLS 1.3 uses.