HKDF-SHA256

HKDF is a key derivation function built on HMAC. It has two phases:

1) Extract: PRK = HMAC(salt, IKM) 2) Expand: OKM = HMAC(PRK, T(0) || info || 0x01) || HMAC(PRK, T(1) || info || 0x02) ...

Where T(0) is empty and output is truncated to the requested length.

Function signatures

func HKDFExtract(salt, ikm []byte) [32]byte
func HKDFExpand(prk [32]byte, info []byte, length int) []byte

Requirements

• If salt is nil or empty, use 32 zero bytes as the salt.
• length can be any non-negative value; return an empty slice for 0.
• Use HMAC-SHA256 provided in the starter (do not import crypto/hmac).

Notes

• HKDF is used for protocol key derivation, not password hashing.