The AuthKey is known as such:
When the registrar receives M1 he generates his public key and rnonce and can compute the
DHKey = SHA-256(gAB mod p, 192)
and subsequently the
KDK = HMAC-SHA-256(DHKey, enonce || EnrolleeMAC|| rnonce)
Finally the AuthKey is derived as part of
AuthKey|| KeyWrapKey|| EMSK = kdf(KDK, “Wi-FiEasy andSecure Key Derivation”, [...