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See Key-derivation key.
NIST SP 800-57 Part 1 Rev. 5
A key used with a key-derivation method to derive additional keys. Sometimes called a master key.
NIST SP 800-57 Part 1 Rev. 5 under Key-derivation key
A key used as an input to a key-derivation method to derive other keys; see SP 800-108.
NIST SP 800-133 Rev. 2 under Key-derivation key
As used in this Recommendation, a key that is used during the key-expansion step of a key-derivation procedure to derive the secret output keying material. This key-derivation key is obtained from a shared secret during the randomness-extraction step.
NIST SP 800-56C Rev. 2 under Key-derivation key