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A person authorized by an FCKMS service provider or FCKMS service-using organization to use a specific key that is managed by the FCKMS.
NIST SP 800-152
One or more entities that are authorized to use a symmetric key or the private key of a key pair.
NIST SP 800-175B Rev. 1 under Owner of a key or key pair
An entity authorized to use a cryptographic key or key pair and whose identifier is associated with a cryptographic key or key pair.
NIST SP 800-57 Part 1 Rev. 5