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A pass-phrase, personal identification number (PIN), biometric data, or other mechanisms of equivalent authentication robustness used to protect access to any use of a private key, except for private keys associated with System or Device certificates.
CNSSI 4009-2015 from CNSSI 1300
Private data, other than keys, that are required to access cryptographic modules (i.e., unlock private keys for signing or decryption events).
NIST SP 800-32 under Activation Data