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A passphrase is a memorized secret consisting of a sequence of words or other text that a claimant uses to authenticate their identity. A passphrase is similar to a password in usage, but is generally longer for added security.
NIST SP 800-63-3
A password used to protect an identity key. After entered by a user or administrator, a passphrase is mathematically converted into large number which serves as a key that is used to encrypt the identity key. In order to decrypt the identity, the passphrase must be entered again so that the same key can be regenerated for decryption.