The interception, alteration, and retransmission of a cipher signal or data in such a way as to mislead the recipient. 2. (AIS) An attempt to gain access to an AIS by posing as an authorized user. 3. Unauthorized use of legitimate Identification and Authentication (I&A) data, however it was obtained, to mimic a subject different from the attacker. Impersonating, masquerading, piggybacking, and mimicking are forms of spoofing.