An illegal number is a number that under some interpretations and under some legislation represents information which is illegal to possess, utter or propagate. Any information that can be represented in binary format is, ipso facto, representable as a number, and therefore if the information itself is illegal in some way, the pure number itself may be called illegal. To date, the idea of a number being illegal has not been tested in the courts.
An illegal number may represent some type of classified information or trade secret, legal to possess only by certain authorized persons. An AACS encryption key that came to prominence in May 2007 is an example of a number claimed to be a secret, and whose publication or inappropriate possession is claimed to be illegal. It allegedly assists in the decryption of any HD DVD or Blu-Ray Disc released before this date. The issuers of a series of cease-and-desist letters claim that the key itself is therefore a copyright circumvention device, and that publishing the key violates Title 1 of the Digital Millennium Copyright Act.