Importance

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Abstract

An IFF system produces a challenge in which some or all of the anti-interference pulses that are inserted between adjacent bits of an encrypted message are replaced with 'information pulses' that convey information which a decryption sub-system uses to decipher the encrypted message. To produce an information pulse that replaces a given anti-interference pulse, the system combines an information bit with the anti-interference pulse. The decryption sub-system recovers the information bit from the information pulse by comparing the state of that pulse with the state of the anti-interference pulse that it replaced. If the states of the two pulses are the same, the information bit is a zero and if the states differ, the information bit is a one. The information included in the information pulses may be, for example, an address that the decryption sub-system uses to select a cryptographic key from one or more stored tables of keys. Alternatively, or in addition, the information pulses may include the time associated with the encryption of the message, which the decryption sub-system uses to update a sub-system clock.

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First Claim