ENCYCLOPEDIA OF Espionage, Intelligence, and Security Volume ...

Cipher Padstructure or pattern; a pseudorandom number sequenceis one that looks like truly random sequence but is in factproduced by a series of arithmetical calculations that canbe repeated at will. Pseudorandom number sequences areeasy to generate in digital computers using arithmeticalprocedures termed pseudorandom number generators(PNGs). The bits produced by a PNG can be strung togetherinto a stream that is as long as any desired message.This stream of bits is termed ”the cryptographic bitstream“ or ”key-stream.“ A message can then be encryptedby performing the EXCLUSIVE OR (XOR) operationpairwise on bits from the message-stream and thekey-stream. The XOR operation for two bits is defined asfollows:it. Because cipher systems of this type work on streams ofbits, they are termed stream ciphers.The discussion so far assumed that the receiver of theencrypted message has access to the same key-stream asthe sender. In a cipher-pad or one-time-tape system, agreementon the key sequence is assured by sending the key(on paper or some other medium) to both ends of the link.In a stream cipher, it is assured by generating the keystreamat both ends of the link. Because the pseudorandombits of the key-stream are generated by a PNG, both endsof the cipher link need only start their PNGs at the samepoint in its series of operations to generate the same keystream.This can be accomplished by transmission to thereceiver of a group of numbers termed a ”seed“ or ”initializingvector.“The following is a message-stream, a key-stream, andthe encrypted bitstream produced by XORing the message-streamand the key-stream together:Message-stream: 1 0 1 1 0 0 0 1Key-stream: 0 1 0 1 0 0 1 1Encrypted bitstream: 1 1 1 0 0 0 1 0It is easy to verify that each bit in the encryptedbitstream is the XOR of the two bits above it.The XOR function is used for encipherment because ithas the following useful property: the XOR of the encryptedbitstream and of the key-stream recovers themessage-stream.Encrypted bitstream: 1 1 1 0 0 0 1 0Key-stream: 0 1 0 1 0 0 1 1Recovered message: 1 0 1 1 0 0 0 1In the example above, it is easy to verify that each bitin the recovered message is the XOR of the two bits aboveQuantum cryptography. Weak points exist even in this system.For example, all PNGs start to repeat themselveseventually, and so do not produce truly random numbers.Also, the initializing vector must be known somehow atboth ends of the cipher link. The answer to these difficultiesmay be resolved using quantum cryptography. Inquantum cryptography, stream ciphering returns to theold idea of sending a key-stream along with the message.However, the key-stream is not sent on a paper tape oreven as a conventional digital message. It is generated bythe sender as a series of truly random subatomic eventsand shared by the sender and receiver using pairs of”entangled“ photons that cannot, by the most fundamentallaws of physics as they are now understood, be interceptedwithout revealing the presence of the eavesdropper.Real-world quantum-cryptographic systems are beingdeveloped rapidly, and proof-of-concept systems havealready been built. Thus, there seems to be no basicobstacle to the development of truly unbreakable quantum-cryptographicsystems, the ultimate development ofthe cipher-pad concept.❚ FURTHER READING:BOOKS:Meyer, Carl H., and Stephen M. Matyas. Cryptography: ANew Dimension in Computer Data Security. New York:John Wiley & Sons, 1982.Mollin, Richard A. An Introduction to Cryptography. NewYork: Chapman & Hall, 2001.PERIODICALS:Bennett, Charles H., and Peter W. Shor. ”Privacy in aQuantum World.“ Science no. 5415 (1999): 747–748.SEE ALSOCodes and CiphersQuantum Physics: Applications to Espionage, Intelligence,and Security Issues208 Encyclopedia of Espionage, Intelligence, and Security