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Information-theoretic security

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Information-theoretic security is a cryptosystem whose security derives purely from information theory. In other words, it cannot be broken even if the adversary had unlimited computing power. The adversary simply does not have enough information to break the encryption and so the cryptosystems are considered cryptanalytically-unbreakable.


Perfect security is an interesting special case. It means that for an encryption algorithm, if there is ciphertext produced that uses it, no information about the plaintext is provided without knowledge of the key. If E is a perfectly secure encryption function, for any fixed message m, there must be, for each ciphertext c, at least one key k such that . It has been proved that any cipher with the perfect secrecy property must use keys with effectively the same requirements as one-time pad keys.[1]



  1. Secret sharing schemes such as Shamir's are information-theoretically secure (and also perfectly secure) in that having less than the requisite number of shares of the secret provides no information about the secret.
  2. More generally, secure multiparty computation protocols often have information-theoretic security.
  4. Reductions between cryptographic primitives or tasks can often be achieved information-theoretically. Such reductions are important from a theoretical perspective because they establish that primitive can be realized if primitive can be realized.


Physical layer encryptionEdit

A weaker notion of security, defined by Aaron D. Wyner, established a now-flourishing area of research that is known as physical layer encryption.[2] It exploits the physical wireless channel for its security by communications, signal processing, and coding techniques. The security is provable, unbreakable, and quantifiable (in bits/second/hertz).

Wyner's initial physical layer encryption work in the 1970s posed the Alice–Bob–Eve problem in which Alice wants to send a message to Bob without Eve decoding it. If the channel from Alice to Bob is statistically better than the channel from Alice to Eve, it had been shown that secure communication is possible.[3] That is intuitive, but Wyner measured the secrecy in information theoretic terms defining secrecy capacity, which essentially is the rate at which Alice can transmit secret information to Bob. Shortly afterward, Imre Csiszár and Körner showed that secret communication was possible even if Eve had a statistically better channel to Alice than Bob did.[4] More recent theoretical results are concerned with determining the secrecy capacity and optimal power allocation in broadcast fading channels.[5][6] There are caveats, as many capacities are not computable unless the assumption is made that Alice knows the channel to Eve. If that were known, Alice could simply place a null in Eve's direction. Secrecy capacity for MIMO and multiple colluding eavesdroppers is more recent and ongoing work,[7][8] and such results still make the non-useful assumption about eavesdropper channel state information knowledge.

Still other work is less theoretical by attempting to compare implementable schemes. One physical layer encryption scheme is to broadcast artificial noise in all directions except that of Bob's channel, which basically jams Eve. One paper by Negi and Goel details its implementation, and Khisti and Wornell computed the secrecy capacity when only statistics about Eve's channel are known.[9][10]

Parallel to that work in the information theory community is work in the antenna community, which has been termed near-field direct antenna modulation or directional modulation.[11] It has been shown that by using a parasitic array, the transmitted modulation in different directions could be controlled independently.[12] Secrecy could be realized by making the modulations in undesired directions difficult to decode. Directional modulation data transmission was experimentally demonstrated using a phased array.[13] Others have demonstrated directional modulation with switched arrays and phase-conjugating lenses.[14][15][16]


Unconditional securityEdit

Information-theoretic security is often used interchangeably with unconditional security. However, the latter term can also refer to systems that do not rely on unproven computational hardness assumptions. Today, such systems are essentially the same as those that are information-theoretically secure. Nevertheless, it does not always have to be that way. One day, RSA might be proved secure, as it relies on the assertion that factoring large numbers is hard, thus becoming unconditionally secure, but it will never be information-theoretically secure because even if no efficient algorithms for factoring large primes exist, it could still be done in principle with unlimited computational power.

See alsoEdit


  1. ^ a b Shannon, Claude E. (October 1949). "Communication Theory of Secrecy Systems" (PDF). Bell System Technical Journal. USA: AT&T Corporation. 28 (4): 656–715. doi:10.1002/j.1538-7305.1949.tb00928.x. Retrieved 2011-12-21. 
  3. ^ Wyner, A. D. (October 1975). "The Wire-Tap Channel" (PDF). Bell System Technical Journal. AT&T Corporation. 54 (8): 1355–1387. doi:10.1002/j.1538-7305.1975.tb02040.x. Retrieved 2013-04-11. 
  4. ^ Csiszár, I.; Körner, J. (May 1978). "Broadcast Channels with Confidential Messages". IEEE Transactions on Information Theory. IEEE. IT-24 (3): 339–348. doi:10.1109/TIT.1978.1055892. 
  5. ^ Liang, Yingbin; Poor, Vincent; Shamai (Shitz), Shlomo (June 2008). "Secure Communication Over Fading Channels". IEEE Transactions on Information Theory. IEEE. 54 (6): 2470–2492. arXiv:cs/0701024 . doi:10.1109/tit.2008.921678. 
  6. ^ Gopala, P.; Lai, L.; El Gamal, H. (October 2008). "On the Secrecy Capacity of Fading Channels". IEEE Transactions on Information Theory. IEEE. 54 (10): 4687–4698. arXiv:cs/0610103 . doi:10.1109/tit.2008.928990. 
  7. ^ Khisti, Ashish; Wornell, Gregory (November 2010). "Secure Transmission with Multiple Antennas II: The MIMOME Wiretap Channel". IEEE Transactions on Information Theory. IEEE. 56 (11): 5515–5532. doi:10.1109/tit.2010.2068852. 
  8. ^ Oggier, F.; Hassibi, B. (August 2011). "The Secrecy Capacity of the MIMO Wiretap Channel". IEEE Transactions on Information Theory. IEEE. 57 (8): 4961–4972. arXiv:0710.1920 . doi:10.1109/tit.2011.2158487. 
  9. ^ Negi, R.; Goel, S. (2008). "Guaranteeing secrecy using artificial noise". IEEE Transactions on Wireless Communications. IEEE. 7 (6): 2180–2189. doi:10.1109/twc.2008.060848. 
  10. ^ Khisti, Ashish; Wornell, Gregory (Jul 2010). "Secure transmission with multiple antennas I: The MISOME wiretap channel". IEEE Transactions on Information Theory. IEEE. 56 (7): 3088–3104. doi:10.1109/tit.2010.2048445. 
  11. ^ Daly, M.P.; Bernhard, J.T. (Sep 2009). "Directional modulation technique for phased arrays". IEEE Transactions on Antennas and Propagation. IEEE. 57: 2633–2640. Bibcode:2009ITAP...57.2633D. doi:10.1109/tap.2009.2027047. 
  12. ^ Babakhani, A.; Rutledge, D.B.; Hajimiri, A. (Dec 2008). "Transmitter architectures based on near-field direct antenna modulation". IEEE Journal Solid-State Circuits. IEEE. 76 (12): 2674–2692. 
  13. ^ Daly, M.P.; Daly, E.L.; Bernhard, J.T. (May 2010). "Demonstration of directional modulation using a phased array". IEEE Transactions on Antennas and Propagation. IEEE. 58: 1545–1550. Bibcode:2010ITAP...58.1545D. doi:10.1109/tap.2010.2044357. 
  16. ^ Malyuskin, O.; Fusco, V. (2012). "Spatial data encryption using phase conjugating lenses". IEEE Transactions on Antennas and Propagation. IEEE. 60 (6): 2913–2920. Bibcode:2012ITAP...60.2913M. doi:10.1109/tap.2012.2194661. 