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Published Tuesday, October 11, Cryptography could get a boost from photonic technology. The encryptions on which our finances, work and national security rely are based on one simple assumption: The public-key infrastructure developed in the s and now practically ubiquitous thanks to its incorporation into protocols, such as those used to protect web-surfing sessions, relies on the intrinsic computational difficulty of a select group of mathematical functions.
The difficulty of these operations ensures the private keys that should be used to unlock the data cannot be reverse-engineered from the encrypted text or the widely disseminated public keys that make this security strategy practical. If they work as expected, quantum computers may render many of the arithmetic techniques redundant. Is there an alternative that is less vulnerable to improvements in algorithms and changes in computer technology? One possibility is to harness the media used to convey data around the world.
Some ,km of fibre-optic cable lie under the oceans, transmitting vast quantities of information at high speed across the globe. Optical security is already a part of everyday life, from the holographic patches on CDs, DVDs and bank notes that allow us to ensure our music, movies and money are authentic, to biometric border control speeding up entry into countries. But what if the properties of propagating light could be used to provide better security for digital systems? Free-space optical security and encryption is an often-missed area of active and intense research that potentially offers just this: Using the properties of light freely propagating in air, space or a vacuum, free-space optics FSO is actually a diverse field with numerous sub-branches.
FSO and optical security are now coming together, as Sheridan describes: With optical encryption for physical storage, the information is encoded into holograms. The authenticity of the message can be validated using optical tests such as ellipsometry or speckle analysis. Security for FSO manipulates certain physical parameters of the optical waves that convey the information. These waves offer a host of hiding places for sensitive data: Called the double random phase encoding DRPE method, it converts an input laser signal, which is encoded with data using changes in phase and amplitude, into what appears to be white noise.
The technique uses phase masks - two random pattern gratings - that alter the light in both the frequency and spatial domains. The researchers calculated the probability to be one in 22,, The encrypted data bits are recorded and stored as a hologram, which can then be recorded by a CCD charge-coupled device camera and reconstructed digitally by a computer, ready to be transmitted.
When the receiver clicks on the attachment in their inbox, the image they see will be white noise. Central to decrypting the image are the phase mask keys - a full description of the two pattern gratings that encrypted the image. If the receiver has physical copies of the phase masks, they can reconstruct the image optically by simply reversing the encryption process. If the receiver instead just has the keys, they can rebuild the image digitally. The underlying approach was to produce a set of ciphertexts that, when analysed in combination, could reverse-engineer the random phase key.
The successful attack on DRPE inspired Javidi to further analyse the method, attacking the encryption himself in nine different ways. Although he proved that DRPE remains robust against brute-force attacks - that is, trying every possible key until finding the correct one - the study revealed weaknesses against chosen- and known-plaintext attacks.
In a chosen-plaintext attack, the attacker has the ability to trick a legitimate user of the system into encrypting particular images of their choosing and can also see the resulting encrypted images. Javidi found that by choosing, at most, three image pairs, an attacker can recover the two encryption keys and break the system. Worryingly, in his cryptanalysis Javidi found that attacks of this kind would only require two image pairs to break the system. The reason DRPE and many related techniques failed against these attacks boils down to one fatal flaw: Sheridan illuminates this best: There is only one possible solution that satisfies all the equations and so, although it might be numerically difficult or tedious [to solve], in general it is possible to find the one good solution, or at least get close to it.
Far from discouraging the community though, this crucial weakness has stimulated ingenious directions in research that can deal with cunning attack strategies. The key problem is one caused by the greater predictability of linear systems.
He and others are now looking to exploit non-linearity in optical processes. Carnicer and Javidi have joined forces to lead the way in advancing one research direction that could overcome the effects of linearity: Importantly, this is a nonlinear transformation of the data. However, since photon counting is performed on the amplitude of the encrypted message, information is lost, so that when decrypting the message the receiver obtains a noisy, unrecognisable image.
Importantly, though, only the amplitude information is modified. The photon-limited encrypted image can be verified from phase information using nonlinear filters. This technique could have important uses in object identification, particularly hardware security.
For example, counterfeit integrated circuits ICs are a growing problem. In , two nuclear operators in the US reported they had unwittingly purchased counterfeit parts for replacements to their control systems. Reported counterfeit parts incidents quadrupled between and Half of all manufacturers have, at some point, encountered bogus components. Adding an optical phase tag to a genuine IC means its authenticity can be confirmed by the eventual buyer simply by illuminating the tag with a laser and capturing the resulting speckle signature with a CCD.
One way of introducing nonlinearity in optical security that Situ is exploring is to change the way light propagates through the encryption device. With a photorefractive crystal placed into a standard DRPE system, laser light can propagate nonlinearly. Other, more speculative possibilities are also being explored.
For instance, the nanoworld offers a raft of potential new ways to hide information. One example is in optical artefact metrics. Artefact metrics use the intrinsic, complicated and hopefully unique characteristics of a physical object for authentication.
A sheet of paper, for example, scanned at the micro-level will have a unique pattern of random, naturally occurring texture imperfections that can be used to watermark or fingerprint a document. Although this pattern is unique, it is only due to current technological limitations that it cannot be copied by a skilled forger. Such techniques may be the ultimate in anti-counterfeiting.
Unlike lone-wolf hackers infiltrating government and corporate digital systems from their bedrooms, optical cryptanalysts work in collaboration and simulate attacks on security systems in the lab. Then we send something into the system and get something out. Using the input and output data, cryptanalysts attempt to establish a connection, and develop suitable computational algorithms to deduce the keys.
Professor John Sheridan from University College Dublin, who has also mounted successful attacks on advanced DRPE-based systems, notes a dearth of people willing to attack optical security systems: This might be the case for several reasons: So while increasingly complex systems are being designed, there is still no way of telling how vulnerable they are to attacks - always leaving a sliver of doubt in the mind of even the most confident cryptographer.
Brute-force attack - as the name suggests, a brute-force attack involves trying every possible combination of data in order to find the key that decrypts an encrypted message. It is usually a last resort and usually not regarded as practical because modern encryptions have huge key spaces that would take hundreds or thousands of years to crack with this method. This is a weak attack because the attacker has little to work with.
Many classic ciphers are susceptible to this type of attack, as were older versions of encrypted ZIP files. Eve registers and starts encrypting chosen files and looks at the resulting ciphertext. The Allies mounted such an attack to decipher messages from the Enigma machine during World War Two, but could only do so once they had captured one.
Chosen-ciphertext attack - including the lunchtime attack and the adaptive chosen-ciphertext attack. This kind of attack is impractical in many situations, but is also the strongest of the above methods. Side-channel attack - unlike other methods, which find weaknesses in the cryptographic algorithms or use brute force, side-channel attacks exploit weaknesses in the physical implementation of the security. Sound, electromagnetic leaks, power use and many more can be exploited to break the system.
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