Chemical key unlocks cryptography
(appeared in May 2016)

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Coding at the molecular level may be the new invisible ink, says S.Ananthanarayanan.

Codes and concealment have been the means to exchange secret information since centuries. A code that was difficult to crack, however, was difficult to compose and also to read. Inks that were visible only after special treatment have lost utility after they were easily discovered. Modern commerce uses codes based on mathematics and huge calculations, but they are difficult to implement. What many present day applications need is a method that is both difficult to crack and easy to use.

Tanmay Sarkar, Karuthapandi Selvakumar, Leila Motiei and David Margulies from the Weizmann Institute of Science in Israel, in the journal, Nature Communications, describe a molecular level answer to this need. The device is a stable chemical base to which the coder adds a combination of common, everyday materials. The resulting mixture helps code the message with great security and knowing the chemicals used for coding allows easy decoding by one’s correspondent.

An elementary coding method is when the letters of the alphabet are replaced by other letters or by symbols. But these codes can be cracked by intelligent inspection. Even when the code is complicated by changing the coding system in a random way, computers and statistical analysis of the coded material can break it down. While the complex codes were difficult to use, invisible ink had the merit of being simple, and it was popular with international spies as lately as during the First World War. The current solution, for banking and e-commerce, which need messages to be conveyed with security, however, is again with the help of complex codes.

These codes generally depend on a device that generates random numbers, which help scramble the letters in a message. The same device used at the other end then unscrambles the code. To prevent others who may have the same machine from eavesdropping, the sender has a secret starting point, which only the intended receiver knows. There are variations of this method including methods based on very long prime numbers as numeric ‘keys’. If two long prime numbers are multiplied together, the product is one from which it is fiendishly difficult to extract the original factors. A number based on the product is then published as a public key, to encode a message meant for the person who has the factors and hence is the only one able to decode the message.

These methods, however, are cumbersome and can be used only for short exchanges or to encode the key to another, conventional encryption. The method described by the Israel group in Nature Communications, in contrast, is an easily usable method that randomly encodes each successive character, including spaces, in a message. And then, the method enables the intended receiver to reproduce the original encoding scheme with equal ease, for the original message to be read out.

Fluorescence

The Israel group’s method is based on fluorescence, or the colours in the emission from some chemicals when they are bathed in light of higher frequency. The domestic tube light is an instance of fluorescence. The electric discharge within the tube generates ultra violet light. When this strikes the coating inside the tube, the material gives off light of longer wavelengths, which add up to nearly white light.

An earlier work by some of the members of the same group had identified a chemical that could be doped with specific additives, to lead to specific differences in fluorescence. , l The three additives would then be the key and the chemical would act like a lock that needs a specific input to open. A three input keypad lock, like the one shown in the picture, can be set in 3x3x3=27 ways and protects against accidental opening. A larger keypad lock, which can code in 10,000 ways, could even provide password protection. But a simple molecular lock, if it has three inputs, would not compare, as it could code only as 123, 132, 213, 231, 312, 321, or in 6 six ways in all.

Although apparently limited in the range of coding, however, a molecular lock has the advantages of very compact dimensions and the robustness of the specific emission patterns, which could be read by a hand-held device and enable very easy coding and decoding. The principle of a chemical binding to different additives to generate a range of distinguishable fluorescence patterns hence suggests extension to complex coding applications with the ease of invisible ink.

The system now described by the group is a further development, of not just a molecular lock, but of a generator of a coding scheme, using a versatile, fluorescent molecule, the molecule scale messaging sensor or m-SMS, which is able to form stable links with a range of easily available reagents and generate unique emission spectra. The coder thus adds a random reagent to the base material, which gives rise to a pattern of emission at different frequencies. Each letter in the message to be encoded is then associated with successive emission levels and a code is generated. Even spaces in the original message are coded like this and anybody intercepting the message cannot even make out where the words or sentences start and terminate.

The particular additive that is used is conveyed to the proper recipient. She can then easily recreate the original emission pattern and use the levels of emission over the wavelengths of light to decode the message. The letters in the message are first coded by a simpler method where the representation of each character has a range or spread. In this way, even errors in the reproduced emission pattern during decoding would be accommodated, as the example in the box would show.

The actual material used is described in the Nature Communications paper as a kind of organic molecule that has multiple branches that take part in the fluorescence action, which is to absorb light at high frequency and then distribute the energy for emission at other frequencies. The molecule also has affinity for, or the capacity to form bonds with, a variety of chemical groups, like sugars, metals, etc. Formation of such bonds affects the energy levels involved in the fluorescence action and versatility in bonding leads to great variability in the frequencies of light that is emitted. The m-SMS, is thus able to combine with a variety of reagents and to respond with a specific emission spectrum for each substance. The possible reagents include easily available soft drinks, kitchen ingredients or medicines or pharmaceuticals, which have the advantage of purity.

A sender of a secret message could thus create her own completely unknown emission spectrum and use this as the basis for encoding a message. The intended receiver of the message, who knows which reagent to use with the m-SMS, can now read out the original emission spectrum with a simple hand-held spectrometer. For longer messages, the reagents could be used in succession, generating a fresh set of codes every time. The particular order of reagents to use could even work as an additional password protection, by using the m-SMS in the form of a keypad lock, the paper says. Apart from security of the code and the password protection, even the chemical used or the m-SMS can be sent across concealed within an ordinary letter.

While coding by m-SMS would ensure secrecy, it would also ensure that the message is genuine. Knowing how the message has been coded would also be evidence of authorship. m-SMS thus satisfies important requirements of encryption systems, in addition to its simplicity and versatility.

Invisible ink

Writing with lemon juice is a perfect way. The trick is to write the message between the lines of an ordinary note in real ink. The lemon juice disappears as soon as it dries, but can be later brought to life either by holding the paper over a candle or pressing with a warm iron.

German spies operating in England during World War I used this method. While correspondence of Germans, especially those under suspicion, was regularly screened by the British War Office, many letters appear to have slipped through. Till Mabel Beatrice Elliot got suspicious of a business letter written to a ‘friend’ in Holland, by a person recently come in from New York. Elliot applied simple chemistry in heating the letter, which revealed writing about Royal Navy ships and forces around London. Many more spies were soon caught, some convicted based on possession of lemons or corroded pen nibs!

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