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The original title: "there is a password hidden in this letter. Can you guess where it is?" "

Source: Unsplash, an encrypted communication, may be able to protect against hackers.

You see, there are some trees whose leaves have fallen off in this picture. The color of each pixel in the picture can be determined by red (R), green (G) and blue (B). If you change the RGB values of each pixel according to specific rules, you will get a completely different picture.

Photo Source: cyp via Wikimedia Commons, the transformation rule is the key. If you use the correct key and brighten the solved picture after completing the transformation, you will find a kitten hidden inside.

Cyp via Wikimedia Commons: in this way, when Xiao Ming sends a picture of the tree to Xiao Hong, even if the data is intercepted by an outsider during transmission, the other person can only look at the tree. Only Xiao Hong, who has the correct key, will know that what Xiao Ming wants to show himself is a cat. This is called steganography (Steganography), which uses some visible information to cover the real message.

In previous wars, people often used codebooks to send keys to the front to ensure encrypted communications. However, in the era of asymmetric encryption (encryption with public key, decryption with private key), once the password book is obtained by the enemy, the content of the communication may no longer be secure.

So, if you use the idea of "steganography" to hide the key in the password book behind the useless information, it will be very difficult for the opponent to find the real key, right? Recently, a group of scientists at the University of Texas at Austin wrote a letter to Professor James James Reuther at the University of Massachusetts at Lowell:

Dear Professor Luthor.

I hope everything will be all right when you receive the letter in Lowell!

Our molecular encryption program is going well. Look forward to chatting with you!

Good luck.

Anslin laboratory

A secret key is implied in the short letter. If you are still reading this letter carefully, please do not give up, because the key is not in the letter, but in the credit ink.

What is magic ink? In recent years, with the increasing imagination of scientists, there are more and more carriers that can be used to store information. For example, storing the contents of a novel in nanoscale DNA molecules may no longer be a surprising operation. After all, a complex and sophisticated machine like the human body is also assembled from information encoded by DNA, and its storage capacity can be imagined.

In contrast, some abiotic polymers cannot store data as densely as DNA, often storing only a few bytes or a word, and it is not easy to read information from it, but researchers are happy to accept the challenge. Eric Anslyn, a chemistry professor at the University of Texas at Austin, and his friends want to try to keep the key of an encrypted file in a sequence-controllable polymer (sequence-defined polymers).

First, scientists used a computer to generate a 256-bit binary key, a string of 256 zeros or ones-used to encrypt the full text of a novel. Such keys can be arranged in 2256 ≈ 1077 possible ways, and it is almost impossible to crack them by computer brute force. Using this string of numbers to encrypt information, the security factor is very high, but only if the key is not leaked.

Then, you need to design the polymer to store the key. The polymer the researchers chose was oligocarbamate (oligourethane), a material that looks a lot like plastic. They split the 256-bit key into eight different oligomer sequences, each of which represents 32 digits.

You may think that each polymer needs 32 monomers. No, in fact, a polymer sequence contains only 10 monomers, and only the middle 8 monomers are responsible for encoding the key. How to represent 32-bit numbers with 8 monomers?

Decimal, hexadecimal, binary (picture source: credit-kalmatthes.medium.com) Don't forget that 32-bit numbers are 32 bits in binary. If you represent 0 and 1 with two different monomers, you only need 32 monomers. But scientists have designed 16 different carbamate monomers, which represent 0,9 and F, respectively, so that binary numbers can be converted into hexadecimal numbers: 16 is the fourth power of 2, and one digit in hexadecimal can represent four digits in binary. In this way, the 32-bit numbers in the binary can be written into eight monomers in the middle of a polymer.

The 256-bit key is written in eight polymer sequences. When the key is distributed, the receiver still needs to know which sequence to read first and which sequence to read later in order to get the correct key. The researchers are also prepared for this. In addition to the eight monomers that represent numbers, there are two monomers in each polymer sequence as placeholders. One of them is a decoding placeholder, which is marked with isotopes like "fingerprints". The marks in each of the eight sequences are different, indicating the order in which they are read.

After the encryption was completed, the team dissolved all eight polymers (500 nanomoles each) in isopropanol and mixed the solution with glycerol and soot. Together, these substances formed a special kind of ink, and scientists poured it into the ballpoint pen. The letter mentioned at the beginning was written with this pen on ordinary printing paper.

Source: 2000 miles away from the original paper, Professor James Luthor and his colleagues in another lab will look for hidden keys after receiving letters.

It's easy to deposit, how to read it? Reading the information encoded in the polymer is probably the most difficult step in molecular storage.

In the ink of the letter, there are altogether eight polymer sequences to be read. In general, when using tandem mass spectrometry (MS / MS), researchers analyze each polymer individually to prevent the spectrum from becoming too complex. It would be nice to analyze the sequences of eight oligomers in one mixture at the same time.

Scientists have come up with a new plan to "cut off" the monomers that make up the polymer one by one, or depolymerize. After eight polymers with hidden keys are extracted from the ink, one monomer can be removed from the end of the polymer each time by heat-induced cyclization. In this way, the eight polymers that have been gradually removed can be sequenced simultaneously by liquid chromatography-mass spectrometry (LC / MS), eliminating the need for separate detection.

The monomer at the end was cut off (Photo Source: original paper) the researchers allowed the macromolecules to disintegrate slowly at 70 ℃. The instrument is sampled at a specified point in time to see where the polymers have been dismantled. At first, the instrument could only detect eight polymers, and that was the initial version. After 550 minutes, almost all the polymers became monomers.

From the 0th minute to the 550th minute, the key has basically been disassembled (picture source: original paper). A total of 8 polymer sequences are used, and each sequence initially has 10 monomers. When a monomer is cut off from the polymer, the mass of the rest is reduced. When the length of the polymer changes from 10 monomers to 9 monomers, 8 monomers, and finally only one monomer is left, scientists will get a total of 80 different masses in the process.

With 80 quality data (picture source: original paper), coupled with eight isotope tags identified by the researchers (used to indicate the reading order), the researchers finally read the string of numbers hidden in the polymer. However, it is still a hexadecimal version, and then converted to binary is a 256-bit key.

If you remember, in a lab in Texas, the research team used this key to encrypt a file. When scientists in Massachusetts unlocked the file with a key, they found that it was the full text of the novel the Wizard of Oz.

Perhaps in the minds of researchers, the espionage game of passing the "password book" is as tortuous as the plot of the novel. And the ending made them very excited. Professor Eric Enslin, one of the lead authors of the study, said it was the first time so much information was stored in this type of polymer, known as sequence-controllable polymers, or SDP, which marked a revolutionary advance in molecular data storage and cryptography.

After all, the super storage capacity of DNA only has four different bases (A, T, C, G). This time, scientists use 16 different monomers to encode information, which means that sequence controllable polymers have great storage potential.

Original paper:

Https://pubs.acs.org/doi/10.1021/acscentsci.2c00460

Reference link:

Https://www.eurekalert.org/news-releases/958890

Https://research.utexas.edu/showcase/articles/view/scientists-encode-wizard-of-oz-in-a-vanishingly-small-plastic

Https://www.researchgate.net/figure/Steganography-used-to-hide-the-image-of-a-cat-in-the-image-of-a-tree_fig2_344459934

This article comes from the official account of Wechat: global Science (ID:huanqiukexue), written by: chestnut, revision: Erqi

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