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steganography

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Subverting Backdoored Encrryption

This is a really interesting research result. This paper proves that two parties can create a secure communications channel using a communications system with a backdoor. It’s a theoretical result, so it doesn’t talk about how easy that channel is to create. And the assumptions on the adversary are pretty reasonable: that each party can create his own randomness, and that the government isn’t literally eavesdropping on every single part of the network at all times.

This result reminds me a lot of the work about subliminal channels from the 1980s and 1990s, and the notions of how to build an anonymous communications system on top of an identified system. Basically, it’s always possible to overlay a system around and outside any closed system.

How to Subvert Backdoored Encryption: Security Against Adversaries that Decrypt All Ciphertexts,” by Thibaut Horel and Sunoo Park and Silas Richelson and Vinod Vaikuntanathan.

Abstract: In this work, we examine the feasibility of secure and undetectable point-to-point communication in a world where governments can read all the encrypted communications of their citizens. We consider a world where the only permitted method of communication is via a government-mandated encryption scheme, instantiated with government-mandated keys. Parties cannot simply encrypt ciphertexts of some other encryption scheme, because citizens caught trying to communicate outside the government’s knowledge (e.g., by encrypting strings which do not appear to be natural language plaintexts) will be arrested. The one guarantee we suppose is that the government mandates an encryption scheme which is semantically secure against outsiders: a perhaps reasonable supposition when a government might consider it advantageous to secure its people’s communication against foreign entities. But then, what good is semantic security against an adversary that holds all the keys and has the power to decrypt?

We show that even in the pessimistic scenario described, citizens can communicate securely and undetectably. In our terminology, this translates to a positive statement: all semantically secure encryption schemes support subliminal communication. Informally, this means that there is a two-party protocol between Alice and Bob where the parties exchange ciphertexts of what appears to be a normal conversation even to someone who knows the secret keys and thus can read the corresponding plaintexts. And yet, at the end of the protocol, Alice will have transmitted her secret message to Bob. Our security definition requires that the adversary not be able to tell whether Alice and Bob are just having a normal conversation using the mandated encryption scheme, or they are using the mandated encryption scheme for subliminal communication.

Our topics may be thought to fall broadly within the realm of steganography: the science of hiding secret communication within innocent-looking messages, or cover objects. However, we deal with the non-standard setting of an adversarially chosen distribution of cover objects (i.e., a stronger-than-usual adversary), and we take advantage of the fact that our cover objects are ciphertexts of a semantically secure encryption scheme to bypass impossibility results which we show for broader classes of steganographic schemes. We give several constructions of subliminal communication schemes under the assumption that key exchange protocols with pseudorandom messages exist (such as Diffie-Hellman, which in fact has truly random messages). Each construction leverages the assumed semantic security of the adversarially chosen encryption scheme, in order to achieve subliminal communication.

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New Research in Invisible Inks

It’s a lot more chemistry than I understand:

Invisible inks based on “smart” fluorescent materials have been shining brightly (if only you could see them) in the data-encryption/decryption arena lately…. But some of the materials are costly or difficult to prepare, and many of these inks remain somewhat visible when illuminated with ambient or ultraviolet light. Liang Li and coworkers at Shanghai Jiao Tong University may have come up with a way to get around those problems. The team prepared a colorless solution of an inexpensive lead-based metal-organic framework (MOF) compound and used it in an ink-jet printer to create completely invisible patterns on paper. Then they exposed the paper to a methylammonium bromide decryption solution…revealing the pattern…. They rendered the pattern invisible again by briefly treating the paper with a polar solvent….

Full paper.

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Hiding Information in Silver and Carbon Ink

Interesting:

“We used silver and carbon ink to print an image consisting of small rods that are about a millimeter long and a couple of hundred microns wide,” said Ajay Nahata from the University of Utah, leader of the research team. “We found that changing the fraction of silver and carbon in each rod changes the conductivity in each rod just slightly, but visually, you can’t see this modification. Passing terahertz radiation at the correct frequency and polarization through the array allows extraction of information encoded into the conductivity.”

Research paper.

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WWW Malware Hides in Images

There’s new malware toolkit that uses steganography to hide in images:

For the past two months, a new exploit kit has been serving malicious code hidden in the pixels of banner ads via a malvertising campaign that has been active on several high profile websites.

Discovered by security researchers from ESET, this new exploit kit is named Stegano, from the word steganography, which is a technique of hiding content inside other files.

In this particular scenario, malvertising campaign operators hid malicious code inside PNG images used for banner ads.

The crooks took a PNG image and altered the transparency value of several pixels. They then packed the modified image as an ad, for which they bought ad displays on several high-profile websites.

Since a large number of advertising networks allow advertisers to deliver JavaScript code with their ads, the crooks also included JS code that would parse the image, extract the pixel transparency values, and using a mathematical formula, convert those values into a character.

Slashdot thread.

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Friday Squid Blogging: More Research Inspired by Squid Skin

Research on color-changing materials:

What do squid and jellyfish skin have in common with human skin? All three have inspired a team of chemists to create materials that change color or texture in response to variations in their surroundings. These materials could be used for encrypting secret messages, creating anti-glare surfaces, or detecting moisture or damage.

They don’t really mean “encrypting”; they mean hiding. But interesting nonetheless.

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Hiding a Morse Code Message in a Pop Song

In Colombia:

The team began experimenting with Morse code using various percussion instruments and a keyboard. They learned that operators skilled in Morse code can often read the signals at a rate of 40 words per minute ­ but played that fast, the beat would sound like a European Dance track. “We discovered the magic number was 20,” says Portela. “You can fit approximately 20 Morse code words into a piece of music the length of a chorus, and it sounds okay.”

[…]

Portela says they played with the Morse code using Reason software, which gives each audio channel or instrument its own dedicated track. With a separate visual lane for certain elements, it was possible to match the code to the beat of the song — and, crucially, blend it in.

Hiding the Morse code took weeks, with constant back-and-forth with Col. Espejo and the military to make sure their men could understand the message. “It was difficult because Morse code is not a musical beat. Sometimes it was too obvious,” says Portela. “Other times the code was not understood. And we had to hide it three times in the song to make sure the message was received.”

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