Is there an unbreakable code?

The question has been central to cryptography for thousands of years, and lies at the heart of efforts to secure private information on the internet. In a new paper, Cornell Tech researchers identified a problem that holds the key to whether all encryption can be broken – as well as a surprising connection to a mathematical concept that aims to define and measure randomness.

“Our result not only shows that cryptography has a natural ‘mother’ problem, it also shows a deep connection between two quite separate areas of mathematics and computer science – cryptography and algorithmic information theory,” said Rafael Pass, professor of computer science at Cornell Tech.

Pass is co-author of “On One-Way Functions and Kolmogorov Complexity,” which will be presented at the IEEE Symposium on Foundations of Computer Science, to be held Nov. 16-19 in Durham, North Carolina.

“The result,” he said, “is that a natural computational problem introduced in the 1960s in the Soviet Union characterizes the feasibility of basic cryptography – private-key encryption, digital signatures and authentication, for example.”

For millennia, cryptography was considered a cycle: Someone invented a code, the code was effective until someone eventually broke it, and the code became ineffective. In the 1970s, researchers seeking a better theory of cryptography introduced the concept of the one-way function – an easy task or problem in one direction that is impossible in the other.

For example, it’s easy to light a match, but impossible to return a burning match to its unlit state without rearranging its atoms – an immensely difficult task.

“The idea was, if we have such a one-way function, maybe that’s a very good starting point for understanding cryptography,” Pass said. “Encrypting the message is very easy. And if you have the key, you can also decrypt it. But someone who doesn’t know the key should have to do the same thing as restoring a lit match.”

But researchers have not been able to prove the existence of a one-way function. The most well-known candidate – which is also the basis of the most commonly used encryption schemes on the internet – relies on integer factorization. It’s easy to multiply two random prime numbers – for instance, 23 and 47 – but significantly harder to find those two factors if only given their product, 1,081.

It is believed that no efficient factoring algorithm exists for large numbers, Pass said, though researchers may not have found the right algorithms yet.

“The central question we’re addressing is: Does it exist? Is there some natural problem that characterizes the existence of one-way functions?” he said. “If it does, that’s the mother of all problems, and if you have a way to solve that problem, you can break all purported one-way functions. And if you don’t know how to solve that problem, you can actually get secure cryptography.”

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