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Breaking RSA with a Quantum Computer
news.movim.eu / Schneier · Tuesday, 3 January  17:38 · 1 minute
A group of Chinese researchers have just published a paper claiming that they can—although they have not yet done so—break 2048bit RSA. This is something to take seriously. It might not be correct, but it’s not obviously wrong.
We have long known from Shor’s algorithm that factoring with a quantum computer is easy. But it takes a big quantum computer, on the orders of millions of qbits, to factor anything resembling the key sizes we use today. What the researchers have done is combine classical lattice reduction factoring techniques with a quantum approximate optimization algorithm. This means that they only need a quantum computer with 372 qbits, which is well within what’s possible today. (IBM will announce a 1000qbit quantum computer in a few months. Others are on their way as well.)
The Chinese group didn’t have that large a quantum computer to work with. They were able to factor 48bit numbers using a 10qbit quantum computer. And while there are always potential problems when scaling something like this up by a factor of 50, there are no obvious barriers.
Honestly, most of the paper is over my head—both the latticereduction math and the quantum physics. And there’s the nagging question of why the Chinese government didn’t classify this research.
But…wow…maybe…and yikes! Or not.
“Factoring integers with sublinear resources on a superconducting quantum processor”
Abstract: Shor’s algorithm has seriously challenged information security based on public key cryptosystems. However, to break the widely used RSA2048 scheme, one needs millions of physical qubits, which is far beyond current technical capabilities. Here, we report a universal quantum algorithm for integer factorization by combining the classical lattice reduction with a quantum approximate optimization algorithm (QAOA). The number of qubits required is O(logN/loglogN ), which is sublinear in the bit length of the integer N , making it the most qubitsaving factorization algorithm to date. We demonstrate the algorithm experimentally by factoring integers up to 48 bits with 10 superconducting qubits, the largest integer factored on a quantum device. We estimate that a quantum circuit with 372 physical qubits and a depth of thousands is necessary to challenge RSA2048 using our algorithm. Our study shows great promise in expediting the application of current noisy quantum computers, and paves the way to factor large integers of realistic cryptographic significance.