A record announced first, explained later
Eric Lu, an engineer at Cognition, posted a 130-digit number on X on September 3, 2026 followed by the words "divides RSA-260", announcing a factor of the 260-digit, 862-bit number from the RSA Factoring Challenge. It was the first RSA challenge number factored since RSA-250 in 2020. Scientific American reported the next day that Lu had given few details of the method. [2][3]
On September 9, Lu's post on Cognition's blog published both 130-digit factors and explained how. Devin agents built a GPU lattice siever as a drop-in replacement for the CPU siever in CADO-NFS, an open-source implementation of the general number field sieve, then modified almost every other stage of the pipeline. Lu says he ran an average of 3 and at most 18 Devin sessions at a time over the three-week project, and that his own role was to set priorities, establish benchmarks and notice when work went off track. [1]
The cost, and what it does not change
Lu estimates the run at about 4,900 GPU-days, or 13.5 GPU-years, about $400,000 at market prices, most of it spent sieving, and says it ran at bottom priority on spare or fragmented nodes that amount to a single-digit percentage of Cognition's compute. He reports essentially no algorithmic advance but says the GPU siever factors numbers at 10 times lower cost than the previous public state of the art. He estimates that factoring RSA-1024 would cost roughly $30 million at market GPU prices, and that RSA-2048 remains roughly a billion times harder than RSA-1024 and is not meaningfully affected. [1]
Atlas interpretation: The factors are easy to check, since multiplying the two published numbers reproduces RSA-260, but the cost, the tenfold claim and the account of Devin's role are Lu's own. Ten days later an Anthropic engineer reported factoring the larger RSA-896 with Claude, also by adapting CADO-NFS to GPUs, so the record changed hands twice in September on the strength of AI-written GPU code. [1][3]
Sources
- Factoring RSA-260
Cognition · Sep 9, 2026
- What’s the tech behind the record-breaking RSA-260 crack?
Scientific American · Sep 4, 2026
- Claude Helped Factor RSA-896, A 270-Digit Encryption Challenge, Says Anthropic Engineer
OfficeChai · Sep 20, 2026