RSA-260 Challenge Number Factored After 35 Years, Setting New Record

Cognition engineer Eric Lu (@penlume) announced on X that he had factored RSA-260, a 260-decimal-digit (about 862-bit) RSA Factoring Challenge number that had stood unbroken for 35 years, setting a new world record for general-purpose integer factoring. He announced the result with a single sentence and posted two 130-digit factors, both passing primality checks. The event matters both as a long-standing cryptographic milestone falling and as a showcase of X as a live gathering place for the cryptography community.

Confirmed

• Eric Lu published two 130-digit factors of RSA-260, both passing primality tests, completing the factorization of the 260-digit (862-bit) challenge number.
• According to @MikePFrank, the factorization took about 7 months.
• The previous record was the 2020 factorization of RSA-250 by an academic team using CADO-NFS; per cryptographer Steve Weis (relayed by @matthewdgreen), it consumed roughly 2,700 core-hours of computation.
• Within two hours of the tweet, a member of the team that factored RSA-155 in 1999 appeared in the replies to congratulate him—an achievement completed "before @penlume was born."
• Per @marvinvonhagen, the relevant Wikipedia entry was updated only minutes after the tweet.

Why it matters

• RSA-260 was an iconic, long-unsolved number in RSA Laboratories' factoring challenge; its factorization directly refreshes the world record for general-purpose integer factoring.
• The congratulation from the 1999 RSA-155 veteran added a community-legacy note that multiple relays (@marvinvonhagen, @k7agar) highlighted.

2026-09-03 ~ 2026-09-05 · 6 related posts

Full story(2 episodes)→

• Episode 1: Claimed Factor of RSA-260 Sparks Speculation of AI Involvement(2026-09-03, 3 posts)
• Episode 2: RSA-260 Challenge Number Factored After 35 Years, Setting New Record(2026-09-03, 6 posts)

• RSA-260 Factored, Setting a New World Record for General-Purpose Number Factorization — marvinvonhagen · 2026-09-03
• RSA-260 factor posted, RSA-155 legend shows up in replies within 2 hours — k7agar · 2026-09-03
• [source] RSA-260 factored after 35 years: Cognition engineer Eric Lu did it by hand in 7 months — MikePFrank · 2026-09-03
• RSA-260 Factor Tweet Draws Congrats From 1999 RSA-155 Team Within 2 Hours — marvinvonhagen · 2026-09-04

2 near-duplicate retellings: matthew_d_green · Daniel_Lerch