Splitting a big number into the smaller numbers that multiply to make it.
📰 Seen in the news
An engineer used the AI Claude to help crack RSA-896 — a 270-digit number that stood as a cryptographic challenge for decades — announced 19 Sep 2026. Factoring big numbers is what keeps online banking secure, so this record matters far beyond maths class.
Multiplying is easy. Un-multiplying is the mountain
3 × 7 = 21 takes a second. But given 21, which two numbers multiply back to it? Now do that with a 270-digit number — that's factorization, and it's so hard that half of internet security (the RSA system guarding your messages and payments) bets on it staying hard. Splitting RSA-896 took about 2,000 GPUs and Claude's help. Easy one way, mountain the other.
Keep it: One-way doors in maths are worth billions. Factorization is the most famous one.
Word of the Day · one word a day, heard in the news