THE GARDEN

FactorDB (Pragmatic Factorization Lookup)

Updated 19 Sep 2026

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  What it is

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    FactorDB (factordb.com) is a public, crowd-sourced database of integer factorizations — anyone can submit a number, and the site remembers (and shares) the result forever, along with running its own background factorization on submitted numbers. In practice, a meaningful fraction of "hard-looking" numbers that show up across many different CTFs, textbooks, and demos have already been factored by someone, at some point, and are sitting in FactorDB's database ready to look up — turning what looks like a serious factorization challenge into a single search-box query.

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  When to apply

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    Always worth a quick check before investing serious time implementing a sophisticated factoring algorithm, especially for numbers that feel "famous" (published in a well-known challenge, paper, or writeup) or suspiciously specific/structured. It's not a substitute for actually understanding the intended technique, but it's a completely legitimate move under competition time pressure — real practitioners do this too, not just CTF players.

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  Worked example

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    For $N=77$, a database lookup may return $7\cdot11$ immediately. Treat that as external evidence: verify $7\cdot11=77$ and primality locally before computing $\varphi(N)$. A missing result says nothing about whether $N$ is factorable.

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  When it won't help

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    Freshly-generated, genuinely random large semiprimes (the vast majority of properly-run RSA challenges) — FactorDB only has what's already been submitted or is small enough for its own background crawler to reach, so it's a lookup tool, not a factoring algorithm.

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  Related

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    Integer Factorization via ECM - Sage's factor()

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  Cards

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    What kind of numbers is FactorDB most likely to already have factored?

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      Numbers that have been publicly shared before — in challenges, papers, or writeups — not freshly-generated random semiprimes.

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    Is checking FactorDB considered a legitimate competition technique?

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      Yes — it's a pragmatic, widely-used shortcut, though it doesn't substitute for understanding the intended cryptographic technique.

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