FactorDB (Pragmatic Factorization Lookup)
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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 , a database lookup may return immediately. Treat that as external evidence: verify and primality locally before computing . A missing result says nothing about whether 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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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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