I'm working on a CTF. The challenge is to get the contents of an encrypted message given the ciphertext and the 2048-bit RSA public key. I did finally get the flag after a few hours, but I'm still not sure why the first step worked.

The first step -- factor n into its two prime factors is from what I can tell, not supposed to be feasible, but it was done in about 1 second on this website. Why was it so easy?



1 Answer 1


That number was so quick to factor because its factors are extremely close together, i.e., it factors as $\left(\lfloor\sqrt{n}\rfloor + 70\right)\left(\lfloor\sqrt{n}\rfloor - 68\right)$.

Some algorithms, like Fermat's, which work best the closer together the factors are, will find this factorization instantly.

  • $\begingroup$ ohh, I see thank you. I thought maybe it was something like n was 2^2048-1, or maybe there was some type of factoring rainbow table, but that makes more sense $\endgroup$ Jun 5, 2021 at 2:06
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    $\begingroup$ @rainbowkitty227 Rainbow tables aren't relevant to integer factorization. $\endgroup$
    – forest
    Jun 5, 2021 at 2:07
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    $\begingroup$ Ryan Sheasby wrote two good articles describing what rainbow tables actually are (and why they're usually useless these days): rsheasby.medium.com/… and rsheasby.medium.com/… $\endgroup$ Jun 5, 2021 at 2:25
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    $\begingroup$ In particular, the site uses a quadratic sieve which is like an extension to Fermat's method. $\endgroup$
    – qwr
    Jun 5, 2021 at 8:54
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    $\begingroup$ IOW, the puzzle designers picked this specific number, just like textbook authors pick nice round numbers. $\endgroup$
    – RonJohn
    Jun 5, 2021 at 23:49

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