The mods might have been a bit hasty in closing this question about the cube-root attack on RSA. The reason for closing was given as follows:

Requests for analyzing or deciphering a block of data are off-topic here, as the results are rarely useful to anyone else.

The question wasn't asking to have a block of data deciphered. In fact the ciphertext doesn't appear anywhere in the question. My understanding is that the OP was just asking for help on how — in general — to go about attacking an RSA block encrypted with a small value of e.

Yes, the question does include a public key, but only to demonstrate the modulus and exponent values. The OP had already given up on factoring n, and was looking for help with a different line of attack.

According to the help pages, questions on cryptanalysis techniques are supposed to be welcome here. So I think the question should be re-opened.

  • 1
    $\begingroup$ We (read: mortal non-mod and non-dev users) can't vote for the re-open, because it was a migrated question that was bounced back to Sec.SE. $\endgroup$
    – SEJPM
    Mar 11, 2016 at 11:43
  • $\begingroup$ @SEJPM Indeed. Good thing is I happened to stumble upon this before the system went into its daily a clean-up frenzy. That way, I could un-bounce, un-lock, and (in contrast to my personal opinion and the opinion of some other close-voters) re-open before it reached the “point of no return”. ;) $\endgroup$
    – e-sushi
    Mar 11, 2016 at 21:19

1 Answer 1


The mods might…

Erm, just for the record: there were also non-mods (aka “regular users”) voting with the same close-reason. So, this wasn’t a case of mods being a bit hasty. It was a community-votes result.

The Question

Anyway, let’s look at the question itself:

I have this RSA public key

-----END PUBLIC KEY-----

From which I get the values of n and e

n = 25227313400403003289291316040964706793284526130307773148182567990181812103363366492141868094341253168514098087977707768352520652692143206065406906643084095767849329466558935239586943914389925429616795559049929569740742883151498832979927915567828657543757871650853272541594015354947860164753295474118247326770430739348498870310114642065925503324951264351013482315725877175994479883134119470270775988034913135302127722176827339225792610659723969082753807504414521250872063048608812550101510336558064093636352586121260624931803298322088622685800438189865267661921807764104595582348335380035988782396542471100276681082383

e = 3

I have given a chipher.bin file which i have to decrypt using these values. I had tried to find the prime factor of n but failed, then I come to know about cube-root RSA attack but didn't able to figure out how to proceed.

To me, this translates to:

I’ve got an encrypted file and want to decrypt it using the above mentioned RSA key and known e and n values. How do I attack this key to decrypt my bin file? I’m not saying where, but I heard about something called “cube-root RSA attack”. As I don’t understand it and have even less of an idea how I would go about actually applying it, I need someone to hold my hand and walk me through this attack from A to Z, practically attacking/breaking the mentioned key.

Even now, this practically boils down to a request for analyzing and attacking an RSA key to me. That was the reason I regarded this – among other people – to be off-topic and voted accordingly.

If you check the comments, you’ll notice others didn’t go that far an interpreted the question to be a simple request to decrypt the bin file itself…

Now, I do understand what you are aiming at, but I fail to really see it in this case. See, it would have been a totally different case if the question would have been something along the lines of:

Hi, I’m trying to figure out how to actually apply a “cube-root RSA attack”. I have done the following research ... and know that ... but when I tried ... I failed to apply the attack successfully. What am I missing or doing wrong?

But when you re-read the quoted question, you’ll have to agree that it strongly differs from that since…

All that has been posted is the key, the e and n value and a request asking how to attack and break the key with the goal to decrypt some unknown file.

As said, I did (and still do) regard that to be a specific request for analyzing and attacking an RSA key and not a general question about “cube-root RSA attacks” which would be helpfull to others.

Your Re-Open Request

Nevertheless, as you have been able to provide an answer which obviously adds some of the value the question itself is lacking, I have decided to re-open the question as you requested.

Yet – should community votes close this question again – I won’t step in to rescue it once more.

Notice: To prevent future references from pointing here for the wrong reasons, I would like to point out and put an emphasis on the fact that the re-opening of this specific, individual question does not mean any or all alike (let’s just call them) “borderlining questions” can or should generally be considered to be on-topic. This is not a Carte Blanche / Blank Cheque to circumvent or undermine any of the well-vetted rules layed out in the Crypto.SE help center.


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