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Informally, the hardness of RSA/Strong RSA assumption lies in the hardness of factoring a large composite number $N$ having two large primes as its factors. If RSA modulus $N$ is a prime number, then the system is trivially breakable.

But, what about this variant of the Strong RSA problem presented in Ben Lynn's PhD dissertation? Here it says, given $c$, it's hard to find a pair $(g,a)$ such that $c = g^a$. RSA modulus $N$ hasn't been talked about here.

Are the above two notions synonymous?

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  • $\begingroup$ What about just outputting $\:(c,\hspace{-0.04 in}1)\;$? $\;\;\;\;$ $\endgroup$ – user991 Jan 26 '15 at 11:46
  • $\begingroup$ @RickyDemer That actually seems to be an error in Ben Lynn's PhD thesis: It talks about $a$ being chosen from $\mathbb Z_{r-1}^+$, which includes $1$. Therefore, the "strong RSA problem" as defined in the thesis is trivial. Do I miss something here? $\endgroup$ – yyyyyyy Jan 26 '15 at 11:49
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The modulus $N$ is implicit in the group $\mathbb Z_N^\ast$ that $c$ and $g$ are chosen from. That is, in this context, $g^a$ is taken to mean "the residue class represented by $x^a\bmod N$ where $x$ denotes some representant of $g$'s residue class".

Therefore, when additionally requiring that $a\geq3$ and odd, this notion is equivalent to the strong RSA assumption as defined in Rivest's paper.

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    $\begingroup$ It sounds like a generic version of Quadratic Residue problem. I wonder whether there's any nomenclature for that. $\endgroup$ – Holmes.Sherlock Jan 26 '15 at 12:36

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