$N=p*q$ is a product of two distinct primes. Show that if $\phi(N)$ and 2N are known, then it is possible to compute p and q in polynomial time.

so, I know that $\phi(N)=(p-1)(q-1)$

Given this, if $\phi(N)=C$ where $C$ is a known constant,



So, I know it is possible to compute p and q. How would I prove that it is possible to compute them in polynomial time?

  • $\begingroup$ All you need is to plug the $p$ you got into $2N=2pq$. Then you get a quadratic in $q$. $\endgroup$
    – N. S.
    Dec 5, 2012 at 20:33

1 Answer 1


$\phi(N)=(p-1)(q-1)=pq-(p+q)+1=N+1-(p+q)$, thus: $$p+q=N+1-\phi(N)$$ $$pq=N$$ You have two equations that will enable you to solve for p,q using the quadratic formula

  • $\begingroup$ So to me, this reduces to $p=\frac{1-2q}{2}$, thus $N=q\bigg(\frac{1-2q}{2}\bigg)$... How would I put this in quadratic form? All I can reduce this to is $N=\frac{-2q^2+q}{2}$ $\endgroup$
    – gfppaste
    Dec 5, 2012 at 22:31
  • $\begingroup$ You can use the quadratic formula to solve this equation $\endgroup$
    – Amr
    Dec 5, 2012 at 22:34

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