# Computing p and q from private key

We are given n (public modulus) where n=pq and e (encryption exponent). Then I was able to crack the private key d, using Wieners attack. So now, I have (n,e,d). My question is, is there a way to calculate p and q from this information? If so, any links and explanation would be much appreciated!

• – user991
Nov 4, 2013 at 16:56

It's actually fairly easy to factor $n$ given $e$ and $d$. Here's the standard way to do this:

• Compute $f = ed - 1$. What's interesting about $f$ is that $x^f \equiv 1\ (\bmod n)$ for (almost) any $x$.

• Write $f$ as $2^s g$ for an odd value $g$.

• Select a random value $a$, and compute $b = a^g \bmod n$.

• If $b = 1$ or $-1$, then go back and select another random value of $a$

• Repeatedly (in practice, up to $s$ times):

• compute $c = b^2 \bmod n$.

• If $c = 1$ then the factors for $n$ are $gcd(n, b-1)$ and $gcd(n, b+1)$

• If $c = -1$, then go back and select another random value of $a$

• Otherwise, set $b = c$, and go through another iteration of the loop.

If you are familiar with the Miller-Rabin primality test, this will look familiar; the logic is the same (except that we use $ed-1$ rather than $n-1$ as the startign place for the exponent)

• Just to clarify, so when we write $f$ as $2^s g$, do you mean $f=2^s g$? Also, is it $2^s g$ or $2^{s g}$? Nov 4, 2013 at 17:09
• @hheluilop: $f = 2^s \times g$; keep on dividing $f$ by two until you get an odd number. Nov 4, 2013 at 17:16
• There is no such value as $\gcd(n+1)$. Did you mean $\gcd(n,b+1)$? May 10, 2015 at 19:33
• @cpast: yes, of course May 11, 2015 at 21:16

Generally, (n,e,d) is sufficient. Using these three it is possible to decrypt, encrypt, sign and verify any message or signature.

If you still need p and q: NIST SP 800-56B: Recommendation for Pair-Wise Key Establishment Schemes Using Integer Factorization Cryptography, Appendix C Prime Factor Recovery (Normative) contains formula for retrieving p and q, when you know (n,e,d). This formula is useful for instance to convert the private key in (n,e,d) format to CRT format.

Even a tool exists for the job: RSA CRT/SFM Converter.

• I will definitely check out the tool later. Do you know if the tool can handle 115-135 digit long integers for n and maybe 30-50 for d? Nov 4, 2013 at 17:24
• If my memory serves me right, it'll handle any usual lengths. Nov 4, 2013 at 17:45
• The algorithm in the NIST document is clear to me except for step 2. How does one calculate $r$? Is brute force the only option? Feb 3, 2015 at 8:36
• This step is fast to calculate because the other value ($2^t$) is multiple of 2. In fact, t is the number of zero bits on the least significant bits of k. An easy way to calculate r in many big number packages is to shift k right t steps. Either calculate the zeroes or shift one bit a time as long as the value is even. Feb 3, 2015 at 17:47
• While seeing formulas is great, sometimes I need to see it in code. Found an implementation in Python that helped me understand it better: github.com/truongkma/ctf-tools/blob/master/… Jun 19, 2019 at 19:59