c# - Calculate primes p and q from private exponent (d), public exponent (e) and the modulus (n)

Question

How do I calculate the p and q parameters from e (publickey), d (privatekey) and modulus?

I have BigInteger keys at hand I can copy paste into code. One publickey, one privatekey and a modulus.

I need to calculate the RSA parameters p and q from this. But I suspect there is a library for that which I was unable to find with google. Any ideas? Thanks.

This does not have to be brute force, since I'm not after the private key. I just have a legacy system which stores a public, private key pair and a modulus and I need to get them into c# to use with RSACryptoServiceProvider.

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So it comes down to calculating (p+q) by

public BigInteger _pPlusq()
    {
        int k = (this.getExponent() * this.getD() / this.getModulus()).IntValue();

        BigInteger phiN = (this.getExponent() * this.getD() - 1) / k;

        return phiN - this.getModulus() - 1;

    }

but this doesn't seem to work. Can you spot the problem?

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5 hours later... :)

Ok. How can I select a random number out of Zn* (http://en.wikipedia.org/wiki/Multiplicative_group_of_integers_modulo_n) in C#?

Answer 1

Let's assume that e is small (that's the common case; the Traditional public exponent is 65537). Let's also suppose that ed = 1 mod phi(n), where phi(n) = (p-1)(q-1) (this is not necessarily the case; the RSA requirements are that ed = 1 mod lcm(p-1,q-1) and phi(n) is only a multiple of lcm(p-1,q-1)).

Now you have ed = k*phi(n)+1 for some integer k. Since d is smaller than phi(n), you know that k < e. So you only have a small number of k to try. Actually, phi(n) is close to n (the difference being on the order of sqrt(n); in other words, when written out in bits, the upper half of phi(n) is identical to that of n) so you can compute k' with: k'=round(ed/n). k' is very close to k (i.e. |k'-k| <= 1) as long as the size of e is no more than half the size of n.

Given k, you easily get phi(n) = (ed-1)/k. It so happens that:

phi(n) = (p-1)(q-1) = pq - (p+q) + 1 = n + 1 - (p+q)

Thus, you get p+q = n + 1 - phi(n). You also have pq. It is time to remember that for all real numbers a and b, a and b are the two solutions of the quadratic equation X²-(a+b)X+ab. So, given p+q and pq, p and q are obtained by solving the quadratic equation:

p = ((p+q) + sqrt((p+q)² - 4*pq))/2

q = ((p+q) - sqrt((p+q)² - 4*pq))/2

In the general case, e and d may have arbitrary sizes (possibly greater than n), because all that RSA needs is that ed = 1 mod (p-1) and ed = 1 mod (q-1). There is a generic (and fast) method which looks a bit like the Miller-Rabin primality test. It is described in the Handbook of Applied Cryptography (chapter 8, section 8.2.2, page 287). That method is conceptually a bit more complex (it involves modular exponentiation) but may be simpler to implement (because there is no square root).