README.md

Pollard's p-1 Factorisation

Prerequisites:

1. Fermat's Little Theorem

2. RSA Encryption/Decryption

Let the modulus be N and its factors be p and q. The factorisation technique described here works only when p-1 and q-1 have very small prime factors.

The idea

We can write from Fermat's Little Theorem: , where p is a prime and a is any positive integer. Thus, we can write for any positive integer k that, where r is a positive integer

If we now take GCD of p*r and N = p*q, we will have: GCD(p*r, N) = p And thus, we have got one of the factors of N using this method. The method is not so easy to implement as we will see in the next section. The idea is to make the exponent a large multiple of p − 1 by making it a number with very many prime factors; generally, we take the product of all prime powers less than some limit B.

The exploit

To understand this attack, you can watch this video by David Wong on Pollard's p-1 Factorisation

Implementation of the above technique:

def pollard(n, B):
    a = 2
    for p in primes(B):
        pp = 1
        while pp*p <= B:
            pp *= p
        a = pow(a, pp, n)   # provided a>=b, GCD(a, b) = GCD(a % b, b)
        g = gcd(a-1, n)
        if 1 < g < n:
            return g
    return None

You can checkout the exploit here

References

1. David Wong- Pollard's p-1 Factorisation

2. Wikipedia- Pollard's p-1 Factorisation