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In the Wikipedia article on Diffie-Hellman, the algorithm calls for a large prime modulus, $p$, and a generator, $g$, which is a primitive root of $p$.

As far as my knowledge of number theory goes, to find a primitive root of a prime number, you first need to determine the prime factors of $\phi(p)$, or $p-1$.

Obviously, 2 divides $\phi(p)$, but after that you are left with the problem of factoring a number that is the same bit size as the original $p$.

I was under the impression that the security of the system was dependent upon the difficulty of finding the prime factors of a big number in the first place.

What am I missing?

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migrated from security.stackexchange.com Feb 23 '16 at 10:08

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  • $\begingroup$ related: crypto.stackexchange.com/questions/820/… $\endgroup$ – cygnusv Feb 23 '16 at 11:06
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    $\begingroup$ We typically use safe primes for Diffie-Hellman, i.e. primes where $\frac{p-1}{2}$ is prime. In that case factoring is trivial. $\endgroup$ – CodesInChaos Feb 23 '16 at 11:27
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Diffie Hellman is not dependent on the difficulty of factorisation of large numbers (sort of*).

It relies on the fact that there is no efficient algorithm know to the human race as of today that can efficiently deduce the discrete logarithm of a number. That is given an element $y \in G$ where G is a cyclic group of prime order $q$ , it is not computationally feasible to find a number $x \in Z^*_q$ such that $y = g^x$.

They key point here is that the order of the group needs to be a prime number. This means that the group $Z^*_p$ is not secure for any prime $p$since its order is obviously not prime.

To deal with this trouble one can choose a prime $p$ such that $p= kq + 1$. This means that there exists a unique subgroup (called schnorr group) of $Z^*_p$ of order $q$ since there exists exactly one subgroup for each divisor of $φ(p) = p-1$.

To find a generator for that group is trivial. Choose a random value $1 <h < p$ such that $h^r \mod{p} \neq 1$. If you find such a number then this inequality means that the order of $h$ is greater than $r$. However the only possible order greater than $r$ is $q$. As a result $h$ has order $q$ and can generate a secure subgroup of $Z^*_p$. If $h^r \mod{p} = 1$ then simply generate another random $h$ and try again until you find a generator.

*One can show that if the order of the group has only small prime divisors then the discrete logarithm problem is much easier to solve than other groups. However by selecting a subgroup of a large prime order as we did above by selecting $p= kq + 1$ means that the subgroup we are generating only has one large factor (itself), which in turn means that you need not worry of these factorisation attacks.

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You're missing that the party which will choose g also got to choose p.

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