Socat: “the hard coded 1024 bit DH p parameter was not prime”
Q: How does p not being a prime => backdoor?<p>A: p not being a prime means two things:<p>* subgroup confinement attacks (where you send a public key made with a fake generator g) should be able to take place if the code is weak -> this is because there must be low order subgroups.<p>* the generator g might not be of great order. This can be easily tested if you know how to factor p: the order of the multiplicative group (Zp)* is the euler's totient function on p. If you know the order of the multiplicative group then you have an algorithm to find the order of your generator: you try all the divisors of the group's order, see the smallest one that works.<p>Unfortunately, if you don't know how to factor p then you can't easily do that.<p>Another question is: How can they know it's not a prime if they don't know the factorization of p? We have efficient provable tests for that: they tell you if p is prime or not and nothing else.