DH Exchange Code / Magic Money comments

[hughes@ah.com (Eric Hughes)]

>To use DH, we need a modulus n and a generator g. Unlike an RSA modulus,
>which is a product of two primes, a DH modulus must be prime. (n-1)/2 must
>also be prime. 

I know I recommended this characteristic for the modulus (and I got it
from Burt Kaliski).  Nevertheless, (n-1)/2 doesn't _have_ to be prime,
it's just much easier to prove that your generator actually is a
generator.  In fact, half the elements in such a ring are
multiplicative generators.  The algorithm to find moduli is simple,
even if it does take a long time.

There are faster ways of looking for moduli. One method is to take a
candidate prime and try to factor n-1, if you can.  (If you can't,
give up and go on.)  If you get a few small factors and one large
probable prime factor, then you can still look for known generators.
The candidate must first be relatively prime to the modulus.  Then one
checks that the candidate raised to each of the factors is not 1.
There are fewer generators in such moduli, but the moduli are easier
to find.

The security of the modulus to a precomputation attack is equal to the
size of its largest prime factor, so while the second method is
ever-so-slightly less secure with the same modulus size, the effective
security can be made the same by increasing the modulus size of the
second method.

>This makes the moduli slightly painful to find, but they can
>be reused indefinitely. 

Be careful about saying "indefinite".  It's not true in the long run,
so far as we can tell now.  As computational power increases, so also
do the lengths required to prevent attacks.

Remember, that every crypto system has a sunset after which there will
be enough computation available to read past traffic, if recorded.  No
cryptosystem is good forever.  One always needs to figure out just how
long one wants one ciphertext to be secure.

Or is that a sunrise? ...

(I pass over arguments about physical limitations of computation, not
because I think they are wrong, but because I'm not convinced that we
know enough to know we're asking the right questions.  Plus these
arguments do not yield key sizes that are yet practical to implement.)

And lastly, you can trust a thousand-bit modulus p where (p-1)/2 is
also prime.  Go ahead and use it.

Eric