Problem - Kevin Tafuro

Some people like to use a fixed modulus shared across all users. We
don’t recommend that approach, but if you insist on using it, be sure
to read RFCs 2631 and 2785.
Diffie-Hellman requires another parameter g, the “generator,” which is a value that
we’ll be exponentiating. For ease of computation, use either 2 or 5. * Note that not
every {prime, generator} pair will work, and you will need to test the generator to
make sure that it has the mathematical properties that Diffie-Hellman requires.
OpenSSLexpects that 2 or 5 will be used as a generator. To select a prime for the
modulus, you can use the function DH_generate_parameters( ), which has the following
signature:
DH *DH_generate_parameters(int prime_len, int g,
void (*callback)(int, int, void *), void *cb_arg);
This function has the following arguments:
prime_len
Size in bits of the prime number for the modulus (n) to be generated.
g
Generator you want to use. It should be either 2 or 5.
callback
Pointer to a callback function that is passed directly to BN_generate_prime( ), as
discussed in Recipe 7.4. It may be specified as NULL, in which case no progress
will be reported.
cb_arg
Application-specific argument that is passed directly to the callback function, if
one is specified.
The result will be a new DH object containing the generated modulus (n) and generator
(g) parameters. When you’re done with the DH object, free it with the function DH_
free( ).
Once parameters are generated, you need to check to make sure the prime and the
generator will work together properly. In OpenSSL, you can do this with DH_check( ):
int *DH_check(DH *ctx, int *err);
This function has the following arguments:
ctx
Pointer to the Diffie-Hellman context object to check.
* It’s possible (but not recommended) to use a nonprime value for n, in which case you need to compute a
suitable value for g. See the Applied Cryptography for an algorithm.
Using Basic Diffie-Hellman Key Agreement | 433
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