Document the DH library, and make some minor changes along the way.

doc/crypto/DH_generate_parameters.pod

@@ -0,0 +1,67 @@
+=pod
+
+=head1 NAME
+
+DH_generate_parameters, DH_check - Generate and check Diffie-Hellman parameters
+
+=head1 SYNOPSIS
+
+ #include <openssl/dh.h>
+
+ DH *DH_generate_parameters(int prime_len, int generator,
+     void (*callback)(int, int, void *), void *cb_arg);
+
+ int DH_check(DH *dh, int *codes);
+
+=head1 DESCRIPTION
+
+DH_generate_parameters() generates Diffie-Hellman parameters that can
+be shared among a group of users, and returns them in a newly
+allocated B<DH> structure. The pseudo-random number generator must be
+seeded prior to calling DH_generate_parameters().
+
+B<prime_len> is the length in bits of the safe prime to be generated.
+B<generator> is a small number E<gt> 1, typically 2 or 5. 
+
+A callback function may be used to provide feedback about the progress
+of the key generation. If B<callback> is not B<NULL>, it will be
+called as described in L<BN_generate_prime(3)> while a random prime
+number is generated, and when a prime has been found, B<callback(3,
+0, cb_arg)> is called.
+
+DH_check() validates Diffie-Hellman parameters. It checks that B<p> is
+a safe prime, and that B<g> is a suitable generator. In the case of an
+error, the bit flags DH_CHECK_P_NOT_SAFE_PRIME or
+DH_NOT_SUITABLE_GENERATOR are set in B<*codes>.
+DH_UNABLE_TO_CHECK_GENERATOR is set if the generator cannot be
+checked, i.e. it does not equal 2 or 5.
+
+=head1 RETURN VALUES
+
+DH_generate_parameters() returns a pointer to the DH structure, or
+NULL if the parameter generation fails. The error codes can be
+obtained by ERR_get_error(3).
+
+DH_check() returns 1 if the check could be performed, 0 otherwise.
+
+=head1 NOTES
+
+DH_generate_parameters() may run for several hours before finding a
+suitable prime.
+
+The parameters generated by DH_generate_parameters() are not to be
+used in signature schemes.
+
+=head1 SEE ALSO
+
+dh(3), err(3), rand(3), DH_free(3)
+
+=head1 HISTORY
+
+DH_check() is available in all versions of SSLeay and OpenSSL.
+The B<cb_arg> argument to DH_generate_parameters() was added in SSLeay 0.9.0.
+
+In versions before OpenSSL 0.9.5, DH_CHECK_P_NOT_STRONG_PRIME is used
+instead of DH_CHECK_P_NOT_SAFE_PRIME.
+
+=cut