NAME

       DH_generate_parameters, DH_check - generate and check Diffie-Hellman parameters

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);

DESCRIPTION

       DH_generate_parameters() generates Diffie-Hellman parameters that can be shared among a
       group of users, and returns them in a newly allocated DH structure. The pseudo-random
       number generator must be seeded prior to calling DH_generate_parameters().

       prime_len is the length in bits of the safe prime to be generated.  generator is a small
       number > 1, typically 2 or 5.

       A callback function may be used to provide feedback about the progress of the key
       generation. If callback is not NULL, it will be called as described in
       BN_generate_prime(3) while a random prime number is generated, and when a prime has been
       found, callback(3, 0, cb_arg) is called.

       DH_check() validates Diffie-Hellman parameters. It checks that p is a safe prime, and that
       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 *codes.
       DH_UNABLE_TO_CHECK_GENERATOR is set if the generator cannot be checked, i.e. it does not
       equal 2 or 5.

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.

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.

BUGS

       If generator is not 2 or 5, dh->g=generator is not a usable generator.

SEE ALSO

       dh(3), ERR_get_error(3), rand(3), DH_free(3)

HISTORY

       DH_check() is available in all versions of SSLeay and OpenSSL.  The 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.