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

The DH_generate_parameters() function 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(). 

The prime_len is the length in bits of the safe prime to be generated. The 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() 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, meaning it does not equal 2 or 5. 

NOTES

The DH_generate_parameters() function 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. 

RESTRICTIONS

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

RETURN VALUES

The DH_generate_parameters() function returns a pointer to the DH structure, or NULL if the parameter generation fails. The error codes can be obtained from ERR_get_error(). 

The DH_check() function returns 1 if the check could be performed, 0 otherwise. 

HISTORY

The DH_check() function 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. 

SEE ALSO

Functions: dh(3), err(3), rand(3), DH_free(3)