DH_GENERATE_PARAMETERS(3)

NAME
DH_generate_parameters_ex, DH_generate_parameters, DH_check - generate and check Diffie-Hellman parameters

SYNOPSIS
 #include <openssl/dh.h>

 int DH_generate_parameters_ex(DH *dh, int prime_len,int generator, BN_GENCB *cb);

 int DH_check(DH *dh, int *codes);

Deprecated:

 DH *DH_generate_parameters(int prime_len, int generator,
     void (*callback)(int, int, void *), void *cb_arg);

DESCRIPTION
DH_generate_parameters_ex() generates Diffie-Hellman parameters that can be shared among a group of users, and stores them in the provided DH structure.

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 cb 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, BN_GENCB_call(cb, 3, 0) is called. See BN_generate_prime(3) for information on the BN_GENCB_call() function.

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_ex() and DH_check() return 1 if the check could be performed, 0 otherwise.

DH_generate_parameters() (deprecated) 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).

NOTES
DH_generate_parameters_ex() and DH_generate_parameters() may run for several hours before finding a suitable prime.

The parameters generated by DH_generate_parameters_ex() and 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.