NAME

DH_generate_parameters_ex, DH_check, DH_check_pub_key, DH_generate_parameters — 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);

int
DH_check_pub_key(const DH *dh, const BIGNUM *pub_key, 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_GENCB_call(3).

DH_check() validates Diffie-Hellman parameters. If no problems are found, *codes is set to zero. Otherwise, one or more of the following bits are set:

DH_CHECK_P_NOT_PRIME: The parameter dh->p is not prime.
DH_CHECK_P_NOT_SAFE_PRIME: The parameter dh->p is not a safe prime.
DH_UNABLE_TO_CHECK_GENERATOR: The generator dh->g cannot be checked for suitability: it is neither 2 nor 5.
DH_NOT_SUITABLE_GENERATOR: The generator dh->g is not suitable.

DH_check_pub_key() checks whether pub_key is a valid public key when using the domain parameters contained in dh. If no problems are found, *codes is set to zero. Otherwise, one or more of the following bits are set:

DH_CHECK_PUBKEY_TOO_SMALL: pub_key is less than or equal to 1.
DH_CHECK_PUBKEY_TOO_LARGE: pub_key is greater than or equal to dh->p - 1.
DH_CHECK_PUBKEY_INVALID: dh->q is set but pub_key to the power of dh->q is not 1 modulo dh->p.

RETURN VALUES

DH_generate_parameters_ex(), DH_check(), and DH_check_pub_key() return 1 if the check could be performed or 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).

HISTORY

DH_check() and DH_generate_parameters() first appeared in SSLeay 0.5.1 and have been available since OpenBSD 2.4.

The cb_arg argument to DH_generate_parameters() was added in SSLeay 0.9.0.

DH_check_pub_key() first appeared in OpenSSL 0.9.8a and has been available since OpenBSD 4.0.

DH_generate_parameters_ex() first appeared in OpenSSL 0.9.8 and has been available since OpenBSD 4.5.

CAVEATS

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.