libressl-DH_set_method - select DH method

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DH_SET_METHOD(3) Library Functions Manual DH_SET_METHOD(3)

NAME

DH_set_default_method, DH_get_default_method, DH_set_method, DH_new_method, DH_OpenSSL — select DH method

SYNOPSIS

library “libcrypto” #include <openssl/dh.h>

void

DH_set_default_method(const DH_METHOD *meth);

const DH_METHOD *

DH_get_default_method(void);

int

DH_set_method(DH *dh, const DH_METHOD *meth);

DH *

DH_new_method(ENGINE *engine);

const DH_METHOD *

DH_OpenSSL(void);

DESCRIPTION

A DH_METHOD object contains pointers to the functions used for Diffie-Hellman operations. By default, the internal implementation returned by DH_OpenSSL() is used. By selecting another method, alternative implementations such as hardware accelerators may be used.

DH_set_default_method() selects meth as the default method for all DH structures created later.

DH_get_default_method() returns a pointer to the current default method.

DH_set_method() selects meth to perform all operations using the key dh. This replaces the DH_METHOD used by the dh key. It is possible to have DH keys that only work with certain DH_METHOD implementations, and in such cases attempting to change the DH_METHOD for the key can have unexpected results.

DH_new_method() allocates and initializes a DH structure. The engine argument is ignored and the default method controlled by DH_set_default_method() is used.

The DH_METHOD structure is defined as follows:

typedef struct dh_meth_st
{
/* name of the implementation */

const char *name;

/* generate private and public DH values for key agreement */
int (*generate_key)(DH *dh);

/* compute shared secret */
int (*compute_key)(unsigned char *key, BIGNUM *pub_key, DH *dh);

/* compute r = a ˆ p mod m (May be NULL for some implementations) */
int (*bn_mod_exp)(DH *dh, BIGNUM *r, BIGNUM *a, const BIGNUM *p,
const BIGNUM *m, BN_CTX *ctx,
BN_MONT_CTX *m_ctx);

/* called at DH_new */
int (*init)(DH *dh);

/* called at DH_free */
int (*finish)(DH *dh);

int flags;

char *app_data; /* ?? */

} DH_METHOD;

RETURN VALUES

DH_OpenSSL() and DH_get_default_method() return pointers to the respective DH_METHOD.

DH_set_method() returns 1 on success or 0 on failure. Currently, it cannot fail.

DH_new_method() returns NULL and sets an error code that can be obtained by ERR_get_error(3) if the allocation fails. Otherwise it returns a pointer to the newly allocated structure.

SEE ALSO

DH_new(3)

HISTORY

DH_set_default_method(), DH_get_default_method(), DH_set_method(), DH_new_method() and DH_OpenSSL() first appeared in OpenSSL 0.9.5 and have been available since OpenBSD 2.7. GNU June 8, 2025 DH_SET_METHOD(3)

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