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 ^ 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.

June 8, 2025 | Debian