DH_set_method(3)

NAME
DH_set_default_openssl_method, DH_get_default_openssl_method, DH_set_method, DH_new_method, DH_OpenSSL - select DH method

SYNOPSIS
 #include <openssl/dh.h>
 #include <openssl/engine.h>
 void DH_set_default_openssl_method(DH_METHOD *meth);
 DH_METHOD *DH_get_default_openssl_method(void);
 int DH_set_method(DH *dh, ENGINE *engine);
 DH *DH_new_method(ENGINE *engine);
 DH_METHOD *DH_OpenSSL(void);

DESCRIPTION
A DH_METHOD specifies the functions that OpenSSL uses for Diffie-Hellman operations. By modifying the method, alternative implementations such as hardware accelerators may be used.

Initially, the default is to use the OpenSSL internal implementation. DH_OpenSSL() returns a pointer to that method.

DH_set_default_openssl_method() makes meth the default method for all DH structures created later. NB: This is true only whilst the default engine for Diffie-Hellman operations remains as "openssl". ENGINEs provide an encapsulation for implementations of one or more algorithms, and all the DH functions mentioned here operate within the scope of the default "openssl" engine.

DH_get_default_openssl_method() returns a pointer to the current default method for the "openssl" engine.

DH_set_method() selects engine as the engine that will be responsible for all operations using the structure dh. If this function completes successfully, then the dh structure will have its own functional reference of engine, so the caller should remember to free their own reference to engine when they are finished with it. NB: An ENGINE's DH_METHOD can be retrieved (or set) by ENGINE_get_DH() or ENGINE_set_DH().

DH_new_method() allocates and initializes a DH structure so that engine will be used for the DH operations. If engine is NULL, the default engine for Diffie-Hellman opertaions is used.

THE DH_METHOD STRUCTURE
 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_openssl_method() return pointers to the respective DH_METHODs.

DH_set_default_openssl_method() returns no value.

DH_set_method() returns non-zero if the ENGINE associated with dh was successfully changed to engine.

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(3), DH_new(3)

HISTORY
DH_set_default_method(), DH_get_default_method(), DH_set_method(), DH_new_method() and DH_OpenSSL() were added in OpenSSL 0.9.4.

DH_set_default_openssl_method() and DH_get_default_openssl_method() replaced DH_set_default_method() and DH_get_default_method() respectively, and DH_set_method() and DH_new_method() were altered to use ENGINEs rather than DH_METHODs during development of OpenSSL 0.9.6.