DH_OpenSSL (3)

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NAME

DH_set_default_method, DH_get_default_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_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 specifies the functions that OpenSSL uses for Diffie-Hellman operations. By modifying the method, alternative implementations such as hardware accelerators may be used. See the section for important information about how these functions are affected by the use of calls.

Initially, the default

is the OpenSSL internal implementation, as returned by DH_OpenSSL().

DH_set_default_method() makes meth the default method for all

structures created later. : This is true only whilst no has been set as a default for so this function is no longer recommended.

DH_get_default_method() returns a pointer to the current default

However, the meaningfulness of this result is dependent on whether the is being used, so this function is no longer recommended.

DH_set_method() selects meth to perform all operations using the key dh. This will replace the

used by the key and if the previous method was supplied by an the handle to that will be released during the change. It is possible to have keys that only work with certain implementations (eg. from an module that supports embedded hardware-protected keys), and in such cases attempting to change the for the key can have unexpected results.

DH_new_method() allocates and initializes a

structure so that engine will be used for the operations. If engine is the default for operations is used, and if no default is set, the controlled by DH_set_default_method() 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_method() return pointers to the respective s.

DH_set_default_method() returns no value.

DH_set_method() returns non-zero if the provided meth was successfully set as the method for dh (including unloading the

handle if the previous method was supplied by an ).

DH_new_method() returns

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.

NOTES

As of version 0.9.7, implementations are grouped together with other algorithmic APIs (eg. etc) in modules. If a default is specified for functionality using an function, that will override any defaults set using the (ie. DH_set_default_method()). For this reason, the is the recommended way to control default implementations for use in and other cryptographic algorithms.

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

s rather than s during development of the engine version of OpenSSL 0.9.6. For 0.9.7, the handling of defaults in the was restructured so that this change was reversed, and behaviour of the other functions resembled more closely the previous behaviour. The behaviour of defaults in the now transparently overrides the behaviour of defaults in the without requiring changing these function prototypes.