Initial Commit

openssl-1.0.2f/doc/crypto/DH_set_method.pod

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+=pod
+
+=head1 NAME
+
+DH_set_default_method, DH_get_default_method,
+DH_set_method, DH_new_method, DH_OpenSSL - select DH method
+
+=head1 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);
+
+=head1 DESCRIPTION
+
+A B<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. IMPORTANT: See the NOTES section for
+important information about how these DH API functions are affected by the use
+of B<ENGINE> API calls.
+
+Initially, the default DH_METHOD is the OpenSSL internal implementation, as
+returned by DH_OpenSSL().
+
+DH_set_default_method() makes B<meth> the default method for all DH
+structures created later. B<NB>: This is true only whilst no ENGINE has been set
+as a default for DH, so this function is no longer recommended.
+
+DH_get_default_method() returns a pointer to the current default DH_METHOD.
+However, the meaningfulness of this result is dependent on whether the ENGINE
+API is being used, so this function is no longer recommended.
+
+DH_set_method() selects B<meth> to perform all operations using the key B<dh>.
+This will replace the DH_METHOD used by the DH key and if the previous method
+was supplied by an ENGINE, the handle to that ENGINE will be released during the
+change. It is possible to have DH keys that only work with certain DH_METHOD
+implementations (eg. from an ENGINE module that supports embedded
+hardware-protected keys), 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 so that B<engine> will
+be used for the DH operations. If B<engine> is NULL, the default ENGINE for DH
+operations is used, and if no default ENGINE is set, the DH_METHOD controlled by
+DH_set_default_method() is used.
+
+=head1 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;
+
+=head1 RETURN VALUES
+
+DH_OpenSSL() and DH_get_default_method() return pointers to the respective
+B<DH_METHOD>s.
+
+DH_set_default_method() returns no value.
+
+DH_set_method() returns non-zero if the provided B<meth> was successfully set as
+the method for B<dh> (including unloading the ENGINE handle if the previous
+method was supplied by an ENGINE).
+
+DH_new_method() returns NULL and sets an error code that can be obtained by
+L<ERR_get_error(3)|ERR_get_error(3)> if the allocation fails. Otherwise it
+returns a pointer to the newly allocated structure.
+
+=head1 NOTES
+
+As of version 0.9.7, DH_METHOD implementations are grouped together with other
+algorithmic APIs (eg. RSA_METHOD, EVP_CIPHER, etc) in B<ENGINE> modules. If a
+default ENGINE is specified for DH functionality using an ENGINE API function,
+that will override any DH defaults set using the DH API (ie.
+DH_set_default_method()). For this reason, the ENGINE API is the recommended way
+to control default implementations for use in DH and other cryptographic
+algorithms.
+
+=head1 SEE ALSO
+
+L<dh(3)|dh(3)>, L<DH_new(3)|DH_new(3)>
+
+=head1 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 B<ENGINE>s rather than
+B<DH_METHOD>s during development of the engine version of OpenSSL 0.9.6. For
+0.9.7, the handling of defaults in the ENGINE API 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 ENGINE API now
+transparently overrides the behaviour of defaults in the DH API without
+requiring changing these function prototypes.
+
+=cut