1=pod 2 3=head1 NAME 4 5DH_set_default_method, DH_get_default_method, 6DH_set_method, DH_new_method, DH_OpenSSL - select DH method 7 8=head1 SYNOPSIS 9 10 #include <openssl/dh.h> 11 #include <openssl/engine.h> 12 13 void DH_set_default_method(const DH_METHOD *meth); 14 15 const DH_METHOD *DH_get_default_method(void); 16 17 int DH_set_method(DH *dh, const DH_METHOD *meth); 18 19 DH *DH_new_method(ENGINE *engine); 20 21 const DH_METHOD *DH_OpenSSL(void); 22 23=head1 DESCRIPTION 24 25A B<DH_METHOD> specifies the functions that OpenSSL uses for Diffie-Hellman 26operations. By modifying the method, alternative implementations 27such as hardware accelerators may be used. IMPORTANT: See the NOTES section for 28important information about how these DH API functions are affected by the use 29of B<ENGINE> API calls. 30 31Initially, the default DH_METHOD is the OpenSSL internal implementation, as 32returned by DH_OpenSSL(). 33 34DH_set_default_method() makes B<meth> the default method for all DH 35structures created later. B<NB>: This is true only whilst no ENGINE has been set 36as a default for DH, so this function is no longer recommended. 37 38DH_get_default_method() returns a pointer to the current default DH_METHOD. 39However, the meaningfulness of this result is dependent on whether the ENGINE 40API is being used, so this function is no longer recommended. 41 42DH_set_method() selects B<meth> to perform all operations using the key B<dh>. 43This will replace the DH_METHOD used by the DH key and if the previous method 44was supplied by an ENGINE, the handle to that ENGINE will be released during the 45change. It is possible to have DH keys that only work with certain DH_METHOD 46implementations (eg. from an ENGINE module that supports embedded 47hardware-protected keys), and in such cases attempting to change the DH_METHOD 48for the key can have unexpected results. 49 50DH_new_method() allocates and initializes a DH structure so that B<engine> will 51be used for the DH operations. If B<engine> is NULL, the default ENGINE for DH 52operations is used, and if no default ENGINE is set, the DH_METHOD controlled by 53DH_set_default_method() is used. 54 55=head1 THE DH_METHOD STRUCTURE 56 57 typedef struct dh_meth_st 58 { 59 /* name of the implementation */ 60 const char *name; 61 62 /* generate private and public DH values for key agreement */ 63 int (*generate_key)(DH *dh); 64 65 /* compute shared secret */ 66 int (*compute_key)(unsigned char *key, BIGNUM *pub_key, DH *dh); 67 68 /* compute r = a ^ p mod m (May be NULL for some implementations) */ 69 int (*bn_mod_exp)(DH *dh, BIGNUM *r, BIGNUM *a, const BIGNUM *p, 70 const BIGNUM *m, BN_CTX *ctx, 71 BN_MONT_CTX *m_ctx); 72 73 /* called at DH_new */ 74 int (*init)(DH *dh); 75 76 /* called at DH_free */ 77 int (*finish)(DH *dh); 78 79 int flags; 80 81 char *app_data; /* ?? */ 82 83 } DH_METHOD; 84 85=head1 RETURN VALUES 86 87DH_OpenSSL() and DH_get_default_method() return pointers to the respective 88B<DH_METHOD>s. 89 90DH_set_default_method() returns no value. 91 92DH_set_method() returns non-zero if the provided B<meth> was successfully set as 93the method for B<dh> (including unloading the ENGINE handle if the previous 94method was supplied by an ENGINE). 95 96DH_new_method() returns NULL and sets an error code that can be obtained by 97L<ERR_get_error(3)|ERR_get_error(3)> if the allocation fails. Otherwise it 98returns a pointer to the newly allocated structure. 99 100=head1 NOTES 101 102As of version 0.9.7, DH_METHOD implementations are grouped together with other 103algorithmic APIs (eg. RSA_METHOD, EVP_CIPHER, etc) in B<ENGINE> modules. If a 104default ENGINE is specified for DH functionality using an ENGINE API function, 105that will override any DH defaults set using the DH API (ie. 106DH_set_default_method()). For this reason, the ENGINE API is the recommended way 107to control default implementations for use in DH and other cryptographic 108algorithms. 109 110=head1 SEE ALSO 111 112L<dh(3)|dh(3)>, L<DH_new(3)|DH_new(3)> 113 114=head1 HISTORY 115 116DH_set_default_method(), DH_get_default_method(), DH_set_method(), 117DH_new_method() and DH_OpenSSL() were added in OpenSSL 0.9.4. 118 119DH_set_default_openssl_method() and DH_get_default_openssl_method() replaced 120DH_set_default_method() and DH_get_default_method() respectively, and 121DH_set_method() and DH_new_method() were altered to use B<ENGINE>s rather than 122B<DH_METHOD>s during development of the engine version of OpenSSL 0.9.6. For 1230.9.7, the handling of defaults in the ENGINE API was restructured so that this 124change was reversed, and behaviour of the other functions resembled more closely 125the previous behaviour. The behaviour of defaults in the ENGINE API now 126transparently overrides the behaviour of defaults in the DH API without 127requiring changing these function prototypes. 128 129=cut 130