1.\" $OpenBSD: DH_set_method.3,v 1.6 2018/03/22 16:06:33 schwarze Exp $ 2.\" OpenSSL b97fdb57 Nov 11 09:33:09 2016 +0100 3.\" 4.\" This file was written by Ulf Moeller <ulf@openssl.org>. 5.\" Copyright (c) 2000, 2002, 2007 The OpenSSL Project. All rights reserved. 6.\" 7.\" Redistribution and use in source and binary forms, with or without 8.\" modification, are permitted provided that the following conditions 9.\" are met: 10.\" 11.\" 1. Redistributions of source code must retain the above copyright 12.\" notice, this list of conditions and the following disclaimer. 13.\" 14.\" 2. Redistributions in binary form must reproduce the above copyright 15.\" notice, this list of conditions and the following disclaimer in 16.\" the documentation and/or other materials provided with the 17.\" distribution. 18.\" 19.\" 3. 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Redistributions of any form whatsoever must retain the following 34.\" acknowledgment: 35.\" "This product includes software developed by the OpenSSL Project 36.\" for use in the OpenSSL Toolkit (http://www.openssl.org/)" 37.\" 38.\" THIS SOFTWARE IS PROVIDED BY THE OpenSSL PROJECT ``AS IS'' AND ANY 39.\" EXPRESSED OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 40.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR 41.\" PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE OpenSSL PROJECT OR 42.\" ITS CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, 43.\" SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT 44.\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; 45.\" LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 46.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, 47.\" STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) 48.\" ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED 49.\" OF THE POSSIBILITY OF SUCH DAMAGE. 50.\" 51.Dd $Mdocdate: March 22 2018 $ 52.Dt DH_SET_METHOD 3 53.Os 54.Sh NAME 55.Nm DH_set_default_method , 56.Nm DH_get_default_method , 57.Nm DH_set_method , 58.Nm DH_new_method , 59.Nm DH_OpenSSL 60.Nd select DH method 61.Sh SYNOPSIS 62.In openssl/dh.h 63.Ft void 64.Fo DH_set_default_method 65.Fa "const DH_METHOD *meth" 66.Fc 67.Ft const DH_METHOD * 68.Fo DH_get_default_method 69.Fa void 70.Fc 71.Ft int 72.Fo DH_set_method 73.Fa "DH *dh" 74.Fa "const DH_METHOD *meth" 75.Fc 76.Ft DH * 77.Fo DH_new_method 78.Fa "ENGINE *engine" 79.Fc 80.Ft const DH_METHOD * 81.Fo DH_OpenSSL 82.Fa void 83.Fc 84.Sh DESCRIPTION 85A 86.Vt DH_METHOD 87specifies the functions that OpenSSL uses for Diffie-Hellman operations. 88By modifying the method, alternative implementations such as hardware 89accelerators may be used. 90See the 91.Sx CAVEATS 92section for how these DH API functions are affected by the use of 93.Xr engine 3 94API calls. 95.Pp 96Initially, the default 97.Vt DH_METHOD 98is the OpenSSL internal implementation as returned by 99.Fn DH_OpenSSL . 100.Pp 101.Fn DH_set_default_method 102makes 103.Fa meth 104the default method for all 105.Vt DH 106structures created later. 107.Sy NB : 108This is true only whilst no 109.Vt ENGINE 110has been set as a default for DH, so this function is no longer 111recommended. 112.Pp 113.Fn DH_get_default_method 114returns a pointer to the current default 115.Vt DH_METHOD . 116However, the meaningfulness of this result is dependent on whether the 117.Xr engine 3 118API is being used, so this function is no longer recommended. 119.Pp 120.Fn DH_set_method 121selects 122.Fa meth 123to perform all operations using the key 124.Fa dh . 125This will replace the 126.Vt DH_METHOD 127used by the 128.Fa dh 129key and if the previous method was supplied by an 130.Vt ENGINE , 131the handle to that 132.Vt ENGINE 133will be released during the change. 134It is possible to have 135.Vt DH 136keys that only work with certain 137.Vt DH_METHOD 138implementations (e.g. from an 139.Vt ENGINE 140module that supports embedded hardware-protected keys), 141and in such cases attempting to change the 142.Vt DH_METHOD 143for the key can have unexpected results. 144.Pp 145.Fn DH_new_method 146allocates and initializes a 147.Vt DH 148structure so that 149.Fa engine 150will be used for the DH operations. 151If 152.Fa engine 153is 154.Dv NULL , 155the default 156.Vt ENGINE 157for DH operations is used and, if no default 158.Vt ENGINE 159is set, the 160.Vt DH_METHOD 161controlled by 162.Fn DH_set_default_method 163is used. 164.Pp 165The 166.Vt DH_METHOD 167structure is defined as follows: 168.Bd -literal 169typedef struct dh_meth_st 170{ 171 /* name of the implementation */ 172 const char *name; 173 174 /* generate private and public DH values for key agreement */ 175 int (*generate_key)(DH *dh); 176 177 /* compute shared secret */ 178 int (*compute_key)(unsigned char *key, BIGNUM *pub_key, DH *dh); 179 180 /* compute r = a ^ p mod m (May be NULL for some implementations) */ 181 int (*bn_mod_exp)(DH *dh, BIGNUM *r, BIGNUM *a, const BIGNUM *p, 182 const BIGNUM *m, BN_CTX *ctx, 183 BN_MONT_CTX *m_ctx); 184 185 /* called at DH_new */ 186 int (*init)(DH *dh); 187 188 /* called at DH_free */ 189 int (*finish)(DH *dh); 190 191 int flags; 192 193 char *app_data; /* ?? */ 194 195} DH_METHOD; 196.Ed 197.Sh RETURN VALUES 198.Fn DH_OpenSSL 199and 200.Fn DH_get_default_method 201return pointers to the respective 202.Sy DH_METHOD Ns s. 203.Pp 204.Fn DH_set_method 205returns non-zero if the provided 206.Fa meth 207was successfully set as the method for 208.Fa dh 209(including unloading the 210.Vt ENGINE 211handle if the previous method was supplied by an 212.Vt ENGINE ) . 213.Pp 214.Fn DH_new_method 215returns 216.Dv NULL 217and sets an error code that can be obtained by 218.Xr ERR_get_error 3 219if the allocation fails. 220Otherwise it returns a pointer to the newly allocated structure. 221.Sh SEE ALSO 222.Xr DH_new 3 223.Sh HISTORY 224.Fn DH_set_default_method , 225.Fn DH_get_default_method , 226.Fn DH_set_method , 227.Fn DH_new_method 228and 229.Fn DH_OpenSSL 230first appeared in OpenSSL 0.9.5 and have been available since 231.Ox 2.7 . 232.Sh CAVEATS 233As of version 0.9.7, 234.Vt DH_METHOD 235implementations are grouped together with other algorithmic APIs 236(e.g. RSA_METHOD, EVP_CIPHER) in 237.Vt ENGINE 238modules. 239If a default 240.Vt ENGINE 241is specified for DH functionality using an 242.Xr engine 3 243API function, that will override any DH defaults set using the DH API 244.Pq i.e. Fn DH_set_default_method . 245For this reason, the 246.Xr engine 3 247API is the recommended way to control default implementations 248for use in DH and other cryptographic algorithms. 249