.\" $OpenBSD: DH_set_method.3,v 1.5 2016/12/10 22:22:59 schwarze Exp $ .\" OpenSSL b97fdb57 Nov 11 09:33:09 2016 +0100 .\" .\" This file was written by Ulf Moeller . .\" Copyright (c) 2000, 2002, 2007 The OpenSSL Project. All rights reserved. .\" .\" Redistribution and use in source and binary forms, with or without .\" modification, are permitted provided that the following conditions .\" are met: .\" .\" 1. Redistributions of source code must retain the above copyright .\" notice, this list of conditions and the following disclaimer. .\" .\" 2. Redistributions in binary form must reproduce the above copyright .\" notice, this list of conditions and the following disclaimer in .\" the documentation and/or other materials provided with the .\" distribution. .\" .\" 3. 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IN NO EVENT SHALL THE OpenSSL PROJECT OR .\" ITS CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, .\" SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT .\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; .\" LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) .\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, .\" STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) .\" ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED .\" OF THE POSSIBILITY OF SUCH DAMAGE. .\" .Dd $Mdocdate: December 10 2016 $ .Dt DH_SET_METHOD 3 .Os .Sh NAME .Nm DH_set_default_method , .Nm DH_get_default_method , .Nm DH_set_method , .Nm DH_new_method , .Nm DH_OpenSSL .Nd select DH method .Sh SYNOPSIS .In openssl/dh.h .Ft void .Fo DH_set_default_method .Fa "const DH_METHOD *meth" .Fc .Ft const DH_METHOD * .Fo DH_get_default_method .Fa void .Fc .Ft int .Fo DH_set_method .Fa "DH *dh" .Fa "const DH_METHOD *meth" .Fc .Ft DH * .Fo DH_new_method .Fa "ENGINE *engine" .Fc .Ft const DH_METHOD * .Fo DH_OpenSSL .Fa void .Fc .Sh DESCRIPTION A .Vt 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. See the .Sx CAVEATS section for how these DH API functions are affected by the use of .Xr engine 3 API calls. .Pp Initially, the default .Vt DH_METHOD is the OpenSSL internal implementation as returned by .Fn DH_OpenSSL . .Pp .Fn DH_set_default_method makes .Fa meth the default method for all .Vt DH structures created later. .Sy NB : This is true only whilst no .Vt ENGINE has been set as a default for DH, so this function is no longer recommended. .Pp .Fn DH_get_default_method returns a pointer to the current default .Vt DH_METHOD . However, the meaningfulness of this result is dependent on whether the .Xr engine 3 API is being used, so this function is no longer recommended. .Pp .Fn DH_set_method selects .Fa meth to perform all operations using the key .Fa dh . This will replace the .Vt DH_METHOD used by the .Fa dh key and if the previous method was supplied by an .Vt ENGINE , the handle to that .Vt ENGINE will be released during the change. It is possible to have .Vt DH keys that only work with certain .Vt DH_METHOD implementations (e.g. from an .Vt ENGINE module that supports embedded hardware-protected keys), and in such cases attempting to change the .Vt DH_METHOD for the key can have unexpected results. .Pp .Fn DH_new_method allocates and initializes a .Vt DH structure so that .Fa engine will be used for the DH operations. If .Fa engine is .Dv NULL , the default .Vt ENGINE for DH operations is used and, if no default .Vt ENGINE is set, the .Vt DH_METHOD controlled by .Fn DH_set_default_method is used. .Pp The .Vt DH_METHOD structure is defined as follows: .Bd -literal 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; .Ed .Sh RETURN VALUES .Fn DH_OpenSSL and .Fn DH_get_default_method return pointers to the respective .Sy DH_METHOD Ns s. .Pp .Fn DH_set_method returns non-zero if the provided .Fa meth was successfully set as the method for .Fa dh (including unloading the .Vt ENGINE handle if the previous method was supplied by an .Vt ENGINE ) . .Pp .Fn DH_new_method returns .Dv NULL and sets an error code that can be obtained by .Xr ERR_get_error 3 if the allocation fails. Otherwise it returns a pointer to the newly allocated structure. .Sh SEE ALSO .Xr DH_new 3 .Sh HISTORY .Fn DH_set_default_method , .Fn DH_get_default_method , .Fn DH_set_method , .Fn DH_new_method and .Fn DH_OpenSSL were added in OpenSSL 0.9.4. .Sh CAVEATS As of version 0.9.7, .Vt DH_METHOD implementations are grouped together with other algorithmic APIs (e.g. RSA_METHOD, EVP_CIPHER) in .Vt ENGINE modules. If a default .Vt ENGINE is specified for DH functionality using an .Xr engine 3 API function, that will override any DH defaults set using the DH API .Pq i.e. Fn DH_set_default_method . For this reason, the .Xr engine 3 API is the recommended way to control default implementations for use in DH and other cryptographic algorithms.