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1 .\" $OpenBSD: SSL_CTX_set_tmp_dh_callback.3,v 1.2 2016/12/01 21:53:42 schwarze Exp $
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51 .Dd $Mdocdate: December 1 2016 $
52 .Dt SSL_CTX_SET_TMP_DH_CALLBACK 3
53 .Os
54 .Sh NAME
55 .Nm SSL_CTX_set_tmp_dh_callback ,
56 .Nm SSL_CTX_set_tmp_dh ,
57 .Nm SSL_set_tmp_dh_callback ,
58 .Nm SSL_set_tmp_dh
59 .Nd handle DH keys for ephemeral key exchange
60 .Sh SYNOPSIS
61 .In openssl/ssl.h
62 .Ft void
63 .Fo SSL_CTX_set_tmp_dh_callback
64 .Fa "SSL_CTX *ctx"
65 .Fa "DH *(*tmp_dh_callback)(SSL *ssl, int is_export, int keylength)"
66 .Fc
67 .Ft long
68 .Fn SSL_CTX_set_tmp_dh "SSL_CTX *ctx" "DH *dh"
69 .Ft void
70 .Fo SSL_set_tmp_dh_callback
71 .Fa "SSL *ssl"
72 .Fa "DH *(*tmp_dh_callback)(SSL *ssl, int is_export, int keylength"
73 .Fc
74 .Ft long
75 .Fn SSL_set_tmp_dh "SSL *ssl" "DH *dh"
76 .Sh DESCRIPTION
77 .Fn SSL_CTX_set_tmp_dh_callback
78 sets the callback function for
79 .Fa ctx
80 to be used when a DH parameters are required to
81 .Fa tmp_dh_callback .
82 The callback is inherited by all
83 .Vt ssl
84 objects created from
85 .Fa ctx .
86 .Pp
87 .Fn SSL_CTX_set_tmp_dh
88 sets DH parameters to be used to be
89 .Sy dh Ns .
90 The key is inherited by all
91 .Fa ssl
92 objects created from
93 .Fa ctx .
94 .Pp
95 .Fn SSL_set_tmp_dh_callback
96 sets the callback only for
97 .Fa ssl .
98 .Pp
99 .Fn SSL_set_tmp_dh
100 sets the parameters only for
101 .Fa ssl .
102 .Pp
103 These functions apply to SSL/TLS servers only.
104 .Pp
105 When using a cipher with RSA authentication,
106 an ephemeral DH key exchange can take place.
107 Ciphers with DSA keys always use ephemeral DH keys as well.
108 In these cases, the session data are negotiated using the ephemeral/temporary
109 DH key and the key supplied and certified by the certificate chain is only used
110 for signing.
111 Anonymous ciphers (without a permanent server key) also use ephemeral DH keys.
112 .Pp
113 Using ephemeral DH key exchange yields forward secrecy,
114 as the connection can only be decrypted when the DH key is known.
115 By generating a temporary DH key inside the server application that is lost
116 when the application is left, it becomes impossible for an attacker to decrypt
117 past sessions, even if he gets hold of the normal (certified) key,
118 as this key was only used for signing.
119 .Pp
120 In order to perform a DH key exchange the server must use a DH group
121 (DH parameters) and generate a DH key.
122 The server will always generate a new DH key during the negotiation.
123 .Pp
124 As generating DH parameters is extremely time consuming, an application should
125 not generate the parameters on the fly but supply the parameters.
126 DH parameters can be reused,
127 as the actual key is newly generated during the negotiation.
128 The risk in reusing DH parameters is that an attacker may specialize on a very
129 often used DH group.
130 Applications should therefore generate their own DH parameters during the
131 installation process using the
132 .Xr openssl 1
133 .Cm dhparam
134 application.
135 This application guarantees that "strong" primes are used.
136 .Pp
137 Files
138 .Pa dh2048.pem
139 and
140 .Pa dh4096.pem
141 in the
142 .Pa apps
143 directory of the current version of the OpenSSL distribution contain the
144 .Sq SKIP
145 DH parameters,
146 which use safe primes and were generated verifiably pseudo-randomly.
147 These files can be converted into C code using the
148 .Fl C
149 option of the
150 .Xr openssl 1
151 .Cm dhparam
152 application.
153 Generation of custom DH parameters during installation should still
154 be preferred to stop an attacker from specializing on a commonly
155 used group.
156 The file
157 .Pa dh1024.pem
158 contains old parameters that must not be used by applications.
159 .Pp
160 An application may either directly specify the DH parameters or can supply the
161 DH parameters via a callback function.
162 .Pp
163 Previous versions of the callback used
164 .Fa is_export
165 and
166 .Fa keylength
167 parameters to control parameter generation for export and non-export
168 cipher suites.
169 Modern servers that do not support export ciphersuites are advised
170 to either use
171 .Fn SSL_CTX_set_tmp_dh
172 or alternatively, use the callback but ignore
173 .Fa keylength
174 and
175 .Fa is_export
176 and simply supply at least 2048-bit parameters in the callback.
177 .Sh RETURN VALUES
178 .Fn SSL_CTX_set_tmp_dh_callback
179 and
180 .Fn SSL_set_tmp_dh_callback
181 do not return diagnostic output.
182 .Pp
183 .Fn SSL_CTX_set_tmp_dh
184 and
185 .Fn SSL_set_tmp_dh
186 do return 1 on success and 0 on failure.
187 Check the error queue to find out the reason of failure.
188 .Sh EXAMPLES
189 Set up DH parameters with a key length of 2048 bits.
190 Error handling is partly left out.
191 .Pp
192 Command-line parameter generation:
193 .Pp
194 .Dl openssl dhparam -out dh_param_2048.pem 2048
195 .Pp
196 Code for setting up parameters during server initialization:
197 .Bd -literal
198 SSL_CTX ctx = SSL_CTX_new();
199 \&...
201 /* Set up ephemeral DH parameters. */
202 DH *dh_2048 = NULL;
203 FILE *paramfile;
204 paramfile = fopen("dh_param_2048.pem", "r");
205 if (paramfile) {
206 dh_2048 = PEM_read_DHparams(paramfile, NULL, NULL, NULL);
207 fclose(paramfile);
208 } else {
209 /* Error. */
210 }
211 if (dh_2048 == NULL) {
212 /* Error. */
213 }
214 if (SSL_CTX_set_tmp_dh(ctx, dh_2048) != 1) {
215 /* Error. */
216 }
217 .Ed
218 .Sh SEE ALSO
219 .Xr openssl 1 ,
220 .Xr ssl 3 ,
221 .Xr SSL_CTX_set_cipher_list 3 ,
222 .Xr SSL_CTX_set_options 3