1=pod
2
3=head1 NAME
4
5SSL_CTX_set_tmp_dh_callback, SSL_CTX_set_tmp_dh, SSL_set_tmp_dh_callback, SSL_set_tmp_dh - handle DH keys for ephemeral key exchange
6
7=head1 SYNOPSIS
8
9 #include <openssl/ssl.h>
10
11 void SSL_CTX_set_tmp_dh_callback(SSL_CTX *ctx,
12                                  DH *(*tmp_dh_callback)(SSL *ssl, int is_export,
13                                                         int keylength));
14 long SSL_CTX_set_tmp_dh(SSL_CTX *ctx, DH *dh);
15
16 void SSL_set_tmp_dh_callback(SSL *ctx,
17                              DH *(*tmp_dh_callback)(SSL *ssl, int is_export,
18                                                     int keylength));
19 long SSL_set_tmp_dh(SSL *ssl, DH *dh)
20
21=head1 DESCRIPTION
22
23SSL_CTX_set_tmp_dh_callback() sets the callback function for B<ctx> to be
24used when a DH parameters are required to B<tmp_dh_callback>.
25The callback is inherited by all B<ssl> objects created from B<ctx>.
26
27SSL_CTX_set_tmp_dh() sets DH parameters to be used to be B<dh>.
28The key is inherited by all B<ssl> objects created from B<ctx>.
29
30SSL_set_tmp_dh_callback() sets the callback only for B<ssl>.
31
32SSL_set_tmp_dh() sets the parameters only for B<ssl>.
33
34These functions apply to SSL/TLS servers only.
35
36=head1 NOTES
37
38When using a cipher with RSA authentication, an ephemeral DH key exchange
39can take place. Ciphers with DSA keys always use ephemeral DH keys as well.
40In these cases, the session data are negotiated using the
41ephemeral/temporary DH key and the key supplied and certified
42by the certificate chain is only used for signing.
43Anonymous ciphers (without a permanent server key) also use ephemeral DH keys.
44
45Using ephemeral DH key exchange yields forward secrecy, as the connection
46can only be decrypted, when the DH key is known. By generating a temporary
47DH key inside the server application that is lost when the application
48is left, it becomes impossible for an attacker to decrypt past sessions,
49even if he gets hold of the normal (certified) key, as this key was
50only used for signing.
51
52In order to perform a DH key exchange the server must use a DH group
53(DH parameters) and generate a DH key. The server will always generate
54a new DH key during the negotiation.
55
56As generating DH parameters is extremely time consuming, an application
57should not generate the parameters on the fly but supply the parameters.
58DH parameters can be reused, as the actual key is newly generated during
59the negotiation. The risk in reusing DH parameters is that an attacker
60may specialize on a very often used DH group. Applications should therefore
61generate their own DH parameters during the installation process using the
62openssl L<dhparam(1)> application. This application
63guarantees that "strong" primes are used.
64
65Files dh2048.pem, and dh4096.pem in the 'apps' directory of the current
66version of the OpenSSL distribution contain the 'SKIP' DH parameters,
67which use safe primes and were generated verifiably pseudo-randomly.
68These files can be converted into C code using the B<-C> option of the
69L<dhparam(1)> application. Generation of custom DH
70parameters during installation should still be preferred to stop an
71attacker from specializing on a commonly used group. File dh1024.pem
72contains old parameters that must not be used by applications.
73
74An application may either directly specify the DH parameters or
75can supply the DH parameters via a callback function.
76
77Previous versions of the callback used B<is_export> and B<keylength>
78parameters to control parameter generation for export and non-export
79cipher suites. Modern servers that do not support export cipher suites
80are advised to either use SSL_CTX_set_tmp_dh() or alternatively, use
81the callback but ignore B<keylength> and B<is_export> and simply
82supply at least 2048-bit parameters in the callback.
83
84=head1 EXAMPLES
85
86Setup DH parameters with a key length of 2048 bits. (Error handling
87partly left out.)
88
89Command-line parameter generation:
90
91 $ openssl dhparam -out dh_param_2048.pem 2048
92
93Code for setting up parameters during server initialization:
94
95 SSL_CTX ctx = SSL_CTX_new();
96
97 DH *dh_2048 = NULL;
98 FILE *paramfile = fopen("dh_param_2048.pem", "r");
99
100 if (paramfile) {
101     dh_2048 = PEM_read_DHparams(paramfile, NULL, NULL, NULL);
102     fclose(paramfile);
103 } else {
104     /* Error. */
105 }
106 if (dh_2048 == NULL)
107     /* Error. */
108 if (SSL_CTX_set_tmp_dh(ctx, dh_2048) != 1)
109     /* Error. */
110 ...
111
112=head1 RETURN VALUES
113
114SSL_CTX_set_tmp_dh_callback() and SSL_set_tmp_dh_callback() do not return
115diagnostic output.
116
117SSL_CTX_set_tmp_dh() and SSL_set_tmp_dh() do return 1 on success and 0
118on failure. Check the error queue to find out the reason of failure.
119
120=head1 SEE ALSO
121
122L<ssl(7)>, L<SSL_CTX_set_cipher_list(3)>,
123L<SSL_CTX_set_options(3)>,
124L<ciphers(1)>, L<dhparam(1)>
125
126=head1 COPYRIGHT
127
128Copyright 2001-2016 The OpenSSL Project Authors. All Rights Reserved.
129
130Licensed under the OpenSSL license (the "License").  You may not use
131this file except in compliance with the License.  You can obtain a copy
132in the file LICENSE in the source distribution or at
133L<https://www.openssl.org/source/license.html>.
134
135=cut
136