SSL_CTX_set_tmp_dh_callback.pod revision 279265 
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, int keylength));
13 long SSL_CTX_set_tmp_dh(SSL_CTX *ctx, DH *dh);
14
15 void SSL_set_tmp_dh_callback(SSL *ctx,
16            DH *(*tmp_dh_callback)(SSL *ssl, int is_export, int keylength));
17 long SSL_set_tmp_dh(SSL *ssl, DH *dh)
18
19=head1 DESCRIPTION
20
21SSL_CTX_set_tmp_dh_callback() sets the callback function for B<ctx> to be
22used when a DH parameters are required to B<tmp_dh_callback>.
23The callback is inherited by all B<ssl> objects created from B<ctx>.
24
25SSL_CTX_set_tmp_dh() sets DH parameters to be used to be B<dh>.
26The key is inherited by all B<ssl> objects created from B<ctx>.
27
28SSL_set_tmp_dh_callback() sets the callback only for B<ssl>.
29
30SSL_set_tmp_dh() sets the parameters only for B<ssl>.
31
32These functions apply to SSL/TLS servers only.
33
34=head1 NOTES
35
36When using a cipher with RSA authentication, an ephemeral DH key exchange
37can take place. Ciphers with DSA keys always use ephemeral DH keys as well.
38In these cases, the session data are negotiated using the
39ephemeral/temporary DH key and the key supplied and certified
40by the certificate chain is only used for signing.
41Anonymous ciphers (without a permanent server key) also use ephemeral DH keys.
42
43Using ephemeral DH key exchange yields forward secrecy, as the connection
44can only be decrypted, when the DH key is known. By generating a temporary
45DH key inside the server application that is lost when the application
46is left, it becomes impossible for an attacker to decrypt past sessions,
47even if he gets hold of the normal (certified) key, as this key was
48only used for signing.
49
50In order to perform a DH key exchange the server must use a DH group
51(DH parameters) and generate a DH key. The server will always generate a new
52DH key during the negotiation, when the DH parameters are supplied via
53callback and/or when the SSL_OP_SINGLE_DH_USE option of
54L<SSL_CTX_set_options(3)|SSL_CTX_set_options(3)> is set. It will
55immediately create a DH key, when DH parameters are supplied via
56SSL_CTX_set_tmp_dh() and SSL_OP_SINGLE_DH_USE is not set. In this case,
57it may happen that a key is generated on initialization without later
58being needed, while on the other hand the computer time during the
59negotiation is being saved.
60
61If "strong" primes were used to generate the DH parameters, it is not strictly
62necessary to generate a new key for each handshake but it does improve forward
63secrecy. If it is not assured, that "strong" primes were used (see especially
64the section about DSA parameters below), SSL_OP_SINGLE_DH_USE must be used
65in order to prevent small subgroup attacks. Always using SSL_OP_SINGLE_DH_USE
66has an impact on the computer time needed during negotiation, but it is not
67very large, so application authors/users should consider to always enable
68this option.
69
70As generating DH parameters is extremely time consuming, an application
71should not generate the parameters on the fly but supply the parameters.
72DH parameters can be reused, as the actual key is newly generated during
73the negotiation. The risk in reusing DH parameters is that an attacker
74may specialize on a very often used DH group. Applications should therefore
75generate their own DH parameters during the installation process using the
76openssl L<dhparam(1)|dhparam(1)> application. In order to reduce the computer
77time needed for this generation, it is possible to use DSA parameters
78instead (see L<dhparam(1)|dhparam(1)>), but in this case SSL_OP_SINGLE_DH_USE
79is mandatory.
80
81Application authors may compile in DH parameters. Files dh512.pem,
82dh1024.pem, dh2048.pem, and dh4096.pem in the 'apps' directory of current
83version of the OpenSSL distribution contain the 'SKIP' DH parameters,
84which use safe primes and were generated verifiably pseudo-randomly.
85These files can be converted into C code using the B<-C> option of the
86L<dhparam(1)|dhparam(1)> application.
87Authors may also generate their own set of parameters using
88L<dhparam(1)|dhparam(1)>, but a user may not be sure how the parameters were
89generated. The generation of DH parameters during installation is therefore
90recommended.
91
92An application may either directly specify the DH parameters or
93can supply the DH parameters via a callback function. The callback approach
94has the advantage, that the callback may supply DH parameters for different
95key lengths.
96
97The B<tmp_dh_callback> is called with the B<keylength> needed and
98the B<is_export> information. The B<is_export> flag is set, when the
99ephemeral DH key exchange is performed with an export cipher.
100
101=head1 EXAMPLES
102
103Handle DH parameters for key lengths of 512 and 1024 bits. (Error handling
104partly left out.)
105
106 ...
107 /* Set up ephemeral DH stuff */
108 DH *dh_512 = NULL;
109 DH *dh_1024 = NULL;
110 FILE *paramfile;
111
112 ...
113 /* "openssl dhparam -out dh_param_512.pem -2 512" */
114 paramfile = fopen("dh_param_512.pem", "r");
115 if (paramfile) {
116   dh_512 = PEM_read_DHparams(paramfile, NULL, NULL, NULL);
117   fclose(paramfile);
118 }
119 /* "openssl dhparam -out dh_param_1024.pem -2 1024" */
120 paramfile = fopen("dh_param_1024.pem", "r");
121 if (paramfile) {
122   dh_1024 = PEM_read_DHparams(paramfile, NULL, NULL, NULL);
123   fclose(paramfile);
124 }
125 ...
126
127 /* "openssl dhparam -C -2 512" etc... */
128 DH *get_dh512() { ... }
129 DH *get_dh1024() { ... }
130
131 DH *tmp_dh_callback(SSL *s, int is_export, int keylength)
132 {
133    DH *dh_tmp=NULL;
134
135    switch (keylength) {
136    case 512:
137      if (!dh_512)
138        dh_512 = get_dh512();
139      dh_tmp = dh_512;
140      break;
141    case 1024:
142      if (!dh_1024) 
143        dh_1024 = get_dh1024();
144      dh_tmp = dh_1024;
145      break;
146    default:
147      /* Generating a key on the fly is very costly, so use what is there */
148      setup_dh_parameters_like_above();
149    }
150    return(dh_tmp);
151 }
152
153=head1 RETURN VALUES
154
155SSL_CTX_set_tmp_dh_callback() and SSL_set_tmp_dh_callback() do not return
156diagnostic output.
157
158SSL_CTX_set_tmp_dh() and SSL_set_tmp_dh() do return 1 on success and 0
159on failure. Check the error queue to find out the reason of failure.
160
161=head1 SEE ALSO
162
163L<ssl(3)|ssl(3)>, L<SSL_CTX_set_cipher_list(3)|SSL_CTX_set_cipher_list(3)>,
164L<SSL_CTX_set_tmp_rsa_callback(3)|SSL_CTX_set_tmp_rsa_callback(3)>,
165L<SSL_CTX_set_options(3)|SSL_CTX_set_options(3)>,
166L<ciphers(1)|ciphers(1)>, L<dhparam(1)|dhparam(1)>
167
168=cut
169