SSL_CTX_set_tmp_dh_callback.pod revision 89838 
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 *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 DH *(*tmp_dh_callback)(SSL *ssl, int is_export, int keylength));
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 a new
54DH key during the negotiation, when the DH parameters are supplied via
55callback and/or when the SSL_OP_SINGLE_DH_USE option of
56L<SSL_CTX_set_options(3)|SSL_CTX_set_options(3)> is set. It will
57immediately create a DH key, when DH parameters are supplied via
58SSL_CTX_set_tmp_dh() and SSL_OP_SINGLE_DH_USE is not set. In this case,
59it may happen that a key is generated on initialization without later
60being needed, while on the other hand the computer time during the
61negotiation is being saved.
62
63If "strong" primes were used to generate the DH parameters, it is not strictly
64necessary to generate a new key for each handshake but it does improve forward
65secrecy. If it is not assured, that "strong" primes were used (see especially
66the section about DSA parameters below), SSL_OP_SINGLE_DH_USE must be used
67in order to prevent small subgroup attacks. Always using SSL_OP_SINGLE_DH_USE
68has an impact on the computer time needed during negotiation, but it is not
69very large, so application authors/users should consider to always enable
70this option.
71
72As generating DH parameters is extremely time consuming, an application
73should not generate the parameters on the fly but supply the parameters.
74DH parameters can be reused, as the actual key is newly generated during
75the negotiation. The risk in reusing DH parameters is that an attacker
76may specialize on a very often used DH group. Applications should therefore
77generate their own DH parameters during the installation process using the
78openssl L<dhparam(1)|dhparam(1)> application. In order to reduce the computer
79time needed for this generation, it is possible to use DSA parameters
80instead (see L<dhparam(1)|dhparam(1)>), but in this case SSL_OP_SINGLE_DH_USE
81is mandatory.
82
83Application authors may compile in DH parameters. Files dh512.pem,
84dh1024.pem, dh2048.pem, and dh4096 in the 'apps' directory of current
85version of the OpenSSL distribution contain the 'SKIP' DH parameters,
86which use safe primes and were generated verifiably pseudo-randomly.
87These files can be converted into C code using the B<-C> option of the
88L<dhparam(1)|dhparam(1)> application.
89Authors may also generate their own set of parameters using
90L<dhparam(1)|dhparam(1)>, but a user may not be sure how the parameters were
91generated. The generation of DH parameters during installation is therefore
92recommended.
93
94An application may either directly specify the DH parameters or
95can supply the DH parameters via a callback function. The callback approach
96has the advantage, that the callback may supply DH parameters for different
97key lengths.
98
99The B<tmp_dh_callback> is called with the B<keylength> needed and
100the B<is_export> information. The B<is_export> flag is set, when the
101ephemeral DH key exchange is performed with an export cipher.
102
103=head1 EXAMPLES
104
105Handle DH parameters for key lengths of 512 and 1024 bits. (Error handling
106partly left out.)
107
108 ...
109 /* Set up ephemeral DH stuff */
110 DH *dh_512 = NULL;
111 DH *dh_1024 = NULL;
112 FILE *paramfile;
113
114 ...
115 /* "openssl dhparam -out dh_param_512.pem -2 512" */
116 paramfile = fopen("dh_param_512.pem", "r");
117 if (paramfile) {
118   dh_512 = PEM_read_DHparams(paramfile, NULL, NULL, NULL);
119   fclose(paramfile);
120 }
121 /* "openssl dhparam -out dh_param_1024.pem -2 1024" */
122 paramfile = fopen("dh_param_1024.pem", "r");
123 if (paramfile) {
124   dh_1024 = PEM_read_DHparams(paramfile, NULL, NULL, NULL);
125   fclose(paramfile);
126 }
127 ...
128
129 /* "openssl dhparam -C -2 512" etc... */
130 DH *get_dh512() { ... }
131 DH *get_dh1024() { ... }
132
133 DH *tmp_dh_callback(SSL *s, int is_export, int keylength)
134 {
135    DH *dh_tmp=NULL;
136
137    switch (keylength) {
138    case 512:
139      if (!dh_512)
140        dh_512 = get_dh512();
141      dh_tmp = dh_512;
142      break;
143    case 1024:
144      if (!dh_1024) 
145        dh_1024 = get_dh1024();
146      dh_tmp = dh_1024;
147      break;
148    default:
149      /* Generating a key on the fly is very costly, so use what is there */
150      setup_dh_parameters_like_above();
151    }
152    return(dh_tmp);
153 }
154
155=head1 RETURN VALUES
156
157SSL_CTX_set_tmp_dh_callback() and SSL_set_tmp_dh_callback() do not return
158diagnostic output.
159
160SSL_CTX_set_tmp_dh() and SSL_set_tmp_dh() do return 1 on success and 0
161on failure. Check the error queue to find out the reason of failure.
162
163=head1 SEE ALSO
164
165L<ssl(3)|ssl(3)>, L<SSL_CTX_set_cipher_list(3)|SSL_CTX_set_cipher_list(3)>,
166L<SSL_CTX_set_tmp_rsa_callback(3)|SSL_CTX_set_tmp_rsa_callback(3)>,
167L<SSL_CTX_set_options(3)|SSL_CTX_set_options(3)>,
168L<ciphers(1)|ciphers(1)>, L<dhparam(1)|dhparam(1)>
169
170=cut
171