SSL_CTX_set_tmp_dh_callback.pod revision 295016 
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
52a new DH key during the negotiation.
53
54As generating DH parameters is extremely time consuming, an application
55should not generate the parameters on the fly but supply the parameters.
56DH parameters can be reused, as the actual key is newly generated during
57the negotiation. The risk in reusing DH parameters is that an attacker
58may specialize on a very often used DH group. Applications should therefore
59generate their own DH parameters during the installation process using the
60openssl L<dhparam(1)|dhparam(1)> application. This application
61guarantees that "strong" primes are used.
62
63Files dh2048.pem, and dh4096.pem in the 'apps' directory of the current
64version of the OpenSSL distribution contain the 'SKIP' DH parameters,
65which use safe primes and were generated verifiably pseudo-randomly.
66These files can be converted into C code using the B<-C> option of the
67L<dhparam(1)|dhparam(1)> application. Generation of custom DH
68parameters during installation should still be preferred to stop an
69attacker from specializing on a commonly used group. Files dh1024.pem
70and dh512.pem contain old parameters that must not be used by
71applications.
72
73An application may either directly specify the DH parameters or
74can supply the DH parameters via a callback function.
75
76Previous versions of the callback used B<is_export> and B<keylength>
77parameters to control parameter generation for export and non-export
78cipher suites. Modern servers that do not support export ciphersuites
79are advised to either use SSL_CTX_set_tmp_dh() or alternatively, use
80the callback but ignore B<keylength> and B<is_export> and simply
81supply at least 2048-bit parameters in the callback.
82
83=head1 EXAMPLES
84
85Setup DH parameters with a key length of 2048 bits. (Error handling
86partly left out.)
87
88 Command-line parameter generation:
89 $ openssl dhparam -out dh_param_2048.pem 2048
90
91 Code for setting up parameters during server initialization:
92
93 ...
94 SSL_CTX ctx = SSL_CTX_new();
95 ...
96
97 /* Set up ephemeral DH parameters. */
98 DH *dh_2048 = NULL;
99 FILE *paramfile;
100 paramfile = fopen("dh_param_2048.pem", "r");
101 if (paramfile) {
102   dh_2048 = PEM_read_DHparams(paramfile, NULL, NULL, NULL);
103   fclose(paramfile);
104 } else {
105   /* Error. */
106 }
107 if (dh_2048 == NULL) {
108  /* Error. */
109 }
110 if (SSL_CTX_set_tmp_dh(ctx, dh_2048) != 1) {
111   /* Error. */
112 }
113 ...
114
115=head1 RETURN VALUES
116
117SSL_CTX_set_tmp_dh_callback() and SSL_set_tmp_dh_callback() do not return
118diagnostic output.
119
120SSL_CTX_set_tmp_dh() and SSL_set_tmp_dh() do return 1 on success and 0
121on failure. Check the error queue to find out the reason of failure.
122
123=head1 SEE ALSO
124
125L<ssl(3)|ssl(3)>, L<SSL_CTX_set_cipher_list(3)|SSL_CTX_set_cipher_list(3)>,
126L<SSL_CTX_set_tmp_rsa_callback(3)|SSL_CTX_set_tmp_rsa_callback(3)>,
127L<SSL_CTX_set_options(3)|SSL_CTX_set_options(3)>,
128L<ciphers(1)|ciphers(1)>, L<dhparam(1)|dhparam(1)>
129
130=cut
131