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