5 SSL_CTX_set_tmp_dh_callback, SSL_CTX_set_tmp_dh,
6 SSL_set_tmp_dh_callback, SSL_set_tmp_dh
7 - handle DH keys for ephemeral key exchange
11 #include <openssl/ssl.h>
13 void SSL_CTX_set_tmp_dh_callback(SSL_CTX *ctx,
14 DH *(*tmp_dh_callback)(SSL *ssl, int is_export,
16 long SSL_CTX_set_tmp_dh(SSL_CTX *ctx, DH *dh);
18 void SSL_set_tmp_dh_callback(SSL *ctx,
19 DH *(*tmp_dh_callback)(SSL *ssl, int is_export,
21 long SSL_set_tmp_dh(SSL *ssl, DH *dh);
25 SSL_CTX_set_tmp_dh_callback() sets the callback function for B<ctx> to be
26 used when a DH parameters are required to B<tmp_dh_callback>.
27 The callback is inherited by all B<ssl> objects created from B<ctx>.
29 SSL_CTX_set_tmp_dh() sets DH parameters to be used to be B<dh>.
30 The key is inherited by all B<ssl> objects created from B<ctx>.
32 SSL_set_tmp_dh_callback() sets the callback only for B<ssl>.
34 SSL_set_tmp_dh() sets the parameters only for B<ssl>.
36 These functions apply to SSL/TLS servers only.
40 When using a cipher with RSA authentication, an ephemeral DH key exchange
41 can take place. Ciphers with DSA keys always use ephemeral DH keys as well.
42 In these cases, the session data are negotiated using the
43 ephemeral/temporary DH key and the key supplied and certified
44 by the certificate chain is only used for signing.
45 Anonymous ciphers (without a permanent server key) also use ephemeral DH keys.
47 Using ephemeral DH key exchange yields forward secrecy, as the connection
48 can only be decrypted, when the DH key is known. By generating a temporary
49 DH key inside the server application that is lost when the application
50 is left, it becomes impossible for an attacker to decrypt past sessions,
51 even if he gets hold of the normal (certified) key, as this key was
52 only used for signing.
54 In order to perform a DH key exchange the server must use a DH group
55 (DH parameters) and generate a DH key. The server will always generate
56 a new DH key during the negotiation.
58 As generating DH parameters is extremely time consuming, an application
59 should not generate the parameters on the fly but supply the parameters.
60 DH parameters can be reused, as the actual key is newly generated during
61 the negotiation. The risk in reusing DH parameters is that an attacker
62 may specialize on a very often used DH group. Applications should therefore
63 generate their own DH parameters during the installation process using the
64 openssl L<openssl-dhparam(1)> application. This application
65 guarantees that "strong" primes are used.
67 An application may either directly specify the DH parameters or
68 can supply the DH parameters via a callback function.
70 Previous versions of the callback used B<is_export> and B<keylength>
71 parameters to control parameter generation for export and non-export
72 cipher suites. Modern servers that do not support export cipher suites
73 are advised to either use SSL_CTX_set_tmp_dh() or alternatively, use
74 the callback but ignore B<keylength> and B<is_export> and simply
75 supply at least 2048-bit parameters in the callback.
79 SSL_CTX_set_tmp_dh_callback() and SSL_set_tmp_dh_callback() do not return
82 SSL_CTX_set_tmp_dh() and SSL_set_tmp_dh() do return 1 on success and 0
83 on failure. Check the error queue to find out the reason of failure.
87 Setup DH parameters with a key length of 2048 bits. (Error handling
90 Command-line parameter generation:
92 $ openssl dhparam -out dh_param_2048.pem 2048
94 Code for setting up parameters during server initialization:
96 SSL_CTX ctx = SSL_CTX_new();
99 FILE *paramfile = fopen("dh_param_2048.pem", "r");
102 dh_2048 = PEM_read_DHparams(paramfile, NULL, NULL, NULL);
109 if (SSL_CTX_set_tmp_dh(ctx, dh_2048) != 1)
115 L<ssl(7)>, L<SSL_CTX_set_cipher_list(3)>,
116 L<SSL_CTX_set_options(3)>,
117 L<openssl-ciphers(1)>, L<openssl-dhparam(1)>
121 Copyright 2001-2020 The OpenSSL Project Authors. All Rights Reserved.
123 Licensed under the Apache License 2.0 (the "License"). You may not use
124 this file except in compliance with the License. You can obtain a copy
125 in the file LICENSE in the source distribution or at
126 L<https://www.openssl.org/source/license.html>.