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, 13 int keylength)); 14 long SSL_CTX_set_tmp_dh(SSL_CTX *ctx, DH *dh); 15 16 void SSL_set_tmp_dh_callback(SSL *ctx, 17 DH *(*tmp_dh_callback)(SSL *ssl, int is_export, 18 int keylength)); 19 long SSL_set_tmp_dh(SSL *ssl, DH *dh) 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 54a new DH key during the negotiation. 55 56As generating DH parameters is extremely time consuming, an application 57should not generate the parameters on the fly but supply the parameters. 58DH parameters can be reused, as the actual key is newly generated during 59the negotiation. The risk in reusing DH parameters is that an attacker 60may specialize on a very often used DH group. Applications should therefore 61generate their own DH parameters during the installation process using the 62openssl L<dhparam(1)> application. This application 63guarantees that "strong" primes are used. 64 65Files dh2048.pem, and dh4096.pem in the 'apps' directory of the current 66version of the OpenSSL distribution contain the 'SKIP' DH parameters, 67which use safe primes and were generated verifiably pseudo-randomly. 68These files can be converted into C code using the B<-C> option of the 69L<dhparam(1)> application. Generation of custom DH 70parameters during installation should still be preferred to stop an 71attacker from specializing on a commonly used group. File dh1024.pem 72contains old parameters that must not be used by applications. 73 74An application may either directly specify the DH parameters or 75can supply the DH parameters via a callback function. 76 77Previous versions of the callback used B<is_export> and B<keylength> 78parameters to control parameter generation for export and non-export 79cipher suites. Modern servers that do not support export cipher suites 80are advised to either use SSL_CTX_set_tmp_dh() or alternatively, use 81the callback but ignore B<keylength> and B<is_export> and simply 82supply at least 2048-bit parameters in the callback. 83 84=head1 RETURN VALUES 85 86SSL_CTX_set_tmp_dh_callback() and SSL_set_tmp_dh_callback() do not return 87diagnostic output. 88 89SSL_CTX_set_tmp_dh() and SSL_set_tmp_dh() do return 1 on success and 0 90on failure. Check the error queue to find out the reason of failure. 91 92=head1 EXAMPLES 93 94Setup DH parameters with a key length of 2048 bits. (Error handling 95partly left out.) 96 97Command-line parameter generation: 98 99 $ openssl dhparam -out dh_param_2048.pem 2048 100 101Code for setting up parameters during server initialization: 102 103 SSL_CTX ctx = SSL_CTX_new(); 104 105 DH *dh_2048 = NULL; 106 FILE *paramfile = fopen("dh_param_2048.pem", "r"); 107 108 if (paramfile) { 109 dh_2048 = PEM_read_DHparams(paramfile, NULL, NULL, NULL); 110 fclose(paramfile); 111 } else { 112 /* Error. */ 113 } 114 if (dh_2048 == NULL) 115 /* Error. */ 116 if (SSL_CTX_set_tmp_dh(ctx, dh_2048) != 1) 117 /* Error. */ 118 ... 119 120=head1 SEE ALSO 121 122L<ssl(7)>, L<SSL_CTX_set_cipher_list(3)>, 123L<SSL_CTX_set_options(3)>, 124L<ciphers(1)>, L<dhparam(1)> 125 126=head1 COPYRIGHT 127 128Copyright 2001-2019 The OpenSSL Project Authors. All Rights Reserved. 129 130Licensed under the OpenSSL license (the "License"). You may not use 131this file except in compliance with the License. You can obtain a copy 132in the file LICENSE in the source distribution or at 133L<https://www.openssl.org/source/license.html>. 134 135=cut 136