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