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. 52The server will always generate a new DH key during the negotiation 53if either the DH parameters are supplied via callback or the 54SSL_OP_SINGLE_DH_USE option of SSL_CTX_set_options(3) is set (or both). 55It will immediately create a DH key if DH parameters are supplied via 56SSL_CTX_set_tmp_dh() and SSL_OP_SINGLE_DH_USE is not set. 57In this case, 58it may happen that a key is generated on initialization without later 59being needed, while on the other hand the computer time during the 60negotiation is being saved. 61 62If "strong" primes were used to generate the DH parameters, it is not strictly 63necessary to generate a new key for each handshake but it does improve forward 64secrecy. If it is not assured that "strong" primes were used, 65SSL_OP_SINGLE_DH_USE must be used in order to prevent small subgroup 66attacks. Always using SSL_OP_SINGLE_DH_USE has an impact on the 67computer time needed during negotiation, but it is not very large, so 68application authors/users should consider always enabling this option. 69The option is required to implement perfect forward secrecy (PFS). 70 71As generating DH parameters is extremely time consuming, an application 72should not generate the parameters on the fly but supply the parameters. 73DH parameters can be reused, as the actual key is newly generated during 74the negotiation. The risk in reusing DH parameters is that an attacker 75may specialize on a very often used DH group. Applications should therefore 76generate their own DH parameters during the installation process using the 77openssl L<dhparam(1)|dhparam(1)> application. This application 78guarantees that "strong" primes are used. 79 80Files dh2048.pem, and dh4096.pem in the 'apps' directory of the current 81version of the OpenSSL distribution contain the 'SKIP' DH parameters, 82which use safe primes and were generated verifiably pseudo-randomly. 83These files can be converted into C code using the B<-C> option of the 84L<dhparam(1)|dhparam(1)> application. Generation of custom DH 85parameters during installation should still be preferred to stop an 86attacker from specializing on a commonly used group. Files dh1024.pem 87and dh512.pem contain old parameters that must not be used by 88applications. 89 90An application may either directly specify the DH parameters or 91can supply the DH parameters via a callback function. 92 93Previous versions of the callback used B<is_export> and B<keylength> 94parameters to control parameter generation for export and non-export 95cipher suites. Modern servers that do not support export ciphersuites 96are advised to either use SSL_CTX_set_tmp_dh() in combination with 97SSL_OP_SINGLE_DH_USE, or alternatively, use the callback but ignore 98B<keylength> and B<is_export> and simply supply at least 2048-bit 99parameters in the callback. 100 101=head1 EXAMPLES 102 103Setup DH parameters with a key length of 2048 bits. (Error handling 104partly left out.) 105 106 Command-line parameter generation: 107 $ openssl dhparam -out dh_param_2048.pem 2048 108 109 Code for setting up parameters during server initialization: 110 111 ... 112 SSL_CTX ctx = SSL_CTX_new(); 113 ... 114 115 /* Set up ephemeral DH parameters. */ 116 DH *dh_2048 = NULL; 117 FILE *paramfile; 118 paramfile = fopen("dh_param_2048.pem", "r"); 119 if (paramfile) { 120 dh_2048 = PEM_read_DHparams(paramfile, NULL, NULL, NULL); 121 fclose(paramfile); 122 } else { 123 /* Error. */ 124 } 125 if (dh_2048 == NULL) { 126 /* Error. */ 127 } 128 if (SSL_CTX_set_tmp_dh(ctx, dh_2048) != 1) { 129 /* Error. */ 130 } 131 SSL_CTX_set_options(ctx, SSL_OP_SINGLE_DH_USE); 132 ... 133 134=head1 RETURN VALUES 135 136SSL_CTX_set_tmp_dh_callback() and SSL_set_tmp_dh_callback() do not return 137diagnostic output. 138 139SSL_CTX_set_tmp_dh() and SSL_set_tmp_dh() do return 1 on success and 0 140on failure. Check the error queue to find out the reason of failure. 141 142=head1 SEE ALSO 143 144L<ssl(3)|ssl(3)>, L<SSL_CTX_set_cipher_list(3)|SSL_CTX_set_cipher_list(3)>, 145L<SSL_CTX_set_tmp_rsa_callback(3)|SSL_CTX_set_tmp_rsa_callback(3)>, 146L<SSL_CTX_set_options(3)|SSL_CTX_set_options(3)>, 147L<ciphers(1)|ciphers(1)>, L<dhparam(1)|dhparam(1)> 148 149=cut 150