Loading doc/ssl/SSL_CTX_set_cipher_list.pod +5 −2 Original line number Diff line number Diff line Loading @@ -39,8 +39,9 @@ certificate and key. A RSA cipher can only be chosen, when a RSA certificate is available. RSA export ciphers with a keylength of 512 bits for the RSA key require a temporary 512 bit RSA key, as typically the supplied key has a length of 1024 bit. RSA ciphers using EDH need a certificate and key and additional DH-parameters. of 1024 bit (see L<SSL_CTX_set_tmp_rsa_callback(3)|SSL_CTX_set_tmp_rsa_callback(3)>). RSA ciphers using EDH need a certificate and key and additional DH-parameters. A DSA cipher can only be chosen, when a DSA certificate is available. DSA ciphers always use DH key exchange and therefore need DH-parameters. Loading @@ -60,6 +61,8 @@ could be selected and 0 on complete failure. L<ssl(3)|ssl(3)>, L<SSL_get_ciphers(3)|SSL_get_ciphers(3)>, L<SSL_CTX_use_certificate(3)|SSL_CTX_use_certificate(3)>, L<SSL_CTX_set_tmp_rsa_callback(3)|SSL_CTX_set_tmp_rsa_callback(3)>, L<SSL_CTX_set_tmp_dh_callback(3)|SSL_CTX_set_tmp_dh_callback(3)>, L<ciphers(1)|ciphers(1)> =cut doc/ssl/SSL_CTX_set_options.pod +7 −3 Original line number Diff line number Diff line Loading @@ -122,11 +122,13 @@ The following B<modifying> options are available: =item SSL_OP_SINGLE_DH_USE Always create a new key when using temporary DH parameters. Always create a new key when using temporary DH parameters (see L<SSL_CTX_set_tmp_dh_callback(3)|SSL_CTX_set_tmp_dh_callback(3)>). =item SSL_OP_EPHEMERAL_RSA Also use the temporary RSA key when doing RSA operations. Also use ephemeral (temporary) RSA key when doing RSA operations (see L<SSL_CTX_set_tmp_rsa_callback(3)|SSL_CTX_set_tmp_rsa_callback(3)>). =item SSL_OP_CIPHER_SERVER_PREFERENCE Loading Loading @@ -182,7 +184,9 @@ SSL_CTX_get_options() and SSL_get_options() return the current bitmask. =head1 SEE ALSO L<ssl(3)|ssl(3)>, L<SSL_new(3)|SSL_new(3)>, L<SSL_clear(3)|SSL_clear(3)> L<ssl(3)|ssl(3)>, L<SSL_new(3)|SSL_new(3)>, L<SSL_clear(3)|SSL_clear(3)>, L<SSL_CTX_set_tmp_dh_callback(3)|SSL_CTX_set_tmp_dh_callback(3)>, L<SSL_CTX_set_tmp_rsa_callback(3)|SSL_CTX_set_tmp_rsa_callback(3)> =head1 HISTORY Loading doc/ssl/SSL_CTX_set_tmp_dh_callback.pod 0 → 100644 +146 −0 Original line number Diff line number Diff line =pod =head1 NAME 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 =head1 SYNOPSIS #include <openssl/ssl.h> void SSL_CTX_set_tmp_dh_callback(SSL_CTX *ctx, DH *(*tmp_dh_callback)(SSL *ssl, int is_export, int keylength)); long SSL_CTX_set_tmp_dh(SSL_CTX *ctx, DH *dh); void SSL_set_tmp_dh_callback(SSL_CTX *ctx, DH *(*tmp_dh_callback)(SSL *ssl, int is_export, int keylength)); long SSL_set_tmp_dh(SSL *ssl, DH *dh) DH *(*tmp_dh_callback)(SSL *ssl, int is_export, int keylength)); =head1 DESCRIPTION SSL_CTX_set_tmp_dh_callback() sets the callback function for B<ctx> to be used when a DH parameters are required to B<tmp_dh_callback>. The callback is inherited by all B<ssl> objects created from B<ctx>. SSL_CTX_set_tmp_dh() sets DH parameters to be used to be B<dh>. The key is inherited by all B<ssl> objects created from B<ctx>. SSL_set_tmp_dh_callback() sets the callback only for B<ssl>. SSL_set_tmp_dh() sets the paramters only for B<ssl>. These functions apply to SSL/TLS servers only. =head1 NOTES When using a cipher with RSA authentication, an ephemeral DH key exchange can take place. Ciphers with DSA keys always use ephemeral DH keys as well as anonymous ciphers. In this case the session data are negotiated using the ephemeral/temporary DH key and the key supplied and certified by the certificate chain is only used for signing. Using ephemeral DH key exchange yields forward secrecy, as the connection can only be decrypted, when the DH key is known. By generating a temporary DH key inside the server application that is lost when the application is left, it becomes impossible for an attacker to decrypt past sessions, even if he gets hold of the normal (certified) key, as this key was only used for signing. In order to perform a DH key exchange the server must use a DH group (DH parameters) and generate a DH key. The server will automatically generate the DH key when required, as it is computationally cheap (retrieve a random number). The server will reuse the DH key for further connections, unless the SSL_OP_SINGLE_DH_USE option of L<SSL_CTX_set_options(3)|SSL_CTX_set_options(3)> is set, in which case a new DH key for each negotiation will be generated. As generating DH parameters is extremely time consuming, an application should not generate the parameters on the fly but supply the parameters. DH parameters can be reused, as the actual key is newly generated during the negotiation. The risk in reusing DH parameters is that an attacker may specialize on a very often used DH group. Therefore application authors should not copy the DH parameters from other applications or the OpenSSL example application, if they compile in parameters, but generate their own set of parameters using e.g. the openssl L<dhparam(1)|dhparam(1)> application with the B<-C> option. An application may also generate its own set of DH parameters during the installation procedure on a specific host, so that each host uses different parameters. An application my either directly specify the DH parameters or can supply the DH parameters via a callback function. The callback approach has the advantage, that the callback may supply DH parameters for different key lengths. The B<tmp_dh_callback> is called with the B<keylength> needed and the B<is_export> information. The B<is_export> flag is set, when the ephemeral DH key exchange is performed with an export cipher. =head1 EXAMPLES Handle DH parameters for key lengths of 512 and 1024 bits. (Error handling partly left out.) ... /* Set up ephemeral DH stuff */ DH *dh_512 = NULL; DH *dh_1024 = NULL; FILE *paramfile; ... /* "openssl dhparam -out dh_param_512.pem -2 512" */ paramfile = fopen("dh_param_512.pem", "r"); if (paramfile) { dh_512 = PEM_read_DHparams(paramfile, NULL, NULL, NULL); fclose(paramfile); } /* "openssl dhparam -out dh_param_1024.pem -2 1024" */ paramfile = fopen("dh_param_1024.pem", "r"); if (paramfile) { dh_1024 = PEM_read_DHparams(paramfile, NULL, NULL, NULL); fclose(paramfile); } ... /* "openssl dhparam -C -2 512" etc... */ DH *get_dh512() { ... } DH *get_dh1024() { ... } DH *tmp_dh_callback(SSL *s, int is_export, int keylength) { DH *dh_tmp=NULL; switch (keylength) { case 512: if (!dh_512) dh_512 = get_dh512(); dh_tmp = dh_512; break; case 1024: if (!dh_1024) dh_1024 = get_dh1024(); dh_tmp = dh_1024; break; default: /* Generating a key on the fly is very costly, so use what is there */ setup_dh_parameters_like_above(); } return(dh_tmp); } =head1 RETURN VALUES SSL_CTX_set_tmp_dh_callback() and SSL_set_tmp_dh_callback() do not return diagnostic output. SSL_CTX_set_tmp_dh() and SSL_set_tmp_dh() do return 1 on success and 0 on failure. Check the error queue to find out the reason of failure. =head1 SEE ALSO L<ssl(3)|ssl(3)>, L<SSL_CTX_set_cipher_list(3)|SSL_CTX_set_cipher_list(3)>, L<SSL_CTX_set_options(3)|SSL_CTX_set_options(3)>, L<ciphers(1)|ciphers(1)>, L<dhparam(1)|dhparam(1)> =cut doc/ssl/SSL_CTX_set_tmp_rsa_callback.pod 0 → 100644 +155 −0 Original line number Diff line number Diff line =pod =head1 NAME SSL_CTX_set_tmp_rsa_callback, SSL_CTX_set_tmp_rsa, SSL_CTX_need_tmp_rsa, SSL_set_tmp_rsa_callback, SSL_set_tmp_rsa, SSL_need_tmp_rsa - handle RSA keys for ephemeral key exchange =head1 SYNOPSIS #include <openssl/ssl.h> void SSL_CTX_set_tmp_rsa_callback(SSL_CTX *ctx, RSA *(*tmp_rsa_callback)(SSL *ssl, int is_export, int keylength)); long SSL_CTX_set_tmp_rsa(SSL_CTX *ctx, RSA *rsa); long SSL_CTX_need_tmp_rsa(SSL_CTX *ctx); void SSL_set_tmp_rsa_callback(SSL_CTX *ctx, RSA *(*tmp_rsa_callback)(SSL *ssl, int is_export, int keylength)); long SSL_set_tmp_rsa(SSL *ssl, RSA *rsa) long SSL_need_tmp_rsa(SSL *ssl) RSA *(*tmp_rsa_callback)(SSL *ssl, int is_export, int keylength)); =head1 DESCRIPTION SSL_CTX_set_tmp_rsa_callback() sets the callback function for B<ctx> to be used when a temporary/ephemeral RSA key is required to B<tmp_rsa_callback>. The callback is inherited by all B<ssl> objects created from B<ctx>. SSL_CTX_set_tmp_rsa() sets the temporary/ephemeral RSA key to be used to be B<rsa>. The key is inherited by all B<ssl> objects created from B<ctx>. SSL_CTX_need_tmp_rsa() returns 1, if a temporay/ephemeral RSA key is needed, because a RSA key with a keysize larger than 512 bits is installed. SSL_set_tmp_rsa_callback() sets the callback only for B<ssl>. SSL_set_tmp_rsa() sets the key only for B<ssl>. SSL_need_tmp_rsa() returns 1, if a temporay/ephemeral RSA key is needed, because a RSA key with a keysize larger than 512 bits is installed. These functions apply to SSL/TLS servers only. =head1 NOTES When using a cipher with RSA authentication, an ephemeral RSA key exchange can take place. In this case the session data are negotiated using the ephemeral/temporary RSA key and the RSA key supplied and certified by the certificate chain is only used for signing. Using ephemeral RSA key exchange yields forward secrecy, as the connection can only be decrypted, when the RSA key is known. By generating a temporary RSA key inside the server application that is lost when the application is left, it becomes impossible for an attacker to decrypt past sessions, even if he gets hold of the normal (certified) RSA key, as this key was only used for signing. The downside is that creating a RSA key is computationally expensive. On OpenSSL servers ephemeral RSA key exchange is therefore disabled by default and must be explicitly enabled using the SSL_OP_EPHEMERAL_RSA option of L<SSL_CTX_set_options(3)|SSL_CTX_set_options(3)>, except for certain export ciphers. Under previous export restrictions, ciphers with RSA keys shorter (512 bits) than the usual key length of 1024 bits were created. To use these ciphers with RSA keys of usual length, an ephemeral key exchange must be performed, as the normal (certified) key cannot be used. An application my either directly specify the key or can supply the key via a callback function. The callback approach has the advantage, that the callback may generate the key only in case it is actually needed. As the generation of a RSA key is however costly, it will lead to a significant delay in the handshake procedure. Another advantage of the callback function is that it can supply keys of different size (e.g. for SSL_OP_EPHEMERAL_RSA usage) while the explicit setting of the key is only useful for key size of 512 bits to satisfy the export restricted ciphers and does give away key length if a longer key would be allowed. The B<tmp_rsa_callback> is called with the B<keylength> needed and the B<is_export> information. The B<is_export> flag is set, when the ephemeral RSA key exchange is performed with an export cipher. =head1 EXAMPLES Generate temporary RSA keys to prepare ephemeral RSA key exchange. As the generation of a RSA key costs a lot of computer time, it is saved for later reuse. For demonstration purposes, two keys for 512 bits and 1024 bits respectively are generated. ... /* Set up ephemeral RSA stuff */ RSA *rsa_512 = NULL; RSA *rsa_1024 = NULL; if (prepare_export_in_advance || always_use_ephemeral_rsa) { rsa_512 = RSA_generate_key(512,RSA_F4,NULL,NULL); if (rsa_512 == NULL) evaluate_error_queue(); } if (always_use_ephemeral_rsa) { /* Only spend the time to generate the key, if it will actually be needed */ rsa_1024 = RSA_generate_key(1024,RSA_F4,NULL,NULL); if (rsa_1024 == NULL) evaluate_error_queue(); SSL_CTX_set_options(SSL_OP_EPHEMERAL_RSA); } ... RSA *tmp_rsa_callback(SSL *s, int is_export, int keylength) { RSA *rsa_tmp=NULL; switch (keylength) { case 512: if (rsa_512) rsa_tmp = rsa_512; else { /* generate on the fly */ rsa_tmp = RSA_generate_key(512,RSA_F4,NULL,NULL); rsa_512 = rsa_tmp; /* Remember for later reuse */ } break; case 1024: if (rsa_1024) rsa_tmp=rsa_1024; else this_should_never_happen_as_we_are_prepared(); break; default: /* Generating a key on the fly is very costly, so use what is there */ if (rsa_1024) rsa_tmp=rsa_1024; else rsa_tmp=rsa_512; /* Use at least a shorter key */ } return(rsa_tmp); } =head1 RETURN VALUES SSL_CTX_set_tmp_rsa_callback() and SSL_set_tmp_rsa_callback() do not return diagnostic output. SSL_CTX_set_tmp_rsa() and SSL_set_tmp_rsa() do return 1 on success and 0 on failure. Check the error queue to find out the reason of failure. SSL_CTX_need_tmp_rsa() and SSL_need_tmp_rsa() return 1 if a temporary RSA key is needed and 0 otherwise. =head1 SEE ALSO L<ssl(3)|ssl(3)>, L<SSL_CTX_set_cipher_list(3)|SSL_CTX_set_cipher_list(3)>, L<SSL_CTX_set_options(3)|SSL_CTX_set_options(3)>, L<ciphers(1)|ciphers(1)> =cut doc/ssl/ssl.pod +2 −0 Original line number Diff line number Diff line Loading @@ -670,6 +670,8 @@ L<SSL_CTX_set_session_cache_mode(3)|SSL_CTX_set_session_cache_mode(3)>, L<SSL_CTX_set_session_id_context(3)|SSL_CTX_set_session_id_context(3)>, L<SSL_CTX_set_ssl_version(3)|SSL_CTX_set_ssl_version(3)>, L<SSL_CTX_set_timeout(3)|SSL_CTX_set_timeout(3)>, L<SSL_CTX_set_tmp_rsa_callback(3)|SSL_CTX_set_tmp_rsa_callback(3)>, L<SSL_CTX_set_tmp_dh_callback(3)|SSL_CTX_set_tmp_dh_callback(3)>, L<SSL_CTX_set_verify(3)|SSL_CTX_set_verify(3)>, L<SSL_CTX_use_certificate(3)|SSL_CTX_use_certificate(3)>, L<SSL_get_ciphers(3)|SSL_get_ciphers(3)>, Loading Loading
doc/ssl/SSL_CTX_set_cipher_list.pod +5 −2 Original line number Diff line number Diff line Loading @@ -39,8 +39,9 @@ certificate and key. A RSA cipher can only be chosen, when a RSA certificate is available. RSA export ciphers with a keylength of 512 bits for the RSA key require a temporary 512 bit RSA key, as typically the supplied key has a length of 1024 bit. RSA ciphers using EDH need a certificate and key and additional DH-parameters. of 1024 bit (see L<SSL_CTX_set_tmp_rsa_callback(3)|SSL_CTX_set_tmp_rsa_callback(3)>). RSA ciphers using EDH need a certificate and key and additional DH-parameters. A DSA cipher can only be chosen, when a DSA certificate is available. DSA ciphers always use DH key exchange and therefore need DH-parameters. Loading @@ -60,6 +61,8 @@ could be selected and 0 on complete failure. L<ssl(3)|ssl(3)>, L<SSL_get_ciphers(3)|SSL_get_ciphers(3)>, L<SSL_CTX_use_certificate(3)|SSL_CTX_use_certificate(3)>, L<SSL_CTX_set_tmp_rsa_callback(3)|SSL_CTX_set_tmp_rsa_callback(3)>, L<SSL_CTX_set_tmp_dh_callback(3)|SSL_CTX_set_tmp_dh_callback(3)>, L<ciphers(1)|ciphers(1)> =cut
doc/ssl/SSL_CTX_set_options.pod +7 −3 Original line number Diff line number Diff line Loading @@ -122,11 +122,13 @@ The following B<modifying> options are available: =item SSL_OP_SINGLE_DH_USE Always create a new key when using temporary DH parameters. Always create a new key when using temporary DH parameters (see L<SSL_CTX_set_tmp_dh_callback(3)|SSL_CTX_set_tmp_dh_callback(3)>). =item SSL_OP_EPHEMERAL_RSA Also use the temporary RSA key when doing RSA operations. Also use ephemeral (temporary) RSA key when doing RSA operations (see L<SSL_CTX_set_tmp_rsa_callback(3)|SSL_CTX_set_tmp_rsa_callback(3)>). =item SSL_OP_CIPHER_SERVER_PREFERENCE Loading Loading @@ -182,7 +184,9 @@ SSL_CTX_get_options() and SSL_get_options() return the current bitmask. =head1 SEE ALSO L<ssl(3)|ssl(3)>, L<SSL_new(3)|SSL_new(3)>, L<SSL_clear(3)|SSL_clear(3)> L<ssl(3)|ssl(3)>, L<SSL_new(3)|SSL_new(3)>, L<SSL_clear(3)|SSL_clear(3)>, L<SSL_CTX_set_tmp_dh_callback(3)|SSL_CTX_set_tmp_dh_callback(3)>, L<SSL_CTX_set_tmp_rsa_callback(3)|SSL_CTX_set_tmp_rsa_callback(3)> =head1 HISTORY Loading
doc/ssl/SSL_CTX_set_tmp_dh_callback.pod 0 → 100644 +146 −0 Original line number Diff line number Diff line =pod =head1 NAME 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 =head1 SYNOPSIS #include <openssl/ssl.h> void SSL_CTX_set_tmp_dh_callback(SSL_CTX *ctx, DH *(*tmp_dh_callback)(SSL *ssl, int is_export, int keylength)); long SSL_CTX_set_tmp_dh(SSL_CTX *ctx, DH *dh); void SSL_set_tmp_dh_callback(SSL_CTX *ctx, DH *(*tmp_dh_callback)(SSL *ssl, int is_export, int keylength)); long SSL_set_tmp_dh(SSL *ssl, DH *dh) DH *(*tmp_dh_callback)(SSL *ssl, int is_export, int keylength)); =head1 DESCRIPTION SSL_CTX_set_tmp_dh_callback() sets the callback function for B<ctx> to be used when a DH parameters are required to B<tmp_dh_callback>. The callback is inherited by all B<ssl> objects created from B<ctx>. SSL_CTX_set_tmp_dh() sets DH parameters to be used to be B<dh>. The key is inherited by all B<ssl> objects created from B<ctx>. SSL_set_tmp_dh_callback() sets the callback only for B<ssl>. SSL_set_tmp_dh() sets the paramters only for B<ssl>. These functions apply to SSL/TLS servers only. =head1 NOTES When using a cipher with RSA authentication, an ephemeral DH key exchange can take place. Ciphers with DSA keys always use ephemeral DH keys as well as anonymous ciphers. In this case the session data are negotiated using the ephemeral/temporary DH key and the key supplied and certified by the certificate chain is only used for signing. Using ephemeral DH key exchange yields forward secrecy, as the connection can only be decrypted, when the DH key is known. By generating a temporary DH key inside the server application that is lost when the application is left, it becomes impossible for an attacker to decrypt past sessions, even if he gets hold of the normal (certified) key, as this key was only used for signing. In order to perform a DH key exchange the server must use a DH group (DH parameters) and generate a DH key. The server will automatically generate the DH key when required, as it is computationally cheap (retrieve a random number). The server will reuse the DH key for further connections, unless the SSL_OP_SINGLE_DH_USE option of L<SSL_CTX_set_options(3)|SSL_CTX_set_options(3)> is set, in which case a new DH key for each negotiation will be generated. As generating DH parameters is extremely time consuming, an application should not generate the parameters on the fly but supply the parameters. DH parameters can be reused, as the actual key is newly generated during the negotiation. The risk in reusing DH parameters is that an attacker may specialize on a very often used DH group. Therefore application authors should not copy the DH parameters from other applications or the OpenSSL example application, if they compile in parameters, but generate their own set of parameters using e.g. the openssl L<dhparam(1)|dhparam(1)> application with the B<-C> option. An application may also generate its own set of DH parameters during the installation procedure on a specific host, so that each host uses different parameters. An application my either directly specify the DH parameters or can supply the DH parameters via a callback function. The callback approach has the advantage, that the callback may supply DH parameters for different key lengths. The B<tmp_dh_callback> is called with the B<keylength> needed and the B<is_export> information. The B<is_export> flag is set, when the ephemeral DH key exchange is performed with an export cipher. =head1 EXAMPLES Handle DH parameters for key lengths of 512 and 1024 bits. (Error handling partly left out.) ... /* Set up ephemeral DH stuff */ DH *dh_512 = NULL; DH *dh_1024 = NULL; FILE *paramfile; ... /* "openssl dhparam -out dh_param_512.pem -2 512" */ paramfile = fopen("dh_param_512.pem", "r"); if (paramfile) { dh_512 = PEM_read_DHparams(paramfile, NULL, NULL, NULL); fclose(paramfile); } /* "openssl dhparam -out dh_param_1024.pem -2 1024" */ paramfile = fopen("dh_param_1024.pem", "r"); if (paramfile) { dh_1024 = PEM_read_DHparams(paramfile, NULL, NULL, NULL); fclose(paramfile); } ... /* "openssl dhparam -C -2 512" etc... */ DH *get_dh512() { ... } DH *get_dh1024() { ... } DH *tmp_dh_callback(SSL *s, int is_export, int keylength) { DH *dh_tmp=NULL; switch (keylength) { case 512: if (!dh_512) dh_512 = get_dh512(); dh_tmp = dh_512; break; case 1024: if (!dh_1024) dh_1024 = get_dh1024(); dh_tmp = dh_1024; break; default: /* Generating a key on the fly is very costly, so use what is there */ setup_dh_parameters_like_above(); } return(dh_tmp); } =head1 RETURN VALUES SSL_CTX_set_tmp_dh_callback() and SSL_set_tmp_dh_callback() do not return diagnostic output. SSL_CTX_set_tmp_dh() and SSL_set_tmp_dh() do return 1 on success and 0 on failure. Check the error queue to find out the reason of failure. =head1 SEE ALSO L<ssl(3)|ssl(3)>, L<SSL_CTX_set_cipher_list(3)|SSL_CTX_set_cipher_list(3)>, L<SSL_CTX_set_options(3)|SSL_CTX_set_options(3)>, L<ciphers(1)|ciphers(1)>, L<dhparam(1)|dhparam(1)> =cut
doc/ssl/SSL_CTX_set_tmp_rsa_callback.pod 0 → 100644 +155 −0 Original line number Diff line number Diff line =pod =head1 NAME SSL_CTX_set_tmp_rsa_callback, SSL_CTX_set_tmp_rsa, SSL_CTX_need_tmp_rsa, SSL_set_tmp_rsa_callback, SSL_set_tmp_rsa, SSL_need_tmp_rsa - handle RSA keys for ephemeral key exchange =head1 SYNOPSIS #include <openssl/ssl.h> void SSL_CTX_set_tmp_rsa_callback(SSL_CTX *ctx, RSA *(*tmp_rsa_callback)(SSL *ssl, int is_export, int keylength)); long SSL_CTX_set_tmp_rsa(SSL_CTX *ctx, RSA *rsa); long SSL_CTX_need_tmp_rsa(SSL_CTX *ctx); void SSL_set_tmp_rsa_callback(SSL_CTX *ctx, RSA *(*tmp_rsa_callback)(SSL *ssl, int is_export, int keylength)); long SSL_set_tmp_rsa(SSL *ssl, RSA *rsa) long SSL_need_tmp_rsa(SSL *ssl) RSA *(*tmp_rsa_callback)(SSL *ssl, int is_export, int keylength)); =head1 DESCRIPTION SSL_CTX_set_tmp_rsa_callback() sets the callback function for B<ctx> to be used when a temporary/ephemeral RSA key is required to B<tmp_rsa_callback>. The callback is inherited by all B<ssl> objects created from B<ctx>. SSL_CTX_set_tmp_rsa() sets the temporary/ephemeral RSA key to be used to be B<rsa>. The key is inherited by all B<ssl> objects created from B<ctx>. SSL_CTX_need_tmp_rsa() returns 1, if a temporay/ephemeral RSA key is needed, because a RSA key with a keysize larger than 512 bits is installed. SSL_set_tmp_rsa_callback() sets the callback only for B<ssl>. SSL_set_tmp_rsa() sets the key only for B<ssl>. SSL_need_tmp_rsa() returns 1, if a temporay/ephemeral RSA key is needed, because a RSA key with a keysize larger than 512 bits is installed. These functions apply to SSL/TLS servers only. =head1 NOTES When using a cipher with RSA authentication, an ephemeral RSA key exchange can take place. In this case the session data are negotiated using the ephemeral/temporary RSA key and the RSA key supplied and certified by the certificate chain is only used for signing. Using ephemeral RSA key exchange yields forward secrecy, as the connection can only be decrypted, when the RSA key is known. By generating a temporary RSA key inside the server application that is lost when the application is left, it becomes impossible for an attacker to decrypt past sessions, even if he gets hold of the normal (certified) RSA key, as this key was only used for signing. The downside is that creating a RSA key is computationally expensive. On OpenSSL servers ephemeral RSA key exchange is therefore disabled by default and must be explicitly enabled using the SSL_OP_EPHEMERAL_RSA option of L<SSL_CTX_set_options(3)|SSL_CTX_set_options(3)>, except for certain export ciphers. Under previous export restrictions, ciphers with RSA keys shorter (512 bits) than the usual key length of 1024 bits were created. To use these ciphers with RSA keys of usual length, an ephemeral key exchange must be performed, as the normal (certified) key cannot be used. An application my either directly specify the key or can supply the key via a callback function. The callback approach has the advantage, that the callback may generate the key only in case it is actually needed. As the generation of a RSA key is however costly, it will lead to a significant delay in the handshake procedure. Another advantage of the callback function is that it can supply keys of different size (e.g. for SSL_OP_EPHEMERAL_RSA usage) while the explicit setting of the key is only useful for key size of 512 bits to satisfy the export restricted ciphers and does give away key length if a longer key would be allowed. The B<tmp_rsa_callback> is called with the B<keylength> needed and the B<is_export> information. The B<is_export> flag is set, when the ephemeral RSA key exchange is performed with an export cipher. =head1 EXAMPLES Generate temporary RSA keys to prepare ephemeral RSA key exchange. As the generation of a RSA key costs a lot of computer time, it is saved for later reuse. For demonstration purposes, two keys for 512 bits and 1024 bits respectively are generated. ... /* Set up ephemeral RSA stuff */ RSA *rsa_512 = NULL; RSA *rsa_1024 = NULL; if (prepare_export_in_advance || always_use_ephemeral_rsa) { rsa_512 = RSA_generate_key(512,RSA_F4,NULL,NULL); if (rsa_512 == NULL) evaluate_error_queue(); } if (always_use_ephemeral_rsa) { /* Only spend the time to generate the key, if it will actually be needed */ rsa_1024 = RSA_generate_key(1024,RSA_F4,NULL,NULL); if (rsa_1024 == NULL) evaluate_error_queue(); SSL_CTX_set_options(SSL_OP_EPHEMERAL_RSA); } ... RSA *tmp_rsa_callback(SSL *s, int is_export, int keylength) { RSA *rsa_tmp=NULL; switch (keylength) { case 512: if (rsa_512) rsa_tmp = rsa_512; else { /* generate on the fly */ rsa_tmp = RSA_generate_key(512,RSA_F4,NULL,NULL); rsa_512 = rsa_tmp; /* Remember for later reuse */ } break; case 1024: if (rsa_1024) rsa_tmp=rsa_1024; else this_should_never_happen_as_we_are_prepared(); break; default: /* Generating a key on the fly is very costly, so use what is there */ if (rsa_1024) rsa_tmp=rsa_1024; else rsa_tmp=rsa_512; /* Use at least a shorter key */ } return(rsa_tmp); } =head1 RETURN VALUES SSL_CTX_set_tmp_rsa_callback() and SSL_set_tmp_rsa_callback() do not return diagnostic output. SSL_CTX_set_tmp_rsa() and SSL_set_tmp_rsa() do return 1 on success and 0 on failure. Check the error queue to find out the reason of failure. SSL_CTX_need_tmp_rsa() and SSL_need_tmp_rsa() return 1 if a temporary RSA key is needed and 0 otherwise. =head1 SEE ALSO L<ssl(3)|ssl(3)>, L<SSL_CTX_set_cipher_list(3)|SSL_CTX_set_cipher_list(3)>, L<SSL_CTX_set_options(3)|SSL_CTX_set_options(3)>, L<ciphers(1)|ciphers(1)> =cut
doc/ssl/ssl.pod +2 −0 Original line number Diff line number Diff line Loading @@ -670,6 +670,8 @@ L<SSL_CTX_set_session_cache_mode(3)|SSL_CTX_set_session_cache_mode(3)>, L<SSL_CTX_set_session_id_context(3)|SSL_CTX_set_session_id_context(3)>, L<SSL_CTX_set_ssl_version(3)|SSL_CTX_set_ssl_version(3)>, L<SSL_CTX_set_timeout(3)|SSL_CTX_set_timeout(3)>, L<SSL_CTX_set_tmp_rsa_callback(3)|SSL_CTX_set_tmp_rsa_callback(3)>, L<SSL_CTX_set_tmp_dh_callback(3)|SSL_CTX_set_tmp_dh_callback(3)>, L<SSL_CTX_set_verify(3)|SSL_CTX_set_verify(3)>, L<SSL_CTX_use_certificate(3)|SSL_CTX_use_certificate(3)>, L<SSL_get_ciphers(3)|SSL_get_ciphers(3)>, Loading
