191094Sdes=pod 2115619Sdes 3228690Sdes=head1 NAME 491094Sdes 591094SdesSSL_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 691094Sdes 799158Sdes=head1 SYNOPSIS 899158Sdes 999158Sdes #include <openssl/ssl.h> 1091094Sdes 1191094Sdes void SSL_CTX_set_tmp_rsa_callback(SSL_CTX *ctx, 1291094Sdes RSA *(*tmp_rsa_callback)(SSL *ssl, int is_export, int keylength)); 1391094Sdes long SSL_CTX_set_tmp_rsa(SSL_CTX *ctx, RSA *rsa); 1491094Sdes long SSL_CTX_need_tmp_rsa(SSL_CTX *ctx); 1591094Sdes 1691094Sdes void SSL_set_tmp_rsa_callback(SSL_CTX *ctx, 1791094Sdes RSA *(*tmp_rsa_callback)(SSL *ssl, int is_export, int keylength)); 1891094Sdes long SSL_set_tmp_rsa(SSL *ssl, RSA *rsa) 1991094Sdes long SSL_need_tmp_rsa(SSL *ssl) 2091094Sdes 2191094Sdes RSA *(*tmp_rsa_callback)(SSL *ssl, int is_export, int keylength); 2291094Sdes 2391094Sdes=head1 DESCRIPTION 2491094Sdes 2591094SdesSSL_CTX_set_tmp_rsa_callback() sets the callback function for B<ctx> to be 2691094Sdesused when a temporary/ephemeral RSA key is required to B<tmp_rsa_callback>. 2791094SdesThe callback is inherited by all SSL objects newly created from B<ctx> 2891094Sdeswith <SSL_new(3)|SSL_new(3)>. Already created SSL objects are not affected. 2991094Sdes 3091094SdesSSL_CTX_set_tmp_rsa() sets the temporary/ephemeral RSA key to be used to be 3191094SdesB<rsa>. The key is inherited by all SSL objects newly created from B<ctx> 3291094Sdeswith <SSL_new(3)|SSL_new(3)>. Already created SSL objects are not affected. 3391094Sdes 3491094SdesSSL_CTX_need_tmp_rsa() returns 1, if a temporary/ephemeral RSA key is needed 35255376Sdesfor RSA-based strength-limited 'exportable' ciphersuites because a RSA key 3691094Sdeswith a keysize larger than 512 bits is installed. 3791094Sdes 38228690SdesSSL_set_tmp_rsa_callback() sets the callback only for B<ssl>. 39228690Sdes 40228690SdesSSL_set_tmp_rsa() sets the key only for B<ssl>. 41228690Sdes 4291094SdesSSL_need_tmp_rsa() returns 1, if a temporary/ephemeral RSA key is needed, 4391094Sdesfor RSA-based strength-limited 'exportable' ciphersuites because a RSA key 4491100Sdeswith a keysize larger than 512 bits is installed. 4591100Sdes 4691100SdesThese functions apply to SSL/TLS servers only. 4791100Sdes 4891100Sdes=head1 NOTES 4991100Sdes 5091100SdesWhen using a cipher with RSA authentication, an ephemeral RSA key exchange 5191094Sdescan take place. In this case the session data are negotiated using the 5291094Sdesephemeral/temporary RSA key and the RSA key supplied and certified 5391094Sdesby the certificate chain is only used for signing. 5491094Sdes 5591094SdesUnder previous export restrictions, ciphers with RSA keys shorter (512 bits) 5691094Sdesthan the usual key length of 1024 bits were created. To use these ciphers 5791094Sdeswith RSA keys of usual length, an ephemeral key exchange must be performed, 5891094Sdesas the normal (certified) key cannot be directly used. 5991094Sdes 60107937SdesUsing ephemeral RSA key exchange yields forward secrecy, as the connection 61107937Sdescan only be decrypted, when the RSA key is known. By generating a temporary 6291094SdesRSA key inside the server application that is lost when the application 6391100Sdesis left, it becomes impossible for an attacker to decrypt past sessions, 6491100Sdeseven if he gets hold of the normal (certified) RSA key, as this key was 6591100Sdesused for signing only. The downside is that creating a RSA key is 6691100Sdescomputationally expensive. 67 68Additionally, the use of ephemeral RSA key exchange is only allowed in 69the TLS standard, when the RSA key can be used for signing only, that is 70for export ciphers. Using ephemeral RSA key exchange for other purposes 71violates the standard and can break interoperability with clients. 72It is therefore strongly recommended to not use ephemeral RSA key 73exchange and use EDH (Ephemeral Diffie-Hellman) key exchange instead 74in order to achieve forward secrecy (see 75L<SSL_CTX_set_tmp_dh_callback(3)|SSL_CTX_set_tmp_dh_callback(3)>). 76 77An application may either directly specify the key or can supply the key via a 78callback function. The callback approach has the advantage, that the callback 79may generate the key only in case it is actually needed. As the generation of a 80RSA key is however costly, it will lead to a significant delay in the handshake 81procedure. Another advantage of the callback function is that it can supply 82keys of different size while the explicit setting of the key is only useful for 83key size of 512 bits to satisfy the export restricted ciphers and does give 84away key length if a longer key would be allowed. 85 86The B<tmp_rsa_callback> is called with the B<keylength> needed and 87the B<is_export> information. The B<is_export> flag is set, when the 88ephemeral RSA key exchange is performed with an export cipher. 89 90=head1 EXAMPLES 91 92Generate temporary RSA keys to prepare ephemeral RSA key exchange. As the 93generation of a RSA key costs a lot of computer time, they saved for later 94reuse. For demonstration purposes, two keys for 512 bits and 1024 bits 95respectively are generated. 96 97 ... 98 /* Set up ephemeral RSA stuff */ 99 RSA *rsa_512 = NULL; 100 RSA *rsa_1024 = NULL; 101 102 rsa_512 = RSA_generate_key(512,RSA_F4,NULL,NULL); 103 if (rsa_512 == NULL) 104 evaluate_error_queue(); 105 106 rsa_1024 = RSA_generate_key(1024,RSA_F4,NULL,NULL); 107 if (rsa_1024 == NULL) 108 evaluate_error_queue(); 109 110 ... 111 112 RSA *tmp_rsa_callback(SSL *s, int is_export, int keylength) 113 { 114 RSA *rsa_tmp=NULL; 115 116 switch (keylength) { 117 case 512: 118 if (rsa_512) 119 rsa_tmp = rsa_512; 120 else { /* generate on the fly, should not happen in this example */ 121 rsa_tmp = RSA_generate_key(keylength,RSA_F4,NULL,NULL); 122 rsa_512 = rsa_tmp; /* Remember for later reuse */ 123 } 124 break; 125 case 1024: 126 if (rsa_1024) 127 rsa_tmp=rsa_1024; 128 else 129 should_not_happen_in_this_example(); 130 break; 131 default: 132 /* Generating a key on the fly is very costly, so use what is there */ 133 if (rsa_1024) 134 rsa_tmp=rsa_1024; 135 else 136 rsa_tmp=rsa_512; /* Use at least a shorter key */ 137 } 138 return(rsa_tmp); 139 } 140 141=head1 RETURN VALUES 142 143SSL_CTX_set_tmp_rsa_callback() and SSL_set_tmp_rsa_callback() do not return 144diagnostic output. 145 146SSL_CTX_set_tmp_rsa() and SSL_set_tmp_rsa() do return 1 on success and 0 147on failure. Check the error queue to find out the reason of failure. 148 149SSL_CTX_need_tmp_rsa() and SSL_need_tmp_rsa() return 1 if a temporary 150RSA key is needed and 0 otherwise. 151 152=head1 SEE ALSO 153 154L<ssl(3)|ssl(3)>, L<SSL_CTX_set_cipher_list(3)|SSL_CTX_set_cipher_list(3)>, 155L<SSL_CTX_set_options(3)|SSL_CTX_set_options(3)>, 156L<SSL_CTX_set_tmp_dh_callback(3)|SSL_CTX_set_tmp_dh_callback(3)>, 157L<SSL_new(3)|SSL_new(3)>, L<ciphers(1)|ciphers(1)> 158 159=cut 160