SSL_CTX_set_tmp_rsa_callback(3)

     SSL_CTX_set_tmp_rsa_cal0.9.6h3(200SSL_CTX_set_tmp_rsa_callback(3)

     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

     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));

     DESCRIPTION
          SSL_CTX_set_tmp_rsa_callback() sets the callback function
          for ctx to be used when a temporary/ephemeral RSA key is
          required to tmp_rsa_callback.  The callback is inherited by
          all SSL objects newly created from ctx with
          <SSL_new(3)|SSL_new(3)>. Already created SSL objects are not
          affected.

          SSL_CTX_set_tmp_rsa() sets the temporary/ephemeral RSA key
          to be used to be rsa. The key is inherited by all SSL
          objects newly created from ctx with <SSL_new(3)|SSL_new(3)>.
          Already created SSL objects are not affected.

          SSL_CTX_need_tmp_rsa() returns 1, if a temporary/ephemeral
          RSA key is needed for RSA-based strength-limited
          'exportable' ciphersuites because a RSA key with a keysize
          larger than 512 bits is installed.

          SSL_set_tmp_rsa_callback() sets the callback only for ssl.

          SSL_set_tmp_rsa() sets the key only for ssl.

          SSL_need_tmp_rsa() returns 1, if a temporary/ephemeral RSA
          key is needed, for RSA-based strength-limited 'exportable'
          ciphersuites because a RSA key with a keysize larger than
          512 bits is installed.

          These functions apply to SSL/TLS servers only.

     NOTES
          When using a cipher with RSA authentication, an ephemeral

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          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.

          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 directly used.

          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 used for signing only. The downside is that
          creating a RSA key is computationally expensive.

          Additionally, the use of ephemeral RSA key exchange is only
          allowed in the TLS standard, when the RSA key can be used
          for signing only, that is for export ciphers. Using
          ephemeral RSA key exchange for other purposes violates the
          standard and can break interoperability with clients.  It is
          therefore strongly recommended to not use ephemeral RSA key
          exchange and use EDH (Ephemeral Diffie-Hellman) key exchange
          instead in order to achieve forward secrecy (see
          SSL_CTX_set_tmp_dh_callback(3)).

          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 SSL_CTX_set_options(3),
          violating the TLS/SSL standard. When ephemeral RSA key
          exchange is required for export ciphers, it will
          automatically be used without this option!

          An application may 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 tmp_rsa_callback is called with the keylength needed and
          the is_export information. The is_export flag is set, when

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          the ephemeral RSA key exchange is performed with an export
          cipher.

     EXAMPLES
          Generate temporary RSA keys to prepare ephemeral RSA key
          exchange. As the generation of a RSA key costs a lot of
          computer time, they 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;

           rsa_512 = RSA_generate_key(512,RSA_F4,NULL,NULL);
           if (rsa_512 == NULL)
               evaluate_error_queue();

           rsa_1024 = RSA_generate_key(1024,RSA_F4,NULL,NULL);
           if (rsa_1024 == NULL)
             evaluate_error_queue();

           ...

           RSA *tmp_rsa_callback(SSL *s, int is_export, int keylength)
           {
              RSA *rsa_tmp=NULL;

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              switch (keylength) {
              case 512:
                if (rsa_512)
                  rsa_tmp = rsa_512;
                else { /* generate on the fly, should not happen in this example */
                  rsa_tmp = RSA_generate_key(keylength,RSA_F4,NULL,NULL);
                  rsa_512 = rsa_tmp; /* Remember for later reuse */
                }
                break;
              case 1024:
                if (rsa_1024)
                  rsa_tmp=rsa_1024;
                else
                  should_not_happen_in_this_example();
                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);
           }

     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.

     SEE ALSO
          ssl(3), SSL_CTX_set_cipher_list(3), SSL_CTX_set_options(3),
          SSL_CTX_set_tmp_dh_callback(3), SSL_new(3), ciphers(1)

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