MirBSD manpage: SSL_CTX_need_tmp_rsa(3), SSL_CTX_set_tmp_rsa(3), SSL_CTX_set_tmp_rsa_callback(3), SSL_need_tmp_rsa(3), SSL_set_tmp_rsa(3), SSL_set_tmp_rsa_callback(3)



     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


      #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));


     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

     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 'export-
     able' 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.


     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 ephem-
     eral RSA key exchange for other purposes violates the stan-
     dard 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

     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 genera-
     tion of a RSA key is however costly, it will lead to a sig-
     nificant delay in the handshake procedure.  Another advan-
     tage of the callback function is that it can supply keys of
     different size while the explicit setting of the key is only
     useful for key size of 512 bits to satisfy the export res-
     tricted 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
     the ephemeral RSA key exchange is performed with an export


     Generate temporary RSA keys to prepare ephemeral RSA key
     exchange. As the generation of a RSA key costs a lot of com-
     puter time, they saved for later reuse. For demonstration
     purposes, two keys for 512 bits and 1024 bits respectively

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     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)

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


      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, should not happen in this example */
             rsa_tmp = RSA_generate_key(keylength,RSA_F4,NULL,NULL);
             rsa_512 = rsa_tmp; /* Remember for later reuse */
         case 1024:
           if (rsa_1024)
           /* Generating a key on the fly is very costly, so use what is there */
           if (rsa_1024)
             rsa_tmp=rsa_512; /* Use at least a shorter key */


     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

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     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.


     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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