openssl/doc/ssl/SSL_CTX_set_tmp_rsa_callback.pod

=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