
SSL_CTX_set_tmp_rsa_callback(3)
Contents

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>
voidSSL_CTX_set_tmp_rsa_callback(
SSL_CTX *ctx,
RSA *(*cb)(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 ); voidSSL_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) );
The SSL_CTX_set_tmp_rsa_callback() and function 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(). Already created SSL objects are
not affected.
The SSL_CTX_set_tmp_rsa() function 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. Already created SSL objects are not affected.
The SSL_CTX_need_tmp_rsa() function returns 1 if a temporary/ephemeral
RSA key is needed for RSAbased, strengthlimited
exportable ciphersuites because an RSA key with a
keysize larger than 512 bits is installed.
The SSL_set_tmp_rsa_callback() function sets the callback
only for ssl.
The SSL_set_tmp_rsa() function sets the key only for ssl.
The SSL_need_tmp_rsa() function returns 1 if a temporary/ephemeral
RSA key is needed for RSAbased, strengthlimited
exportable ciphersuites because an 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
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 used directly.
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 an 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. We
recommend using the EDH (Ephemeral DiffieHellman) key
exchange instead of the ephemeral RSA key exchange in
order to achieve forward secrecy. (See
SSL_CTX_set_tmp_dh_callback().)
On OpenSSL servers, ephemeral RSA key exchange is disabled
by default and must be explicitly enabled using the
SSL_OP_EPHEMERAL_RSA option of SSL_CTX_set_options(), violating
the TLS/SSL standard. When ephemeral RSA key
exchange is required for export ciphers it automatically
will 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. Because the
generation of an RSA key is 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 the ephemeral RSA key exchange is performed with an
export cipher.
Generate temporary RSA keys to prepare ephemeral RSA key
exchange. As the generation of an RSA key costs a lot of
computer time, they are 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 =3D NULL;
RSA *rsa_1024 =3D NULL;
rsa_512 =3D RSA_generate_key(512,RSA_F4,NULL,NULL);
if (rsa_512 =3D=3D NULL)
evaluate_error_queue();
rsa_1024 =3D RSA_generate_key(1024,RSA_F4,NULL,NULL);
if (rsa_1024 =3D=3D NULL)
evaluate_error_queue();
...
RSA *tmp_rsa_callback(SSL *s, int is_export, int
keylength)
{
RSA *rsa_tmp=3DNULL;
switch (keylength) {
case 512:
if (rsa_512)
rsa_tmp =3D rsa_512;
else { /* generate on the fly, should not happen
in this example =
*/
rsa_tmp =3D RSA_generate_key(keylength,RSA_F4,NULL,NULL);
rsa_512 =3D rsa_tmp; /* Remember for later reuse
*/
}
break;
case 1024:
if (rsa_1024)
rsa_tmp=3Drsa_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=3Drsa_1024;
else
rsa_tmp=3Drsa_512; /* Use at least a shorter key
*/
}
return(rsa_tmp);
}
The SSL_CTX_set_tmp_rsa_callback() and
SSL_set_tmp_rsa_callback() functions do not return diagnostic
output.
The SSL_CTX_set_tmp_rsa() and SSL_set_tmp_rsa() functions
do return 1 on success and 0 on failure. Check the error
queue to find out the reason of failure.
The SSL_CTX_need_tmp_rsa() and SSL_need_tmp_rsa() funcitons
return 1 if a temporary RSA key is needed and 0 otherwise.
Commands: ciphers(1)
Functions: ssl(3), SSL_CTX_set_cipher_list(3)
,SSL_CTX_set_options(3) , SSL_CTX_set_tmp_dh_callback(3),,
SSL_new(3)
SSL_CTX_set_tmp_rsa_callback(3)
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