SSL_CTX_set_tmp_rsa_callback(3)SSL_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>
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) );
DESCRIPTION
The SSL_CTX_set_tmp_rsa_callback() and function sets the callback func‐
tion 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 RSA-based, strength-limited exportable cipher‐
suites 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 RSA-based, strength-limited exportable ciphersuites
because an 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 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 cer‐
tified 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 connec‐
tion 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 pur‐
poses violates the standard and can break interoperability with
clients. We recommend using the EDH (Ephemeral Diffie-Hellman) 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 automati‐
cally 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.
EXAMPLES
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 exam‐
ple =
*/
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);
}
RETURN VALUES
The SSL_CTX_set_tmp_rsa_callback() and SSL_set_tmp_rsa_callback() func‐
tions 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 rea‐
son 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.
SEE ALSO
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)
