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 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 DHE (Ephemeral
Diffie-Hellman) key exchange instead in order to achieve forward secrecy (see
SSL_CTX_set_tmp_dh_callback(3)).
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 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 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;
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)