SSL_CTX_set_tmp_rsa_callback(OpenSSSL_CTX_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
libcrypto, lcrypto
#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 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 RSAbased strengthlimited
'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 RSAbased strengthlimited '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
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 EDH (Ephemeral DiffieHellman) key
exchange instead in order to achieve forward secrecy (see
SSL_CTX_set_tmp_dh_callback(3)).
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 SSL_CTX_set_options(3),
violating the TLS/SSL standard. When ephemeral RSA key
exchange is required for export ciphers, it will automatically
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. 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 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 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);
}
SSL_CTX_set_tmp_rsa_callback() and SSL_set_tmp_rsa_call
back() 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.
ssl(3), SSL_CTX_set_cipher_list(3),
SSL_CTX_set_options(3), SSL_CTX_set_tmp_dh_callback(3),
SSL_new(3), openssl_ciphers(1)
20020805 0.9.SSL_CTX_set_tmp_rsa_callback(3)
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