SSL_CTX_set_tmp_rsa_callback(3) OpenSSL 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>
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 strengthlimited
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 strengthlimited
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 EDH (Ephemeral Diffie-Hellman) 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.
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)
0.9.7l 2001-09-06 SSL_CTX_set_tmp_rsa_callback(3)
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