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 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.
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 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.
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)