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138.\" ======================================================================
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140.IX Title "SSL_CTX_set_tmp_rsa_callback 3"
141.TH SSL_CTX_set_tmp_rsa_callback 3 "0.9.7a" "2003-02-19" "OpenSSL"
142.UC
143.SH "NAME"
144SSL_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 \s-1RSA\s0 keys for ephemeral key exchange
145.SH "SYNOPSIS"
146.IX Header "SYNOPSIS"
147.Vb 1
148\& #include <openssl/ssl.h>
149.Ve
150.Vb 4
151\& void SSL_CTX_set_tmp_rsa_callback(SSL_CTX *ctx,
152\& RSA *(*tmp_rsa_callback)(SSL *ssl, int is_export, int keylength));
153\& long SSL_CTX_set_tmp_rsa(SSL_CTX *ctx, RSA *rsa);
154\& long SSL_CTX_need_tmp_rsa(SSL_CTX *ctx);
155.Ve
156.Vb 4
157\& void SSL_set_tmp_rsa_callback(SSL_CTX *ctx,
158\& RSA *(*tmp_rsa_callback)(SSL *ssl, int is_export, int keylength));
159\& long SSL_set_tmp_rsa(SSL *ssl, RSA *rsa)
160\& long SSL_need_tmp_rsa(SSL *ssl)
161.Ve
162.Vb 1
163\& RSA *(*tmp_rsa_callback)(SSL *ssl, int is_export, int keylength));
164.Ve
165.SH "DESCRIPTION"
166.IX Header "DESCRIPTION"
167\&\fISSL_CTX_set_tmp_rsa_callback()\fR sets the callback function for \fBctx\fR to be
168used when a temporary/ephemeral \s-1RSA\s0 key is required to \fBtmp_rsa_callback\fR.
169The callback is inherited by all \s-1SSL\s0 objects newly created from \fBctx\fR
170with <\fISSL_new\fR\|(3)|\fISSL_new\fR\|(3)>. Already created \s-1SSL\s0 objects are not affected.
171.PP
172\&\fISSL_CTX_set_tmp_rsa()\fR sets the temporary/ephemeral \s-1RSA\s0 key to be used to be
173\&\fBrsa\fR. The key is inherited by all \s-1SSL\s0 objects newly created from \fBctx\fR
174with <\fISSL_new\fR\|(3)|\fISSL_new\fR\|(3)>. Already created \s-1SSL\s0 objects are not affected.
175.PP
176\&\fISSL_CTX_need_tmp_rsa()\fR returns 1, if a temporary/ephemeral \s-1RSA\s0 key is needed
177for RSA-based strength-limited 'exportable' ciphersuites because a \s-1RSA\s0 key
178with a keysize larger than 512 bits is installed.
179.PP
180\&\fISSL_set_tmp_rsa_callback()\fR sets the callback only for \fBssl\fR.
181.PP
182\&\fISSL_set_tmp_rsa()\fR sets the key only for \fBssl\fR.
183.PP
184\&\fISSL_need_tmp_rsa()\fR returns 1, if a temporary/ephemeral \s-1RSA\s0 key is needed,
185for RSA-based strength-limited 'exportable' ciphersuites because a \s-1RSA\s0 key
186with a keysize larger than 512 bits is installed.
187.PP
188These functions apply to \s-1SSL/TLS\s0 servers only.
189.SH "NOTES"
190.IX Header "NOTES"
191When using a cipher with \s-1RSA\s0 authentication, an ephemeral \s-1RSA\s0 key exchange
192can take place. In this case the session data are negotiated using the
193ephemeral/temporary \s-1RSA\s0 key and the \s-1RSA\s0 key supplied and certified
194by the certificate chain is only used for signing.
195.PP
196Under previous export restrictions, ciphers with \s-1RSA\s0 keys shorter (512 bits)
197than the usual key length of 1024 bits were created. To use these ciphers
198with \s-1RSA\s0 keys of usual length, an ephemeral key exchange must be performed,
199as the normal (certified) key cannot be directly used.
200.PP
201Using ephemeral \s-1RSA\s0 key exchange yields forward secrecy, as the connection
202can only be decrypted, when the \s-1RSA\s0 key is known. By generating a temporary
203\&\s-1RSA\s0 key inside the server application that is lost when the application
204is left, it becomes impossible for an attacker to decrypt past sessions,
205even if he gets hold of the normal (certified) \s-1RSA\s0 key, as this key was
206used for signing only. The downside is that creating a \s-1RSA\s0 key is
207computationally expensive.
208.PP
209Additionally, the use of ephemeral \s-1RSA\s0 key exchange is only allowed in
210the \s-1TLS\s0 standard, when the \s-1RSA\s0 key can be used for signing only, that is
211for export ciphers. Using ephemeral \s-1RSA\s0 key exchange for other purposes
212violates the standard and can break interoperability with clients.
213It is therefore strongly recommended to not use ephemeral \s-1RSA\s0 key
214exchange and use \s-1EDH\s0 (Ephemeral Diffie-Hellman) key exchange instead
215in order to achieve forward secrecy (see
216SSL_CTX_set_tmp_dh_callback(3)).
217.PP
218On OpenSSL servers ephemeral \s-1RSA\s0 key exchange is therefore disabled by default
219and must be explicitly enabled using the \s-1SSL_OP_EPHEMERAL_RSA\s0 option of
220SSL_CTX_set_options(3), violating the \s-1TLS/SSL\s0
221standard. When ephemeral \s-1RSA\s0 key exchange is required for export ciphers,
222it will automatically be used without this option!
223.PP
224An application may either directly specify the key or can supply the key via
225a callback function. The callback approach has the advantage, that the
226callback may generate the key only in case it is actually needed. As the
227generation of a \s-1RSA\s0 key is however costly, it will lead to a significant
228delay in the handshake procedure. Another advantage of the callback function
229is that it can supply keys of different size (e.g. for \s-1SSL_OP_EPHEMERAL_RSA\s0
230usage) while the explicit setting of the key is only useful for key size of
231512 bits to satisfy the export restricted ciphers and does give away key length
232if a longer key would be allowed.
233.PP
234The \fBtmp_rsa_callback\fR is called with the \fBkeylength\fR needed and
235the \fBis_export\fR information. The \fBis_export\fR flag is set, when the
236ephemeral \s-1RSA\s0 key exchange is performed with an export cipher.
237.SH "EXAMPLES"
238.IX Header "EXAMPLES"
239Generate temporary \s-1RSA\s0 keys to prepare ephemeral \s-1RSA\s0 key exchange. As the
240generation of a \s-1RSA\s0 key costs a lot of computer time, they saved for later
241reuse. For demonstration purposes, two keys for 512 bits and 1024 bits
242respectively are generated.
243.PP
244.Vb 4
245\& ...
246\& /* Set up ephemeral RSA stuff */
247\& RSA *rsa_512 = NULL;
248\& RSA *rsa_1024 = NULL;
249.Ve
250.Vb 3
251\& rsa_512 = RSA_generate_key(512,RSA_F4,NULL,NULL);
252\& if (rsa_512 == NULL)
253\& evaluate_error_queue();
254.Ve
255.Vb 3
256\& rsa_1024 = RSA_generate_key(1024,RSA_F4,NULL,NULL);
257\& if (rsa_1024 == NULL)
258\& evaluate_error_queue();
259.Ve
260.Vb 1
261\& ...
262.Ve
263.Vb 3
264\& RSA *tmp_rsa_callback(SSL *s, int is_export, int keylength)
265\& {
266\& RSA *rsa_tmp=NULL;
267.Ve
268.Vb 24
269\& switch (keylength) {
270\& case 512:
271\& if (rsa_512)
272\& rsa_tmp = rsa_512;
273\& else { /* generate on the fly, should not happen in this example */
274\& rsa_tmp = RSA_generate_key(keylength,RSA_F4,NULL,NULL);
275\& rsa_512 = rsa_tmp; /* Remember for later reuse */
276\& }
277\& break;
278\& case 1024:
279\& if (rsa_1024)
280\& rsa_tmp=rsa_1024;
281\& else
282\& should_not_happen_in_this_example();
283\& break;
284\& default:
285\& /* Generating a key on the fly is very costly, so use what is there */
286\& if (rsa_1024)
287\& rsa_tmp=rsa_1024;
288\& else
289\& rsa_tmp=rsa_512; /* Use at least a shorter key */
290\& }
291\& return(rsa_tmp);
292\& }
293.Ve
294.SH "RETURN VALUES"
295.IX Header "RETURN VALUES"
296\&\fISSL_CTX_set_tmp_rsa_callback()\fR and \fISSL_set_tmp_rsa_callback()\fR do not return
297diagnostic output.
298.PP
299\&\fISSL_CTX_set_tmp_rsa()\fR and \fISSL_set_tmp_rsa()\fR do return 1 on success and 0
300on failure. Check the error queue to find out the reason of failure.
301.PP
302\&\fISSL_CTX_need_tmp_rsa()\fR and \fISSL_need_tmp_rsa()\fR return 1 if a temporary
303\&\s-1RSA\s0 key is needed and 0 otherwise.
304.SH "SEE ALSO"
305.IX Header "SEE ALSO"
306ssl(3), SSL_CTX_set_cipher_list(3),
307SSL_CTX_set_options(3),
308SSL_CTX_set_tmp_dh_callback(3),
309SSL_new(3), ciphers(1)