Merge branch 'vendor/OPENSSL'
[dragonfly.git] / secure / lib / libssl / man / SSL_CTX_set_tmp_rsa_callback.3
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132.\" ========================================================================
133.\"
134.IX Title "SSL_CTX_set_tmp_rsa_callback 3"
fc468453 135.TH SSL_CTX_set_tmp_rsa_callback 3 "2010-02-27" "0.9.8m" "OpenSSL"
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136.\" For nroff, turn off justification. Always turn off hyphenation; it makes
137.\" way too many mistakes in technical documents.
138.if n .ad l
139.nh
984263bc 140.SH "NAME"
a7d27d5a 141SSL_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
984263bc 142.SH "SYNOPSIS"
e056f0e0 143.IX Header "SYNOPSIS"
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144.Vb 1
145\& #include <openssl/ssl.h>
e257b235 146\&
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147\& void SSL_CTX_set_tmp_rsa_callback(SSL_CTX *ctx,
148\& RSA *(*tmp_rsa_callback)(SSL *ssl, int is_export, int keylength));
149\& long SSL_CTX_set_tmp_rsa(SSL_CTX *ctx, RSA *rsa);
150\& long SSL_CTX_need_tmp_rsa(SSL_CTX *ctx);
e257b235 151\&
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152\& void SSL_set_tmp_rsa_callback(SSL_CTX *ctx,
153\& RSA *(*tmp_rsa_callback)(SSL *ssl, int is_export, int keylength));
154\& long SSL_set_tmp_rsa(SSL *ssl, RSA *rsa)
155\& long SSL_need_tmp_rsa(SSL *ssl)
e257b235 156\&
edae4a78 157\& RSA *(*tmp_rsa_callback)(SSL *ssl, int is_export, int keylength);
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158.Ve
159.SH "DESCRIPTION"
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160.IX Header "DESCRIPTION"
161\&\fISSL_CTX_set_tmp_rsa_callback()\fR sets the callback function for \fBctx\fR to be
162used when a temporary/ephemeral \s-1RSA\s0 key is required to \fBtmp_rsa_callback\fR.
163The callback is inherited by all \s-1SSL\s0 objects newly created from \fBctx\fR
164with <\fISSL_new\fR\|(3)|\fISSL_new\fR\|(3)>. Already created \s-1SSL\s0 objects are not affected.
984263bc 165.PP
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166\&\fISSL_CTX_set_tmp_rsa()\fR sets the temporary/ephemeral \s-1RSA\s0 key to be used to be
167\&\fBrsa\fR. The key is inherited by all \s-1SSL\s0 objects newly created from \fBctx\fR
168with <\fISSL_new\fR\|(3)|\fISSL_new\fR\|(3)>. Already created \s-1SSL\s0 objects are not affected.
984263bc 169.PP
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170\&\fISSL_CTX_need_tmp_rsa()\fR returns 1, if a temporary/ephemeral \s-1RSA\s0 key is needed
171for RSA-based strength-limited 'exportable' ciphersuites because a \s-1RSA\s0 key
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172with a keysize larger than 512 bits is installed.
173.PP
e056f0e0 174\&\fISSL_set_tmp_rsa_callback()\fR sets the callback only for \fBssl\fR.
984263bc 175.PP
e056f0e0 176\&\fISSL_set_tmp_rsa()\fR sets the key only for \fBssl\fR.
984263bc 177.PP
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178\&\fISSL_need_tmp_rsa()\fR returns 1, if a temporary/ephemeral \s-1RSA\s0 key is needed,
179for RSA-based strength-limited 'exportable' ciphersuites because a \s-1RSA\s0 key
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180with a keysize larger than 512 bits is installed.
181.PP
e056f0e0 182These functions apply to \s-1SSL/TLS\s0 servers only.
984263bc 183.SH "NOTES"
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184.IX Header "NOTES"
185When using a cipher with \s-1RSA\s0 authentication, an ephemeral \s-1RSA\s0 key exchange
984263bc 186can take place. In this case the session data are negotiated using the
e056f0e0 187ephemeral/temporary \s-1RSA\s0 key and the \s-1RSA\s0 key supplied and certified
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188by the certificate chain is only used for signing.
189.PP
e056f0e0 190Under previous export restrictions, ciphers with \s-1RSA\s0 keys shorter (512 bits)
984263bc 191than the usual key length of 1024 bits were created. To use these ciphers
e056f0e0 192with \s-1RSA\s0 keys of usual length, an ephemeral key exchange must be performed,
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193as the normal (certified) key cannot be directly used.
194.PP
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195Using ephemeral \s-1RSA\s0 key exchange yields forward secrecy, as the connection
196can only be decrypted, when the \s-1RSA\s0 key is known. By generating a temporary
197\&\s-1RSA\s0 key inside the server application that is lost when the application
984263bc 198is left, it becomes impossible for an attacker to decrypt past sessions,
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199even if he gets hold of the normal (certified) \s-1RSA\s0 key, as this key was
200used for signing only. The downside is that creating a \s-1RSA\s0 key is
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201computationally expensive.
202.PP
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203Additionally, the use of ephemeral \s-1RSA\s0 key exchange is only allowed in
204the \s-1TLS\s0 standard, when the \s-1RSA\s0 key can be used for signing only, that is
205for export ciphers. Using ephemeral \s-1RSA\s0 key exchange for other purposes
984263bc 206violates the standard and can break interoperability with clients.
e056f0e0 207It is therefore strongly recommended to not use ephemeral \s-1RSA\s0 key
e257b235 208exchange and use \s-1EDH\s0 (Ephemeral Diffie-Hellman) key exchange instead
984263bc 209in order to achieve forward secrecy (see
e056f0e0 210\&\fISSL_CTX_set_tmp_dh_callback\fR\|(3)).
984263bc 211.PP
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212On OpenSSL servers ephemeral \s-1RSA\s0 key exchange is therefore disabled by default
213and must be explicitly enabled using the \s-1SSL_OP_EPHEMERAL_RSA\s0 option of
214\&\fISSL_CTX_set_options\fR\|(3), violating the \s-1TLS/SSL\s0
215standard. When ephemeral \s-1RSA\s0 key exchange is required for export ciphers,
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216it will automatically be used without this option!
217.PP
218An application may either directly specify the key or can supply the key via
219a callback function. The callback approach has the advantage, that the
220callback may generate the key only in case it is actually needed. As the
e056f0e0 221generation of a \s-1RSA\s0 key is however costly, it will lead to a significant
984263bc 222delay in the handshake procedure. Another advantage of the callback function
e056f0e0 223is that it can supply keys of different size (e.g. for \s-1SSL_OP_EPHEMERAL_RSA\s0
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224usage) while the explicit setting of the key is only useful for key size of
225512 bits to satisfy the export restricted ciphers and does give away key length
226if a longer key would be allowed.
227.PP
228The \fBtmp_rsa_callback\fR is called with the \fBkeylength\fR needed and
229the \fBis_export\fR information. The \fBis_export\fR flag is set, when the
e056f0e0 230ephemeral \s-1RSA\s0 key exchange is performed with an export cipher.
984263bc 231.SH "EXAMPLES"
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232.IX Header "EXAMPLES"
233Generate temporary \s-1RSA\s0 keys to prepare ephemeral \s-1RSA\s0 key exchange. As the
234generation of a \s-1RSA\s0 key costs a lot of computer time, they saved for later
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235reuse. For demonstration purposes, two keys for 512 bits and 1024 bits
236respectively are generated.
237.PP
238.Vb 4
239\& ...
240\& /* Set up ephemeral RSA stuff */
241\& RSA *rsa_512 = NULL;
242\& RSA *rsa_1024 = NULL;
e257b235 243\&
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244\& rsa_512 = RSA_generate_key(512,RSA_F4,NULL,NULL);
245\& if (rsa_512 == NULL)
246\& evaluate_error_queue();
e257b235 247\&
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248\& rsa_1024 = RSA_generate_key(1024,RSA_F4,NULL,NULL);
249\& if (rsa_1024 == NULL)
250\& evaluate_error_queue();
e257b235 251\&
984263bc 252\& ...
e257b235 253\&
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254\& RSA *tmp_rsa_callback(SSL *s, int is_export, int keylength)
255\& {
256\& RSA *rsa_tmp=NULL;
e257b235 257\&
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258\& switch (keylength) {
259\& case 512:
260\& if (rsa_512)
261\& rsa_tmp = rsa_512;
262\& else { /* generate on the fly, should not happen in this example */
263\& rsa_tmp = RSA_generate_key(keylength,RSA_F4,NULL,NULL);
264\& rsa_512 = rsa_tmp; /* Remember for later reuse */
265\& }
266\& break;
267\& case 1024:
268\& if (rsa_1024)
269\& rsa_tmp=rsa_1024;
270\& else
271\& should_not_happen_in_this_example();
272\& break;
273\& default:
274\& /* Generating a key on the fly is very costly, so use what is there */
275\& if (rsa_1024)
276\& rsa_tmp=rsa_1024;
277\& else
278\& rsa_tmp=rsa_512; /* Use at least a shorter key */
279\& }
280\& return(rsa_tmp);
281\& }
282.Ve
283.SH "RETURN VALUES"
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284.IX Header "RETURN VALUES"
285\&\fISSL_CTX_set_tmp_rsa_callback()\fR and \fISSL_set_tmp_rsa_callback()\fR do not return
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286diagnostic output.
287.PP
e056f0e0 288\&\fISSL_CTX_set_tmp_rsa()\fR and \fISSL_set_tmp_rsa()\fR do return 1 on success and 0
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289on failure. Check the error queue to find out the reason of failure.
290.PP
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291\&\fISSL_CTX_need_tmp_rsa()\fR and \fISSL_need_tmp_rsa()\fR return 1 if a temporary
292\&\s-1RSA\s0 key is needed and 0 otherwise.
984263bc 293.SH "SEE ALSO"
a7d27d5a 294.IX Header "SEE ALSO"
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295\&\fIssl\fR\|(3), \fISSL_CTX_set_cipher_list\fR\|(3),
296\&\fISSL_CTX_set_options\fR\|(3),
297\&\fISSL_CTX_set_tmp_dh_callback\fR\|(3),
298\&\fISSL_new\fR\|(3), \fIciphers\fR\|(1)