openssl: Adjust manual pages for 1.0.1m.
[dragonfly.git] / secure / lib / libssl / man / SSL_CTX_set_tmp_dh_callback.3
... / ...
1.\" Automatically generated by Pod::Man 2.27 (Pod::Simple 3.28)
3.\" Standard preamble:
4.\" ======================================================================== Sp \" Vertical space (when we can't use .PP)
6.if t .sp .5v
7.if n .sp
8.. Vb \" Begin verbatim text
10.ft CW \\$1
13.. Ve \" End verbatim text
15.ft R
18.\" Set up some character translations and predefined strings. \*(-- will
19.\" give an unbreakable dash, \*(PI will give pi, \*(L" will give a left
20.\" double quote, and \*(R" will give a right double quote. \*(C+ will
21.\" give a nicer C++. Capital omega is used to do unbreakable dashes and
22.\" therefore won't be available. \*(C` and \*(C' expand to `' in nroff,
23.\" nothing in troff, for use with C<>. \(*W-
25.ds C+ C\v'-.1v'\h'-1p'\s-2+\h'-1p'+\s0\v'.1v'\h'-1p' n \{\
27. ds -- \(*W-
28. ds PI pi
29. if (\n(.H=4u)&(1m=24u) .ds -- \(*W\h'-12u'\(*W\h'-12u'-\" diablo 10 pitch
30. if (\n(.H=4u)&(1m=20u) .ds -- \(*W\h'-12u'\(*W\h'-8u'-\" diablo 12 pitch
31. ds L" ""
32. ds R" ""
33. ds C` ""
34. ds C' ""
37. ds -- \|\(em\|
38. ds PI \(*p
39. ds L" ``
40. ds R" ''
41. ds C`
42. ds C'
45.\" Escape single quotes in literal strings from groff's Unicode transform. \n(.g .ds Aq \(aq
47.el .ds Aq '
49.\" If the F register is turned on, we'll generate index entries on stderr for
50.\" titles (.TH), headers (.SH), subsections (.SS), items (.Ip), and index
51.\" entries marked with X<> in POD. Of course, you'll have to process the
52.\" output yourself in some meaningful fashion.
54.\" Avoid warning from groff about undefined register 'F'. IX
56.. rF 0
58.if \n(.g .if rF .nr rF 1
59.if (\n(rF:(\n(.g==0)) \{
60. if \nF \{
61. de IX
62. tm Index:\\$1\t\\n%\t"\\$2"
64. if !\nF==2 \{
65. nr % 0
66. nr F 2
67. \}
68. \}
70.rr rF
72.\" Accent mark definitions (@(#)ms.acc 1.5 88/02/08 SMI; from UCB 4.2).
73.\" Fear. Run. Save yourself. No user-serviceable parts.
74. \" fudge factors for nroff and troff
75.if n \{\
76. ds #H 0
77. ds #V .8m
78. ds #F .3m
79. ds #[ \f1
80. ds #] \fP
82.if t \{\
83. ds #H ((1u-(\\\\n(.fu%2u))*.13m)
84. ds #V .6m
85. ds #F 0
86. ds #[ \&
87. ds #] \&
89. \" simple accents for nroff and troff
90.if n \{\
91. ds ' \&
92. ds ` \&
93. ds ^ \&
94. ds , \&
95. ds ~ ~
96. ds /
98.if t \{\
99. ds ' \\k:\h'-(\\n(.wu*8/10-\*(#H)'\'\h"|\\n:u"
100. ds ` \\k:\h'-(\\n(.wu*8/10-\*(#H)'\`\h'|\\n:u'
101. ds ^ \\k:\h'-(\\n(.wu*10/11-\*(#H)'^\h'|\\n:u'
102. ds , \\k:\h'-(\\n(.wu*8/10)',\h'|\\n:u'
103. ds ~ \\k:\h'-(\\n(.wu-\*(#H-.1m)'~\h'|\\n:u'
104. ds / \\k:\h'-(\\n(.wu*8/10-\*(#H)'\z\(sl\h'|\\n:u'
106. \" troff and (daisy-wheel) nroff accents
107.ds : \\k:\h'-(\\n(.wu*8/10-\*(#H+.1m+\*(#F)'\v'-\*(#V'\z.\h'.2m+\*(#F'.\h'|\\n:u'\v'\*(#V'
108.ds 8 \h'\*(#H'\(*b\h'-\*(#H'
109.ds o \\k:\h'-(\\n(.wu+\w'\(de'u-\*(#H)/2u'\v'-.3n'\*(#[\z\(de\v'.3n'\h'|\\n:u'\*(#]
110.ds d- \h'\*(#H'\(pd\h'-\w'~'u'\v'-.25m'\f2\(hy\fP\v'.25m'\h'-\*(#H'
111.ds D- D\\k:\h'-\w'D'u'\v'-.11m'\z\(hy\v'.11m'\h'|\\n:u'
112.ds th \*(#[\v'.3m'\s+1I\s-1\v'-.3m'\h'-(\w'I'u*2/3)'\s-1o\s+1\*(#]
113.ds Th \*(#[\s+2I\s-2\h'-\w'I'u*3/5'\v'-.3m'o\v'.3m'\*(#]
114.ds ae a\h'-(\w'a'u*4/10)'e
115.ds Ae A\h'-(\w'A'u*4/10)'E
116. \" corrections for vroff
117.if v .ds ~ \\k:\h'-(\\n(.wu*9/10-\*(#H)'\s-2\u~\d\s+2\h'|\\n:u'
118.if v .ds ^ \\k:\h'-(\\n(.wu*10/11-\*(#H)'\v'-.4m'^\v'.4m'\h'|\\n:u'
119. \" for low resolution devices (crt and lpr)
120.if \n(.H>23 .if \n(.V>19 \
122. ds : e
123. ds 8 ss
124. ds o a
125. ds d- d\h'-1'\(ga
126. ds D- D\h'-1'\(hy
127. ds th \o'bp'
128. ds Th \o'LP'
129. ds ae ae
130. ds Ae AE
132.rm #[ #] #H #V #F C
133.\" ========================================================================
135.IX Title "SSL_CTX_set_tmp_dh_callback 3"
136.TH SSL_CTX_set_tmp_dh_callback 3 "2015-03-19" "1.0.1m" "OpenSSL"
137.\" For nroff, turn off justification. Always turn off hyphenation; it makes
138.\" way too many mistakes in technical documents.
139.if n .ad l
141.SH "NAME"
142SSL_CTX_set_tmp_dh_callback, SSL_CTX_set_tmp_dh, SSL_set_tmp_dh_callback, SSL_set_tmp_dh \- handle DH keys for ephemeral key exchange
144.IX Header "SYNOPSIS"
145.Vb 1
146\& #include <openssl/ssl.h>
148\& void SSL_CTX_set_tmp_dh_callback(SSL_CTX *ctx,
149\& DH *(*tmp_dh_callback)(SSL *ssl, int is_export, int keylength));
150\& long SSL_CTX_set_tmp_dh(SSL_CTX *ctx, DH *dh);
152\& void SSL_set_tmp_dh_callback(SSL *ctx,
153\& DH *(*tmp_dh_callback)(SSL *ssl, int is_export, int keylength));
154\& long SSL_set_tmp_dh(SSL *ssl, DH *dh)
158\&\fISSL_CTX_set_tmp_dh_callback()\fR sets the callback function for \fBctx\fR to be
159used when a \s-1DH\s0 parameters are required to \fBtmp_dh_callback\fR.
160The callback is inherited by all \fBssl\fR objects created from \fBctx\fR.
162\&\fISSL_CTX_set_tmp_dh()\fR sets \s-1DH\s0 parameters to be used to be \fBdh\fR.
163The key is inherited by all \fBssl\fR objects created from \fBctx\fR.
165\&\fISSL_set_tmp_dh_callback()\fR sets the callback only for \fBssl\fR.
167\&\fISSL_set_tmp_dh()\fR sets the parameters only for \fBssl\fR.
169These functions apply to \s-1SSL/TLS\s0 servers only.
170.SH "NOTES"
171.IX Header "NOTES"
172When using a cipher with \s-1RSA\s0 authentication, an ephemeral \s-1DH\s0 key exchange
173can take place. Ciphers with \s-1DSA\s0 keys always use ephemeral \s-1DH\s0 keys as well.
174In these cases, the session data are negotiated using the
175ephemeral/temporary \s-1DH\s0 key and the key supplied and certified
176by the certificate chain is only used for signing.
177Anonymous ciphers (without a permanent server key) also use ephemeral \s-1DH\s0 keys.
179Using ephemeral \s-1DH\s0 key exchange yields forward secrecy, as the connection
180can only be decrypted, when the \s-1DH\s0 key is known. By generating a temporary
181\&\s-1DH\s0 key inside the server application that is lost when the application
182is left, it becomes impossible for an attacker to decrypt past sessions,
183even if he gets hold of the normal (certified) key, as this key was
184only used for signing.
186In order to perform a \s-1DH\s0 key exchange the server must use a \s-1DH\s0 group
187(\s-1DH\s0 parameters) and generate a \s-1DH\s0 key.
188The server will always generate a new \s-1DH\s0 key during the negotiation
189if either the \s-1DH\s0 parameters are supplied via callback or the
190\&\s-1SSL_OP_SINGLE_DH_USE\s0 option of \fISSL_CTX_set_options\fR\|(3) is set (or both).
191It will immediately create a \s-1DH\s0 key if \s-1DH\s0 parameters are supplied via
192\&\fISSL_CTX_set_tmp_dh()\fR and \s-1SSL_OP_SINGLE_DH_USE\s0 is not set.
193In this case,
194it may happen that a key is generated on initialization without later
195being needed, while on the other hand the computer time during the
196negotiation is being saved.
198If \*(L"strong\*(R" primes were used to generate the \s-1DH\s0 parameters, it is not strictly
199necessary to generate a new key for each handshake but it does improve forward
200secrecy. If it is not assured, that \*(L"strong\*(R" primes were used (see especially
201the section about \s-1DSA\s0 parameters below), \s-1SSL_OP_SINGLE_DH_USE\s0 must be used
202in order to prevent small subgroup attacks. Always using \s-1SSL_OP_SINGLE_DH_USE\s0
203has an impact on the computer time needed during negotiation, but it is not
204very large, so application authors/users should consider to always enable
205this option.
207As generating \s-1DH\s0 parameters is extremely time consuming, an application
208should not generate the parameters on the fly but supply the parameters.
209\&\s-1DH\s0 parameters can be reused, as the actual key is newly generated during
210the negotiation. The risk in reusing \s-1DH\s0 parameters is that an attacker
211may specialize on a very often used \s-1DH\s0 group. Applications should therefore
212generate their own \s-1DH\s0 parameters during the installation process using the
213openssl \fIdhparam\fR\|(1) application. In order to reduce the computer
214time needed for this generation, it is possible to use \s-1DSA\s0 parameters
215instead (see \fIdhparam\fR\|(1)), but in this case \s-1SSL_OP_SINGLE_DH_USE\s0
216is mandatory.
218Application authors may compile in \s-1DH\s0 parameters. Files dh512.pem,
219dh1024.pem, dh2048.pem, and dh4096.pem in the 'apps' directory of current
220version of the OpenSSL distribution contain the '\s-1SKIP\s0' \s-1DH\s0 parameters,
221which use safe primes and were generated verifiably pseudo-randomly.
222These files can be converted into C code using the \fB\-C\fR option of the
223\&\fIdhparam\fR\|(1) application.
224Authors may also generate their own set of parameters using
225\&\fIdhparam\fR\|(1), but a user may not be sure how the parameters were
226generated. The generation of \s-1DH\s0 parameters during installation is therefore
229An application may either directly specify the \s-1DH\s0 parameters or
230can supply the \s-1DH\s0 parameters via a callback function. The callback approach
231has the advantage, that the callback may supply \s-1DH\s0 parameters for different
232key lengths.
234The \fBtmp_dh_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-1DH\s0 key exchange is performed with an export cipher.
238.IX Header "EXAMPLES"
239Handle \s-1DH\s0 parameters for key lengths of 512 and 1024 bits. (Error handling
240partly left out.)
242.Vb 5
243\& ...
244\& /* Set up ephemeral DH stuff */
245\& DH *dh_512 = NULL;
246\& DH *dh_1024 = NULL;
247\& FILE *paramfile;
249\& ...
250\& /* "openssl dhparam \-out dh_param_512.pem \-2 512" */
251\& paramfile = fopen("dh_param_512.pem", "r");
252\& if (paramfile) {
253\& dh_512 = PEM_read_DHparams(paramfile, NULL, NULL, NULL);
254\& fclose(paramfile);
255\& }
256\& /* "openssl dhparam \-out dh_param_1024.pem \-2 1024" */
257\& paramfile = fopen("dh_param_1024.pem", "r");
258\& if (paramfile) {
259\& dh_1024 = PEM_read_DHparams(paramfile, NULL, NULL, NULL);
260\& fclose(paramfile);
261\& }
262\& ...
264\& /* "openssl dhparam \-C \-2 512" etc... */
265\& DH *get_dh512() { ... }
266\& DH *get_dh1024() { ... }
268\& DH *tmp_dh_callback(SSL *s, int is_export, int keylength)
269\& {
270\& DH *dh_tmp=NULL;
272\& switch (keylength) {
273\& case 512:
274\& if (!dh_512)
275\& dh_512 = get_dh512();
276\& dh_tmp = dh_512;
277\& break;
278\& case 1024:
279\& if (!dh_1024)
280\& dh_1024 = get_dh1024();
281\& dh_tmp = dh_1024;
282\& break;
283\& default:
284\& /* Generating a key on the fly is very costly, so use what is there */
285\& setup_dh_parameters_like_above();
286\& }
287\& return(dh_tmp);
288\& }
292\&\fISSL_CTX_set_tmp_dh_callback()\fR and \fISSL_set_tmp_dh_callback()\fR do not return
293diagnostic output.
295\&\fISSL_CTX_set_tmp_dh()\fR and \fISSL_set_tmp_dh()\fR do return 1 on success and 0
296on failure. Check the error queue to find out the reason of failure.
298.IX Header "SEE ALSO"
299\&\fIssl\fR\|(3), \fISSL_CTX_set_cipher_list\fR\|(3),
302\&\fIciphers\fR\|(1), \fIdhparam\fR\|(1)