Standard preamble:
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.... Set up some character translations and predefined strings. \*(-- will
give an unbreakable dash, \*(PI will give pi, \*(L" will give a left
double quote, and \*(R" will give a right double quote. \*(C+ will
give a nicer C++. Capital omega is used to do unbreakable dashes and
therefore won't be available. \*(C` and \*(C' expand to `' in nroff,
nothing in troff, for use with C<>.
.tr \(*W- . ds -- \(*W- . ds PI pi . if (\n(.H=4u)&(1m=24u) .ds -- \(*W\h'-12u'\(*W\h'-12u'-\" diablo 10 pitch . if (\n(.H=4u)&(1m=20u) .ds -- \(*W\h'-12u'\(*W\h'-8u'-\" diablo 12 pitch . ds L" "" . ds R" "" . ds C` "" . ds C' "" 'br\} . ds -- \|\(em\| . ds PI \(*p . ds L" `` . ds R" '' . ds C` . ds C' 'br\}
Escape single quotes in literal strings from groff's Unicode transform.
If the F register is turned on, we'll generate index entries on stderr for
titles (.TH), headers (.SH), subsections (.SS), items (.Ip), and index
entries marked with X<> in POD. Of course, you'll have to process the
output yourself in some meaningful fashion.
Avoid warning from groff about undefined register 'F'.
.. .nr rF 0 . if \nF \{ . de IX . tm Index:\\$1\t\\n%\t"\\$2" .. . if !\nF==2 \{ . nr % 0 . nr F 2 . \} . \} .\} .rr rF
Accent mark definitions (@(#)ms.acc 1.5 88/02/08 SMI; from UCB 4.2).
Fear. Run. Save yourself. No user-serviceable parts.
. \" fudge factors for nroff and troff . ds #H 0 . ds #V .8m . ds #F .3m . ds #[ \f1 . ds #] .\} . ds #H ((1u-(\\\\n(.fu%2u))*.13m) . ds #V .6m . ds #F 0 . ds #[ \& . ds #] \& .\} . \" simple accents for nroff and troff . ds ' \& . ds ` \& . ds ^ \& . ds , \& . ds ~ ~ . ds / .\} . ds ' \\k:\h'-(\\n(.wu*8/10-\*(#H)'\'\h"|\\n:u" . ds ` \\k:\h'-(\\n(.wu*8/10-\*(#H)'\`\h'|\\n:u' . ds ^ \\k:\h'-(\\n(.wu*10/11-\*(#H)'^\h'|\\n:u' . ds , \\k:\h'-(\\n(.wu*8/10)',\h'|\\n:u' . ds ~ \\k:\h'-(\\n(.wu-\*(#H-.1m)'~\h'|\\n:u' . ds / \\k:\h'-(\\n(.wu*8/10-\*(#H)'\z\(sl\h'|\\n:u' .\} . \" troff and (daisy-wheel) nroff accents . \" corrections for vroff . \" for low resolution devices (crt and lpr) \{\ . ds : e . ds 8 ss . ds o a . ds d- d\h'-1'\(ga . ds D- D\h'-1'\(hy . ds th \o'bp' . ds Th \o'LP' . ds ae ae . ds Ae AE .\} ========================================================================
Title "SSL_CTX_set_tmp_dh_callback 3"
way too many mistakes in technical documents.
\fISSL_CTX_set_tmp_dh() sets \s-1DH\s0 parameters to be used to be dh. The key is inherited by all ssl objects created from ctx.
\fISSL_set_tmp_dh_callback() sets the callback only for ssl.
\fISSL_set_tmp_dh() sets the parameters only for ssl.
These functions apply to \s-1SSL/TLS\s0 servers only.
Using ephemeral \s-1DH\s0 key exchange yields forward secrecy, as the connection can only be decrypted, when the \s-1DH\s0 key is known. By generating a temporary \s-1DH\s0 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) key, as this key was only used for signing.
In order to perform a \s-1DH\s0 key exchange the server must use a \s-1DH\s0 group (\s-1DH\s0 parameters) and generate a \s-1DH\s0 key. The server will always generate a new \s-1DH\s0 key during the negotiation, when the \s-1DH\s0 parameters are supplied via callback and/or when the \s-1SSL_OP_SINGLE_DH_USE\s0 option of \fISSL_CTX_set_options\|(3) is set. It will immediately create a \s-1DH\s0 key, when \s-1DH\s0 parameters are supplied via \fISSL_CTX_set_tmp_dh() and \s-1SSL_OP_SINGLE_DH_USE\s0 is not set. In this case, it may happen that a key is generated on initialization without later being needed, while on the other hand the computer time during the negotiation is being saved.
If \*(L"strong\*(R" primes were used to generate the \s-1DH\s0 parameters, it is not strictly necessary to generate a new key for each handshake but it does improve forward secrecy. If it is not assured, that \*(L"strong\*(R" primes were used (see especially the section about \s-1DSA\s0 parameters below), \s-1SSL_OP_SINGLE_DH_USE\s0 must be used in order to prevent small subgroup attacks. Always using \s-1SSL_OP_SINGLE_DH_USE\s0 has an impact on the computer time needed during negotiation, but it is not very large, so application authors/users should consider to always enable this option.
As generating \s-1DH\s0 parameters is extremely time consuming, an application should not generate the parameters on the fly but supply the parameters. \s-1DH\s0 parameters can be reused, as the actual key is newly generated during the negotiation. The risk in reusing \s-1DH\s0 parameters is that an attacker may specialize on a very often used \s-1DH\s0 group. Applications should therefore generate their own \s-1DH\s0 parameters during the installation process using the openssl dhparam\|(1) application. In order to reduce the computer time needed for this generation, it is possible to use \s-1DSA\s0 parameters instead (see dhparam\|(1)), but in this case \s-1SSL_OP_SINGLE_DH_USE\s0 is mandatory.
Application authors may compile in \s-1DH\s0 parameters. Files dh512.pem, dh1024.pem, dh2048.pem, and dh4096.pem in the 'apps' directory of current version of the OpenSSL distribution contain the '\s-1SKIP\s0' \s-1DH\s0 parameters, which use safe primes and were generated verifiably pseudo-randomly. These files can be converted into C code using the -C option of the \fIdhparam\|(1) application. Authors may also generate their own set of parameters using \fIdhparam\|(1), but a user may not be sure how the parameters were generated. The generation of \s-1DH\s0 parameters during installation is therefore recommended.
An application may either directly specify the \s-1DH\s0 parameters or can supply the \s-1DH\s0 parameters via a callback function. The callback approach has the advantage, that the callback may supply \s-1DH\s0 parameters for different key lengths.
The tmp_dh_callback is called with the keylength needed and the is_export information. The is_export flag is set, when the ephemeral \s-1DH\s0 key exchange is performed with an export cipher.
.Vb 5 ... /* Set up ephemeral DH stuff */ DH *dh_512 = NULL; DH *dh_1024 = NULL; FILE *paramfile; \& ... /* "openssl dhparam -out dh_param_512.pem -2 512" */ paramfile = fopen("dh_param_512.pem", "r"); if (paramfile) { dh_512 = PEM_read_DHparams(paramfile, NULL, NULL, NULL); fclose(paramfile); } /* "openssl dhparam -out dh_param_1024.pem -2 1024" */ paramfile = fopen("dh_param_1024.pem", "r"); if (paramfile) { dh_1024 = PEM_read_DHparams(paramfile, NULL, NULL, NULL); fclose(paramfile); } ... \& /* "openssl dhparam -C -2 512" etc... */ DH *get_dh512() { ... } DH *get_dh1024() { ... } \& DH *tmp_dh_callback(SSL *s, int is_export, int keylength) { DH *dh_tmp=NULL; \& switch (keylength) { case 512: if (!dh_512) dh_512 = get_dh512(); dh_tmp = dh_512; break; case 1024: if (!dh_1024) dh_1024 = get_dh1024(); dh_tmp = dh_1024; break; default: /* Generating a key on the fly is very costly, so use what is there */ setup_dh_parameters_like_above(); } return(dh_tmp); } .Ve
\fISSL_CTX_set_tmp_dh() and SSL_set_tmp_dh() do return 1 on success and 0 on failure. Check the error queue to find out the reason of failure.