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Diffstat (limited to 'secure/lib/libcrypto/man/SSL_CTX_set_tmp_dh_callback.3')
-rw-r--r-- | secure/lib/libcrypto/man/SSL_CTX_set_tmp_dh_callback.3 | 312 |
1 files changed, 312 insertions, 0 deletions
diff --git a/secure/lib/libcrypto/man/SSL_CTX_set_tmp_dh_callback.3 b/secure/lib/libcrypto/man/SSL_CTX_set_tmp_dh_callback.3 new file mode 100644 index 0000000..5c9d0e7 --- /dev/null +++ b/secure/lib/libcrypto/man/SSL_CTX_set_tmp_dh_callback.3 @@ -0,0 +1,312 @@ +.\" Automatically generated by Pod::Man version 1.15 +.\" Thu May 9 13:22:24 2002 +.\" +.\" Standard preamble: +.\" ====================================================================== +.de Sh \" Subsection heading +.br +.if t .Sp +.ne 5 +.PP +\fB\\$1\fR +.PP +.. +.de Sp \" Vertical space (when we can't use .PP) +.if t .sp .5v +.if n .sp +.. +.de Ip \" List item +.br +.ie \\n(.$>=3 .ne \\$3 +.el .ne 3 +.IP "\\$1" \\$2 +.. +.de Vb \" Begin verbatim text +.ft CW +.nf +.ne \\$1 +.. +.de Ve \" End verbatim text +.ft R + +.fi +.. +.\" 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. | will give a +.\" real vertical bar. \*(C+ will give a nicer C++. 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Ciphers with \s-1DSA\s0 keys always use ephemeral \s-1DH\s0 keys as well. +In these cases, the session data are negotiated using the +ephemeral/temporary \s-1DH\s0 key and the key supplied and certified +by the certificate chain is only used for signing. +Anonymous ciphers (without a permanent server key) also use ephemeral \s-1DH\s0 keys. +.PP +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. +.PP +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 +SSL_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()\fR 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. +.PP +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. +.PP +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. +.PP +Application authors may compile in \s-1DH\s0 parameters. Files dh512.pem, +dh1024.pem, dh2048.pem, and dh4096 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 \fB\-C\fR option of the +dhparam(1) application. +Authors may also generate their own set of parameters using +dhparam(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. +.PP +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. +.PP +The \fBtmp_dh_callback\fR is called with the \fBkeylength\fR needed and +the \fBis_export\fR information. The \fBis_export\fR flag is set, when the +ephemeral \s-1DH\s0 key exchange is performed with an export cipher. +.SH "EXAMPLES" +.IX Header "EXAMPLES" +Handle \s-1DH\s0 parameters for key lengths of 512 and 1024 bits. (Error handling +partly left out.) +.PP +.Vb 5 +\& ... +\& /* Set up ephemeral DH stuff */ +\& DH *dh_512 = NULL; +\& DH *dh_1024 = NULL; +\& FILE *paramfile; +.Ve +.Vb 14 +\& ... +\& /* "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); +\& } +\& ... +.Ve +.Vb 3 +\& /* "openssl dhparam -C -2 512" etc... */ +\& DH *get_dh512() { ... } +\& DH *get_dh1024() { ... } +.Ve +.Vb 3 +\& DH *tmp_dh_callback(SSL *s, int is_export, int keylength) +\& { +\& DH *dh_tmp=NULL; +.Ve +.Vb 17 +\& 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 +.SH "RETURN VALUES" +.IX Header "RETURN VALUES" +\&\fISSL_CTX_set_tmp_dh_callback()\fR and \fISSL_set_tmp_dh_callback()\fR do not return +diagnostic output. +.PP +\&\fISSL_CTX_set_tmp_dh()\fR and \fISSL_set_tmp_dh()\fR do return 1 on success and 0 +on failure. Check the error queue to find out the reason of failure. +.SH "SEE ALSO" +.IX Header "SEE ALSO" +ssl(3), SSL_CTX_set_cipher_list(3), +SSL_CTX_set_tmp_rsa_callback(3), +SSL_CTX_set_options(3), +ciphers(1), dhparam(1) |