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.\" ========================================================================
.\"
.IX Title "SSL_CTX_set_tmp_dh_callback 3"
.TH SSL_CTX_set_tmp_dh_callback 3 "2005-07-06" "0.9.8" "OpenSSL"
.SH "NAME"
SSL_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
.SH "SYNOPSIS"
.IX Header "SYNOPSIS"
.Vb 1
\& #include <openssl/ssl.h>
.Ve
.PP
.Vb 3
\& void SSL_CTX_set_tmp_dh_callback(SSL_CTX *ctx,
\&            DH *(*tmp_dh_callback)(SSL *ssl, int is_export, int keylength));
\& long SSL_CTX_set_tmp_dh(SSL_CTX *ctx, DH *dh);
.Ve
.PP
.Vb 3
\& void SSL_set_tmp_dh_callback(SSL_CTX *ctx,
\&            DH *(*tmp_dh_callback)(SSL *ssl, int is_export, int keylength));
\& long SSL_set_tmp_dh(SSL *ssl, DH *dh)
.Ve
.PP
.Vb 1
\& DH *(*tmp_dh_callback)(SSL *ssl, int is_export, int keylength));
.Ve
.SH "DESCRIPTION"
.IX Header "DESCRIPTION"
\&\fISSL_CTX_set_tmp_dh_callback()\fR sets the callback function for \fBctx\fR to be
used when a \s-1DH\s0 parameters are required to \fBtmp_dh_callback\fR.
The callback is inherited by all \fBssl\fR objects created from \fBctx\fR.
.PP
\&\fISSL_CTX_set_tmp_dh()\fR sets \s-1DH\s0 parameters to be used to be \fBdh\fR.
The key is inherited by all \fBssl\fR objects created from \fBctx\fR.
.PP
\&\fISSL_set_tmp_dh_callback()\fR sets the callback only for \fBssl\fR.
.PP
\&\fISSL_set_tmp_dh()\fR sets the parameters only for \fBssl\fR.
.PP
These functions apply to \s-1SSL/TLS\s0 servers only.
.SH "NOTES"
.IX Header "NOTES"
When using a cipher with \s-1RSA\s0 authentication, an ephemeral \s-1DH\s0 key exchange
can take place. 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
\&\fISSL_CTX_set_options\fR\|(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 \fIdhparam\fR\|(1) application. In order to reduce the computer
time needed for this generation, it is possible to use \s-1DSA\s0 parameters
instead (see \fIdhparam\fR\|(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
\&\fIdhparam\fR\|(1) application.
Authors may also generate their own set of parameters using
\&\fIdhparam\fR\|(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
.PP
.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
.PP
.Vb 3
\& /* "openssl dhparam -C -2 512" etc... */
\& DH *get_dh512() { ... }
\& DH *get_dh1024() { ... }
.Ve
.PP
.Vb 3
\& DH *tmp_dh_callback(SSL *s, int is_export, int keylength)
\& {
\&    DH *dh_tmp=NULL;
.Ve
.PP
.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"
\&\fIssl\fR\|(3), \fISSL_CTX_set_cipher_list\fR\|(3),
\&\fISSL_CTX_set_tmp_rsa_callback\fR\|(3),
\&\fISSL_CTX_set_options\fR\|(3),
\&\fIciphers\fR\|(1), \fIdhparam\fR\|(1)