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.\" ========================================================================
.\"
.IX Title "SSL_CTX_set_tmp_dh_callback 3"
.TH SSL_CTX_set_tmp_dh_callback 3 "2019-08-14" "1.0.1e" "OpenSSL"
.\" For nroff, turn off justification.  Always turn off hyphenation; it makes
.\" way too many mistakes in technical documents.
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.nh
.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>
\&
\& 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);
\&
\& void SSL_set_tmp_dh_callback(SSL *ctx,
\&            DH *(*tmp_dh_callback)(SSL *ssl, int is_export, int keylength));
\& long SSL_set_tmp_dh(SSL *ssl, DH *dh)
.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
if either the \s-1DH\s0 parameters are supplied via callback or the
\&\s-1SSL_OP_SINGLE_DH_USE\s0 option of \fISSL_CTX_set_options\fR\|(3) is set (or both).
It will  immediately create a \s-1DH\s0 key if \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,
\&\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 always enabling this option.
The option is required to implement perfect forward secrecy (\s-1PFS\s0).
.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. This application
guarantees that \*(L"strong\*(R" primes are used.
.PP
Files dh2048.pem, and dh4096.pem in the 'apps' directory of the 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. Generation of custom \s-1DH\s0
parameters during installation should still be preferred to stop an
attacker from specializing on a commonly used group. Files dh1024.pem
and dh512.pem contain old parameters that must not be used by
applications.
.PP
An application may either directly specify the \s-1DH\s0 parameters or
can supply the \s-1DH\s0 parameters via a callback function.
.PP
Previous versions of the callback used \fBis_export\fR and \fBkeylength\fR
parameters to control parameter generation for export and non-export
cipher suites. Modern servers that do not support export ciphersuites
are advised to either use \fISSL_CTX_set_tmp_dh()\fR in combination with
\&\s-1SSL_OP_SINGLE_DH_USE\s0, or alternatively, use the callback but ignore
\&\fBkeylength\fR and \fBis_export\fR and simply supply at least 2048\-bit
parameters in the callback.
.SH "EXAMPLES"
.IX Header "EXAMPLES"
Setup \s-1DH\s0 parameters with a key length of 2048 bits. (Error handling
partly left out.)
.PP
.Vb 2
\& Command\-line parameter generation:
\& $ openssl dhparam \-out dh_param_2048.pem 2048
\&
\& Code for setting up parameters during server initialization:
\&
\& ...
\& SSL_CTX ctx = SSL_CTX_new();
\& ...
\&
\& /* Set up ephemeral DH parameters. */
\& DH *dh_2048 = NULL;
\& FILE *paramfile;
\& paramfile = fopen("dh_param_2048.pem", "r");
\& if (paramfile) {
\&   dh_2048 = PEM_read_DHparams(paramfile, NULL, NULL, NULL);
\&   fclose(paramfile);
\& } else {
\&   /* Error. */
\& }
\& if (dh_2048 == NULL) {
\&  /* Error. */
\& }
\& if (SSL_CTX_set_tmp_dh(ctx, dh_2048) != 1) {
\&   /* Error. */
\& }
\& SSL_CTX_set_options(ctx, SSL_OP_SINGLE_DH_USE);
\& ...
.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)
