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Zhi Guan
2015-08-15 15:02:15 +08:00
parent 06df2fab54
commit 3bdc0ea895
2536 changed files with 417052 additions and 271997 deletions
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123
doc/ssl/SSL_CTX_set_tmp_dh_callback.pod
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@@ -12,12 +12,10 @@ SSL_CTX_set_tmp_dh_callback, SSL_CTX_set_tmp_dh, SSL_set_tmp_dh_callback, SSL_se
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 *ctx,
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)
DH *(*tmp_dh_callback)(SSL *ssl, int is_export, int keylength));
=head1 DESCRIPTION
SSL_CTX_set_tmp_dh_callback() sets the callback function for B<ctx> to be
@@ -50,24 +48,25 @@ even if he gets hold of the normal (certified) key, as this key was
only used for signing.
In order to perform a DH key exchange the server must use a DH group
(DH parameters) and generate a DH key. The server will always generate a new
DH key during the negotiation, when the DH parameters are supplied via
callback and/or when the SSL_OP_SINGLE_DH_USE option of
L<SSL_CTX_set_options(3)|SSL_CTX_set_options(3)> is set. It will
immediately create a DH key, when DH parameters are supplied via
SSL_CTX_set_tmp_dh() and SSL_OP_SINGLE_DH_USE is not set. In this case,
(DH parameters) and generate a DH key.
The server will always generate a new DH key during the negotiation
if either the DH parameters are supplied via callback or the
SSL_OP_SINGLE_DH_USE option of SSL_CTX_set_options(3) is set (or both).
It will immediately create a DH key if DH parameters are supplied via
SSL_CTX_set_tmp_dh() and SSL_OP_SINGLE_DH_USE 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 "strong" primes were used to generate the DH 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 "strong" primes were used (see especially
the section about DSA parameters below), SSL_OP_SINGLE_DH_USE must be used
in order to prevent small subgroup attacks. Always using SSL_OP_SINGLE_DH_USE
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.
secrecy. If it is not assured that "strong" primes were used,
SSL_OP_SINGLE_DH_USE must be used in order to prevent small subgroup
attacks. Always using SSL_OP_SINGLE_DH_USE 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 (PFS).
As generating DH parameters is extremely time consuming, an application
should not generate the parameters on the fly but supply the parameters.
@@ -75,83 +74,63 @@ DH parameters can be reused, as the actual key is newly generated during
the negotiation. The risk in reusing DH parameters is that an attacker
may specialize on a very often used DH group. Applications should therefore
generate their own DH parameters during the installation process using the
openssl L<dhparam(1)|dhparam(1)> application. In order to reduce the computer
time needed for this generation, it is possible to use DSA parameters
instead (see L<dhparam(1)|dhparam(1)>), but in this case SSL_OP_SINGLE_DH_USE
is mandatory.
openssl L<dhparam(1)|dhparam(1)> application. This application
guarantees that "strong" primes are used.
Application authors may compile in DH parameters. Files dh512.pem,
dh1024.pem, dh2048.pem, and dh4096 in the 'apps' directory of current
Files dh2048.pem, and dh4096.pem in the 'apps' directory of the current
version of the OpenSSL distribution contain the 'SKIP' DH parameters,
which use safe primes and were generated verifiably pseudo-randomly.
These files can be converted into C code using the B<-C> option of the
L<dhparam(1)|dhparam(1)> application.
Authors may also generate their own set of parameters using
L<dhparam(1)|dhparam(1)>, but a user may not be sure how the parameters were
generated. The generation of DH parameters during installation is therefore
recommended.
L<dhparam(1)|dhparam(1)> application. Generation of custom DH
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.
An application may either directly specify the DH parameters or
can supply the DH parameters via a callback function. The callback approach
has the advantage, that the callback may supply DH parameters for different
key lengths.
can supply the DH parameters via a callback function.
The B<tmp_dh_callback> is called with the B<keylength> needed and
the B<is_export> information. The B<is_export> flag is set, when the
ephemeral DH key exchange is performed with an export cipher.
Previous versions of the callback used B<is_export> and B<keylength>
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 SSL_CTX_set_tmp_dh() in combination with
SSL_OP_SINGLE_DH_USE, or alternatively, use the callback but ignore
B<keylength> and B<is_export> and simply supply at least 2048-bit
parameters in the callback.
=head1 EXAMPLES
Handle DH parameters for key lengths of 512 and 1024 bits. (Error handling
Setup DH parameters with a key length of 2048 bits. (Error handling
partly left out.)
Command-line parameter generation:
$ openssl dhparam -out dh_param_2048.pem 2048
Code for setting up parameters during server initialization:
...
/* Set up ephemeral DH stuff */
DH *dh_512 = NULL;
DH *dh_1024 = NULL;
SSL_CTX ctx = SSL_CTX_new();
...
/* Set up ephemeral DH parameters. */
DH *dh_2048 = NULL;
FILE *paramfile;
...
/* "openssl dhparam -out dh_param_512.pem -2 512" */
paramfile = fopen("dh_param_512.pem", "r");
paramfile = fopen("dh_param_2048.pem", "r");
if (paramfile) {
dh_512 = PEM_read_DHparams(paramfile, NULL, NULL, NULL);
dh_2048 = PEM_read_DHparams(paramfile, NULL, NULL, NULL);
fclose(paramfile);
} else {
/* Error. */
}
/* "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);
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);
...
/* "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);
}
=head1 RETURN VALUES
SSL_CTX_set_tmp_dh_callback() and SSL_set_tmp_dh_callback() do not return