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# Conflicts:
#	README.md
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Zhi Guan
2017-02-14 16:12:29 +08:00
parent d2254170b8
commit 43fed1108d
3503 changed files with 320546 additions and 408546 deletions
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56
doc/ssl/SSL_CTX_set_tmp_dh_callback.pod
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@@ -48,25 +48,8 @@ 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
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,
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).
(DH parameters) and generate a DH key. The server will always generate
a new DH key during the negotiation.
As generating DH parameters is extremely time consuming, an application
should not generate the parameters on the fly but supply the parameters.
@@ -74,18 +57,17 @@ 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. This application
openssl L<dhparam(1)> application. This application
guarantees that "strong" primes are used.
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. Generation of custom DH
L<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.
attacker from specializing on a commonly used group. File dh1024.pem
contains 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.
@@ -93,10 +75,9 @@ can supply the DH parameters via a callback function.
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.
are advised to either use SSL_CTX_set_tmp_dh() 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
@@ -123,12 +104,11 @@ partly left out.)
/* Error. */
}
if (dh_2048 == NULL) {
/* Error. */
/* Error. */
}
if (SSL_CTX_set_tmp_dh(ctx, dh_2048) != 1) {
/* Error. */
}
SSL_CTX_set_options(ctx, SSL_OP_SINGLE_DH_USE);
...
=head1 RETURN VALUES
@@ -141,9 +121,17 @@ on failure. Check the error queue to find out the reason of failure.
=head1 SEE ALSO
L<ssl(3)|ssl(3)>, L<SSL_CTX_set_cipher_list(3)|SSL_CTX_set_cipher_list(3)>,
L<SSL_CTX_set_tmp_rsa_callback(3)|SSL_CTX_set_tmp_rsa_callback(3)>,
L<SSL_CTX_set_options(3)|SSL_CTX_set_options(3)>,
L<ciphers(1)|ciphers(1)>, L<dhparam(1)|dhparam(1)>
L<ssl(3)>, L<SSL_CTX_set_cipher_list(3)>,
L<SSL_CTX_set_options(3)>,
L<ciphers(1)>, L<dhparam(1)>
=head1 COPYRIGHT
Copyright 2001-2016 The OpenSSL Project Authors. All Rights Reserved.
Licensed under the OpenSSL license (the "License"). You may not use
this file except in compliance with the License. You can obtain a copy
in the file LICENSE in the source distribution or at
L<https://www.openssl.org/source/license.html>.
=cut