=head1 DESCRIPTION
SSL_CTX_set_options() adds the options set via bitmask in B<options> to B<ctx>.
-Options already set before are not cleared.
+Options already set before are not cleared!
SSL_set_options() adds the options set via bitmask in B<options> to B<ssl>.
-Options already set before are not cleared.
+Options already set before are not cleared!
SSL_CTX_get_options() returns the options set for B<ctx>.
The options are coded as bitmasks and can be combined by a logical B<or>
operation (|). Options can only be added but can never be reset.
-During a handshake, the option settings of the SSL object used. When
+SSL_CTX_set_options() and SSL_set_options() affect the (external)
+protocol behaviour of the SSL library. The (internal) behaviour of
+the API can be changed by using the similar
+L<SSL_CTX_set_modes(3)|SSL_CTX_set_modes(3)> and SSL_set_modes() functions.
+
+During a handshake, the option settings of the SSL object are used. When
a new SSL object is created from a context using SSL_new(), the current
option setting is copied. Changes to B<ctx> do not affect already created
SSL objects. SSL_clear() does not affect the settings.
=item SSL_OP_SINGLE_DH_USE
-Always create a new key when using temporary DH parameters
+Always create a new key when using temporary/ephemeral DH parameters
(see L<SSL_CTX_set_tmp_dh_callback(3)|SSL_CTX_set_tmp_dh_callback(3)>).
+This option must be used to prevent small subgroup attacks, when
+the DH parameters were not generated using "strong" primes
+(e.g. when using DSA-parameters, see L<dhparam(1)|dhparam(1)>).
+If "strong" primes were used, it is not strictly necessary to generate
+a new DH key during each handshake but it is also recommendet.
+SSL_OP_SINGLE_DH_USE should therefore be enabled whenever
+temporary/ephemeral DH parameters are used.
=item SSL_OP_EPHEMERAL_RSA
-Also use ephemeral (temporary) RSA key when doing RSA operations
+Always use ephemeral (temporary) RSA key when doing RSA operations
(see L<SSL_CTX_set_tmp_rsa_callback(3)|SSL_CTX_set_tmp_rsa_callback(3)>).
+According to the specifications this is only done, when a RSA key
+can only be used for signature operations (namely under export ciphers
+with restricted RSA keylength). By setting this option, ephemeral
+RSA keys are always used. This option breaks compatibility with the
+SSL/TLS specifications and may lead to interoperability problems with
+clients and should therefore never be used. Ciphers with EDH (ephemeral
+Diffie-Hellman) key exchange should be used instead.
=item SSL_OP_CIPHER_SERVER_PREFERENCE
non-self-sighed CA which does not have it's CA in netscape, and the
browser has a cert, it will crash/hang. Works for 3.x and 4.xbeta
-=item SSL_OP_NON_EXPORT_FIRST
-
-On servers try to use non-export (stronger) ciphers first. This option does
-not work under all circumstances (in the code it is declared "broken").
-
=item SSL_OP_NETSCAPE_DEMO_CIPHER_CHANGE_BUG
...
L<ssl(3)|ssl(3)>, L<SSL_new(3)|SSL_new(3)>, L<SSL_clear(3)|SSL_clear(3)>,
L<SSL_CTX_set_tmp_dh_callback(3)|SSL_CTX_set_tmp_dh_callback(3)>,
-L<SSL_CTX_set_tmp_rsa_callback(3)|SSL_CTX_set_tmp_rsa_callback(3)>
+L<SSL_CTX_set_tmp_rsa_callback(3)|SSL_CTX_set_tmp_rsa_callback(3)>,
+L<dhparam(1)|dhparam(1)>
=head1 HISTORY
=head1 NOTES
When using a cipher with RSA authentication, an ephemeral DH key exchange
-can take place. Ciphers with DSA keys always use ephemeral DH keys as well
-as anonymous ciphers. In this case the session data are negotiated using the
+can take place. Ciphers with DSA keys always use ephemeral DH keys as well.
+In these cases, the session data are negotiated using the
ephemeral/temporary DH 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 DH keys.
Using ephemeral DH key exchange yields forward secrecy, as the connection
can only be decrypted, when the DH key is known. By generating a temporary
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 automatically
-generate the DH key when required, as it is computationally cheap
-(retrieve a random number). The server will reuse the DH key for further
-connections, unless the SSL_OP_SINGLE_DH_USE option of
-L<SSL_CTX_set_options(3)|SSL_CTX_set_options(3)> is set, in which case
-a new DH key for each negotiation will be generated.
+(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
+immediatly 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,
+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.
As generating DH parameters is extremely time consuming, an application
should not generate the parameters on the fly but supply the parameters.
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. Therefore application authors
-should not copy the DH parameters from other applications or the OpenSSL
-example application, if they compile in parameters, but generate their
-own set of parameters using e.g. the openssl L<dhparam(1)|dhparam(1)>
-application with the B<-C> option. An application may also generate
-its own set of DH parameters during the installation procedure on a specific
-host, so that each host uses different parameters.
-
-An application my either directly specify the DH parameters or
+may specialize on a very often used DH group. Applications should therefore
+generate their own DH paramaters 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.
+
+Application authors may compile in DH parameters. Files dh512.pem,
+dh1024.pem, dh2048.pem, and dh4096 in the 'apps' directory of 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.
+
+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.
=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)>
SSL_CTX_set_tmp_rsa_callback() sets the callback function for B<ctx> to be
used when a temporary/ephemeral RSA key is required to B<tmp_rsa_callback>.
-The callback is inherited by all B<ssl> objects created from B<ctx>.
+The callback is inherited by all SSL objects newly created from B<ctx>
+with <SSL_new(3)|SSL_new(3)>. Already created SSL objects are not affected.
SSL_CTX_set_tmp_rsa() sets the temporary/ephemeral RSA key to be used to be
-B<rsa>. The key is inherited by all B<ssl> objects created from B<ctx>.
+B<rsa>. The key is inherited by all SSL objects newly created from B<ctx>
+with <SSL_new(3)|SSL_new(3)>. Already created SSL objects are not affected.
-SSL_CTX_need_tmp_rsa() returns 1, if a temporay/ephemeral RSA key is needed,
-because a RSA key with a keysize larger than 512 bits is installed.
+SSL_CTX_need_tmp_rsa() returns 1, if a temporay/ephemeral RSA key is needed
+for RSA-based strength-limited 'exportable' ciphersuites because a RSA key
+with a keysize larger than 512 bits is installed.
SSL_set_tmp_rsa_callback() sets the callback only for B<ssl>.
SSL_set_tmp_rsa() sets the key only for B<ssl>.
SSL_need_tmp_rsa() returns 1, if a temporay/ephemeral RSA key is needed,
-because a RSA key with a keysize larger than 512 bits is installed.
+for RSA-based strength-limited 'exportable' ciphersuites because a RSA key
+with a keysize larger than 512 bits is installed.
These functions apply to SSL/TLS servers only.
ephemeral/temporary RSA key and the RSA key supplied and certified
by the certificate chain is only used for signing.
+Under previous export restrictions, ciphers with RSA keys shorter (512 bits)
+than the usual key length of 1024 bits were created. To use these ciphers
+with RSA keys of usual length, an ephemeral key exchange must be performed,
+as the normal (certified) key cannot be directly used.
+
Using ephemeral RSA key exchange yields forward secrecy, as the connection
can only be decrypted, when the RSA key is known. By generating a temporary
RSA 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) RSA key, as this key was
-only used for signing. The downside is that creating a RSA key is
-computationally expensive. On OpenSSL servers ephemeral RSA key exchange
-is therefore disabled by default and must be explicitly enabled using the
-SSL_OP_EPHEMERAL_RSA option of
-L<SSL_CTX_set_options(3)|SSL_CTX_set_options(3)>, except for certain
-export ciphers.
-
-Under previous export restrictions, ciphers with RSA keys shorter (512 bits)
-than the usual key length of 1024 bits were created. To use these ciphers
-with RSA keys of usual length, an ephemeral key exchange must be performed,
-as the normal (certified) key cannot be used.
-
-An application my either directly specify the key or
-can supply the key via a callback function. The callback approach has
-the advantage, that the callback may generate the key only in case it is
-actually needed. As the generation of a RSA key is however costly, it
-will lead to a significant delay in the handshake procedure.
-Another advantage of the callback function is that it can supply keys
-of different size (e.g. for SSL_OP_EPHEMERAL_RSA usage) while the
-explicit setting of the key is only useful for key size of 512 bits
-to satisfy the export restricted ciphers and does give away key length
+used for signing only. The downside is that creating a RSA key is
+computationally expensive.
+
+Additionally, the use of ephemeral RSA key exchange is only allowed in
+the TLS standard, when the RSA key can be used for signing only, that is
+for export ciphers. Using ephemeral RSA key exchange for other purposes
+violates the standard and can break interoperability with clients.
+It is therefore strongly recommended to not use ephemeral RSA key
+exchange and use EDH (Ephemeral Diffie-Hellman) key exchange instead
+in order to achieve forward secrecy (see
+L<SSL_CTX_set_tmp_dh_callback(3)|SSL_CTX_set_tmp_dh_callback(3)>).
+
+On OpenSSL servers ephemeral RSA key exchange is therefore disabled by default
+and must be explicitly enabled using the SSL_OP_EPHEMERAL_RSA option of
+L<SSL_CTX_set_options(3)|SSL_CTX_set_options(3)>, violating the TLS/SSL
+standard. When ephemeral RSA key exchange is required for export ciphers,
+it will automatically be used without this option!
+
+An application may either directly specify the key or can supply the key via
+a callback function. The callback approach has the advantage, that the
+callback may generate the key only in case it is actually needed. As the
+generation of a RSA key is however costly, it will lead to a significant
+delay in the handshake procedure. Another advantage of the callback function
+is that it can supply keys of different size (e.g. for SSL_OP_EPHEMERAL_RSA
+usage) while the explicit setting of the key is only useful for key size of
+512 bits to satisfy the export restricted ciphers and does give away key length
if a longer key would be allowed.
The B<tmp_rsa_callback> is called with the B<keylength> needed and
=head1 EXAMPLES
Generate temporary RSA keys to prepare ephemeral RSA key exchange. As the
-generation of a RSA key costs a lot of computer time, it is saved for later
+generation of a RSA key costs a lot of computer time, they saved for later
reuse. For demonstration purposes, two keys for 512 bits and 1024 bits
respectively are generated.
/* Set up ephemeral RSA stuff */
RSA *rsa_512 = NULL;
RSA *rsa_1024 = NULL;
- if (prepare_export_in_advance || always_use_ephemeral_rsa) {
- rsa_512 = RSA_generate_key(512,RSA_F4,NULL,NULL);
- if (rsa_512 == NULL)
- evaluate_error_queue();
- }
- if (always_use_ephemeral_rsa) {
- /* Only spend the time to generate the key, if it will actually be
- needed */
- rsa_1024 = RSA_generate_key(1024,RSA_F4,NULL,NULL);
- if (rsa_1024 == NULL)
+
+ rsa_512 = RSA_generate_key(512,RSA_F4,NULL,NULL);
+ if (rsa_512 == NULL)
evaluate_error_queue();
- SSL_CTX_set_options(SSL_OP_EPHEMERAL_RSA);
- }
+
+ rsa_1024 = RSA_generate_key(1024,RSA_F4,NULL,NULL);
+ if (rsa_1024 == NULL)
+ evaluate_error_queue();
+
...
RSA *tmp_rsa_callback(SSL *s, int is_export, int keylength)
case 512:
if (rsa_512)
rsa_tmp = rsa_512;
- else { /* generate on the fly */
- rsa_tmp = RSA_generate_key(512,RSA_F4,NULL,NULL);
+ else { /* generate on the fly, should not happen in this example */
+ rsa_tmp = RSA_generate_key(keylength,RSA_F4,NULL,NULL);
rsa_512 = rsa_tmp; /* Remember for later reuse */
}
break;
if (rsa_1024)
rsa_tmp=rsa_1024;
else
- this_should_never_happen_as_we_are_prepared();
+ should_not_happen_in_this_example();
break;
default:
/* Generating a key on the fly is very costly, so use what is there */
L<ssl(3)|ssl(3)>, L<SSL_CTX_set_cipher_list(3)|SSL_CTX_set_cipher_list(3)>,
L<SSL_CTX_set_options(3)|SSL_CTX_set_options(3)>,
-L<ciphers(1)|ciphers(1)>
+L<SSL_CTX_set_tmp_dh_callback(3)|SSL_CTX_set_tmp_dh_callback(3)>,
+L<SSL_new(3)|SSL_new(3)>, L<ciphers(1)|ciphers(1)>
=cut
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