347 lines
12 KiB
Plaintext
347 lines
12 KiB
Plaintext
=pod
|
|
|
|
=head1 NAME
|
|
|
|
SSL_CTX_set_options, SSL_set_options, SSL_CTX_clear_options, SSL_clear_options, SSL_CTX_get_options, SSL_get_options, SSL_get_secure_renegotiation_support - manipulate SSL options
|
|
|
|
=head1 SYNOPSIS
|
|
|
|
#include <openssl/ssl.h>
|
|
|
|
long SSL_CTX_set_options(SSL_CTX *ctx, long options);
|
|
long SSL_set_options(SSL *ssl, long options);
|
|
|
|
long SSL_CTX_clear_options(SSL_CTX *ctx, long options);
|
|
long SSL_clear_options(SSL *ssl, long options);
|
|
|
|
long SSL_CTX_get_options(SSL_CTX *ctx);
|
|
long SSL_get_options(SSL *ssl);
|
|
|
|
long SSL_get_secure_renegotiation_support(SSL *ssl);
|
|
|
|
=head1 DESCRIPTION
|
|
|
|
Note: all these functions are implemented using macros.
|
|
|
|
SSL_CTX_set_options() adds the options set via bitmask in B<options> to B<ctx>.
|
|
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!
|
|
|
|
SSL_CTX_clear_options() clears the options set via bitmask in B<options>
|
|
to B<ctx>.
|
|
|
|
SSL_clear_options() clears the options set via bitmask in B<options> to B<ssl>.
|
|
|
|
SSL_CTX_get_options() returns the options set for B<ctx>.
|
|
|
|
SSL_get_options() returns the options set for B<ssl>.
|
|
|
|
SSL_get_secure_renegotiation_support() indicates whether the peer supports
|
|
secure renegotiation.
|
|
|
|
=head1 NOTES
|
|
|
|
The behaviour of the SSL library can be changed by setting several options.
|
|
The options are coded as bitmasks and can be combined by a logical B<or>
|
|
operation (|).
|
|
|
|
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_mode(3)|SSL_CTX_set_mode(3)> and SSL_set_mode() 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.
|
|
|
|
The following B<bug workaround> options are available:
|
|
|
|
=over 4
|
|
|
|
=item SSL_OP_MICROSOFT_SESS_ID_BUG
|
|
|
|
www.microsoft.com - when talking SSLv2, if session-id reuse is
|
|
performed, the session-id passed back in the server-finished message
|
|
is different from the one decided upon.
|
|
|
|
=item SSL_OP_NETSCAPE_CHALLENGE_BUG
|
|
|
|
Netscape-Commerce/1.12, when talking SSLv2, accepts a 32 byte
|
|
challenge but then appears to only use 16 bytes when generating the
|
|
encryption keys. Using 16 bytes is ok but it should be ok to use 32.
|
|
According to the SSLv3 spec, one should use 32 bytes for the challenge
|
|
when operating in SSLv2/v3 compatibility mode, but as mentioned above,
|
|
this breaks this server so 16 bytes is the way to go.
|
|
|
|
=item SSL_OP_NETSCAPE_REUSE_CIPHER_CHANGE_BUG
|
|
|
|
As of OpenSSL 0.9.8q and 1.0.0c, this option has no effect.
|
|
|
|
=item SSL_OP_SSLREF2_REUSE_CERT_TYPE_BUG
|
|
|
|
...
|
|
|
|
=item SSL_OP_MICROSOFT_BIG_SSLV3_BUFFER
|
|
|
|
...
|
|
|
|
=item SSL_OP_MSIE_SSLV2_RSA_PADDING
|
|
|
|
As of OpenSSL 0.9.7h and 0.9.8a, this option has no effect.
|
|
|
|
=item SSL_OP_SSLEAY_080_CLIENT_DH_BUG
|
|
|
|
...
|
|
|
|
=item SSL_OP_TLS_D5_BUG
|
|
|
|
...
|
|
|
|
=item SSL_OP_TLS_BLOCK_PADDING_BUG
|
|
|
|
...
|
|
|
|
=item SSL_OP_DONT_INSERT_EMPTY_FRAGMENTS
|
|
|
|
Disables a countermeasure against a SSL 3.0/TLS 1.0 protocol
|
|
vulnerability affecting CBC ciphers, which cannot be handled by some
|
|
broken SSL implementations. This option has no effect for connections
|
|
using other ciphers.
|
|
|
|
=item SSL_OP_ALL
|
|
|
|
All of the above bug workarounds.
|
|
|
|
=back
|
|
|
|
It is usually safe to use B<SSL_OP_ALL> to enable the bug workaround
|
|
options if compatibility with somewhat broken implementations is
|
|
desired.
|
|
|
|
The following B<modifying> options are available:
|
|
|
|
=over 4
|
|
|
|
=item SSL_OP_TLS_ROLLBACK_BUG
|
|
|
|
Disable version rollback attack detection.
|
|
|
|
During the client key exchange, the client must send the same information
|
|
about acceptable SSL/TLS protocol levels as during the first hello. Some
|
|
clients violate this rule by adapting to the server's answer. (Example:
|
|
the client sends a SSLv2 hello and accepts up to SSLv3.1=TLSv1, the server
|
|
only understands up to SSLv3. In this case the client must still use the
|
|
same SSLv3.1=TLSv1 announcement. Some clients step down to SSLv3 with respect
|
|
to the server's answer and violate the version rollback protection.)
|
|
|
|
=item SSL_OP_SINGLE_DH_USE
|
|
|
|
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 recommended.
|
|
B<SSL_OP_SINGLE_DH_USE> should therefore be enabled whenever
|
|
temporary/ephemeral DH parameters are used.
|
|
|
|
=item SSL_OP_EPHEMERAL_RSA
|
|
|
|
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
|
|
|
|
When choosing a cipher, use the server's preferences instead of the client
|
|
preferences. When not set, the SSL server will always follow the clients
|
|
preferences. When set, the SSLv3/TLSv1 server will choose following its
|
|
own preferences. Because of the different protocol, for SSLv2 the server
|
|
will send its list of preferences to the client and the client chooses.
|
|
|
|
=item SSL_OP_PKCS1_CHECK_1
|
|
|
|
...
|
|
|
|
=item SSL_OP_PKCS1_CHECK_2
|
|
|
|
...
|
|
|
|
=item SSL_OP_NETSCAPE_CA_DN_BUG
|
|
|
|
If we accept a netscape connection, demand a client cert, have a
|
|
non-self-signed CA which does not have its CA in netscape, and the
|
|
browser has a cert, it will crash/hang. Works for 3.x and 4.xbeta
|
|
|
|
=item SSL_OP_NETSCAPE_DEMO_CIPHER_CHANGE_BUG
|
|
|
|
...
|
|
|
|
=item SSL_OP_NO_SSLv2
|
|
|
|
Do not use the SSLv2 protocol.
|
|
|
|
=item SSL_OP_NO_SSLv3
|
|
|
|
Do not use the SSLv3 protocol.
|
|
|
|
=item SSL_OP_NO_TLSv1
|
|
|
|
Do not use the TLSv1 protocol.
|
|
|
|
=item SSL_OP_NO_SESSION_RESUMPTION_ON_RENEGOTIATION
|
|
|
|
When performing renegotiation as a server, always start a new session
|
|
(i.e., session resumption requests are only accepted in the initial
|
|
handshake). This option is not needed for clients.
|
|
|
|
=item SSL_OP_NO_TICKET
|
|
|
|
Normally clients and servers will, where possible, transparently make use
|
|
of RFC4507bis tickets for stateless session resumption.
|
|
|
|
If this option is set this functionality is disabled and tickets will
|
|
not be used by clients or servers.
|
|
|
|
=item SSL_OP_ALLOW_UNSAFE_LEGACY_RENEGOTIATION
|
|
|
|
Allow legacy insecure renegotiation between OpenSSL and unpatched clients or
|
|
servers. See the B<SECURE RENEGOTIATION> section for more details.
|
|
|
|
=item SSL_OP_LEGACY_SERVER_CONNECT
|
|
|
|
Allow legacy insecure renegotiation between OpenSSL and unpatched servers
|
|
B<only>: this option is currently set by default. See the
|
|
B<SECURE RENEGOTIATION> section for more details.
|
|
|
|
=back
|
|
|
|
=head1 SECURE RENEGOTIATION
|
|
|
|
OpenSSL 0.9.8m and later always attempts to use secure renegotiation as
|
|
described in RFC5746. This counters the prefix attack described in
|
|
CVE-2009-3555 and elsewhere.
|
|
|
|
The deprecated and highly broken SSLv2 protocol does not support
|
|
renegotiation at all: its use is B<strongly> discouraged.
|
|
|
|
This attack has far reaching consequences which application writers should be
|
|
aware of. In the description below an implementation supporting secure
|
|
renegotiation is referred to as I<patched>. A server not supporting secure
|
|
renegotiation is referred to as I<unpatched>.
|
|
|
|
The following sections describe the operations permitted by OpenSSL's secure
|
|
renegotiation implementation.
|
|
|
|
=head2 Patched client and server
|
|
|
|
Connections and renegotiation are always permitted by OpenSSL implementations.
|
|
|
|
=head2 Unpatched client and patched OpenSSL server
|
|
|
|
The initial connection suceeds but client renegotiation is denied by the
|
|
server with a B<no_renegotiation> warning alert if TLS v1.0 is used or a fatal
|
|
B<handshake_failure> alert in SSL v3.0.
|
|
|
|
If the patched OpenSSL server attempts to renegotiate a fatal
|
|
B<handshake_failure> alert is sent. This is because the server code may be
|
|
unaware of the unpatched nature of the client.
|
|
|
|
If the option B<SSL_OP_ALLOW_UNSAFE_LEGACY_RENEGOTIATION> is set then
|
|
renegotiation B<always> succeeds.
|
|
|
|
B<NB:> a bug in OpenSSL clients earlier than 0.9.8m (all of which are
|
|
unpatched) will result in the connection hanging if it receives a
|
|
B<no_renegotiation> alert. OpenSSL versions 0.9.8m and later will regard
|
|
a B<no_renegotiation> alert as fatal and respond with a fatal
|
|
B<handshake_failure> alert. This is because the OpenSSL API currently has
|
|
no provision to indicate to an application that a renegotiation attempt
|
|
was refused.
|
|
|
|
=head2 Patched OpenSSL client and unpatched server.
|
|
|
|
If the option B<SSL_OP_LEGACY_SERVER_CONNECT> or
|
|
B<SSL_OP_ALLOW_UNSAFE_LEGACY_RENEGOTIATION> is set then initial connections
|
|
and renegotiation between patched OpenSSL clients and unpatched servers
|
|
succeeds. If neither option is set then initial connections to unpatched
|
|
servers will fail.
|
|
|
|
The option B<SSL_OP_LEGACY_SERVER_CONNECT> is currently set by default even
|
|
though it has security implications: otherwise it would be impossible to
|
|
connect to unpatched servers (i.e. all of them initially) and this is clearly
|
|
not acceptable. Renegotiation is permitted because this does not add any
|
|
additional security issues: during an attack clients do not see any
|
|
renegotiations anyway.
|
|
|
|
As more servers become patched the option B<SSL_OP_LEGACY_SERVER_CONNECT> will
|
|
B<not> be set by default in a future version of OpenSSL.
|
|
|
|
OpenSSL client applications wishing to ensure they can connect to unpatched
|
|
servers should always B<set> B<SSL_OP_LEGACY_SERVER_CONNECT>
|
|
|
|
OpenSSL client applications that want to ensure they can B<not> connect to
|
|
unpatched servers (and thus avoid any security issues) should always B<clear>
|
|
B<SSL_OP_LEGACY_SERVER_CONNECT> using SSL_CTX_clear_options() or
|
|
SSL_clear_options().
|
|
|
|
The difference between the B<SSL_OP_LEGACY_SERVER_CONNECT> and
|
|
B<SSL_OP_ALLOW_UNSAFE_LEGACY_RENEGOTIATION> options is that
|
|
B<SSL_OP_LEGACY_SERVER_CONNECT> enables initial connections and secure
|
|
renegotiation between OpenSSL clients and unpatched servers B<only>, while
|
|
B<SSL_OP_ALLOW_UNSAFE_LEGACY_RENEGOTIATION> allows initial connections
|
|
and renegotiation between OpenSSL and unpatched clients or servers.
|
|
|
|
=head1 RETURN VALUES
|
|
|
|
SSL_CTX_set_options() and SSL_set_options() return the new options bitmask
|
|
after adding B<options>.
|
|
|
|
SSL_CTX_clear_options() and SSL_clear_options() return the new options bitmask
|
|
after clearing B<options>.
|
|
|
|
SSL_CTX_get_options() and SSL_get_options() return the current bitmask.
|
|
|
|
SSL_get_secure_renegotiation_support() returns 1 is the peer supports
|
|
secure renegotiation and 0 if it does not.
|
|
|
|
=head1 SEE ALSO
|
|
|
|
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<dhparam(1)|dhparam(1)>
|
|
|
|
=head1 HISTORY
|
|
|
|
B<SSL_OP_CIPHER_SERVER_PREFERENCE> and
|
|
B<SSL_OP_NO_SESSION_RESUMPTION_ON_RENEGOTIATION> have been added in
|
|
OpenSSL 0.9.7.
|
|
|
|
B<SSL_OP_TLS_ROLLBACK_BUG> has been added in OpenSSL 0.9.6 and was automatically
|
|
enabled with B<SSL_OP_ALL>. As of 0.9.7, it is no longer included in B<SSL_OP_ALL>
|
|
and must be explicitly set.
|
|
|
|
B<SSL_OP_DONT_INSERT_EMPTY_FRAGMENTS> has been added in OpenSSL 0.9.6e.
|
|
Versions up to OpenSSL 0.9.6c do not include the countermeasure that
|
|
can be disabled with this option (in OpenSSL 0.9.6d, it was always
|
|
enabled).
|
|
|
|
SSL_CTX_clear_options() and SSL_clear_options() were first added in OpenSSL
|
|
0.9.8m.
|
|
|
|
B<SSL_OP_ALLOW_UNSAFE_LEGACY_RENEGOTIATION>, B<SSL_OP_LEGACY_SERVER_CONNECT>
|
|
and the function SSL_get_secure_renegotiation_support() were first added in
|
|
OpenSSL 0.9.8m.
|
|
|
|
=cut
|