=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>
+The options are coded as bitmasks and can be combined by a bitwise B<or>
operation (|).
SSL_CTX_set_options() and SSL_set_options() affect the (external)
=item SSL_OP_NETSCAPE_REUSE_CIPHER_CHANGE_BUG
-ssl3.netscape.com:443, first a connection is established with RC4-MD5.
-If it is then resumed, we end up using DES-CBC3-SHA. It should be
-RC4-MD5 according to 7.6.1.3, 'cipher_suite'.
-
-Netscape-Enterprise/2.01 (https://merchant.netscape.com) has this bug.
-It only really shows up when connecting via SSLv2/v3 then reconnecting
-via SSLv3. The cipher list changes....
-
-NEW INFORMATION. Try connecting with a cipher list of just
-DES-CBC-SHA:RC4-MD5. For some weird reason, each new connection uses
-RC4-MD5, but a re-connect tries to use DES-CBC-SHA. So netscape, when
-doing a re-connect, always takes the first cipher in the cipher list.
+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_MSIE_SSLV2_RSA_PADDING
+=item SSL_OP_SAFARI_ECDHE_ECDSA_BUG
-As of OpenSSL 0.9.7h and 0.9.8a, this option has no effect.
+Don't prefer ECDHE-ECDSA ciphers when the client appears to be Safari on OS X.
+OS X 10.8..10.8.3 has broken support for ECDHE-ECDSA ciphers.
=item SSL_OP_SSLEAY_080_CLIENT_DH_BUG
broken SSL implementations. This option has no effect for connections
using other ciphers.
+=item SSL_OP_TLSEXT_PADDING
+
+Adds a padding extension to ensure the ClientHello size is never between
+256 and 511 bytes in length. This is needed as a workaround for some
+implementations.
+
=item SSL_OP_ALL
All of the above bug workarounds.
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
+clients and should therefore never be used. Ciphers with DHE (ephemeral
Diffie-Hellman) key exchange should be used instead.
=item SSL_OP_CIPHER_SERVER_PREFERENCE
=item SSL_OP_ALLOW_UNSAFE_LEGACY_RENEGOTIATION
-See the B<SECURE RENEGOTIATION> section for a discussion of the purpose of
-this option
+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 draft-ietf-tls-renegotiation (FIXME: replace by RFC). This
-counters a prefix attack described in the draft and elsewhere (FIXME: need full
-reference).
+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>.
-If an unpatched client attempts to connect to a patched OpenSSL server then
-the attempt will succeed but renegotiation is not permitted. As required
-by the standard a B<no_renegotiation> alert is sent back to the client if
-the TLS v1.0 protocol is used. If SSLv3.0 is used then renegotiation results
-in a fatal B<handshake_failed> alert.
+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.
-If a patched OpenSSL client attempts to connect to an unpatched server
-then the connection will fail because it is not possible to determine
-whether an attack is taking place.
+=head2 Unpatched client and patched OpenSSL server
-If the option B<SSL_OP_ALLOW_UNSAFE_LEGACY_RENEGOTIATION> is set then the
-above restrictions are relaxed. Renegotiation is permissible and initial
-connections to unpatched servers will succeed.
+The initial connection succeeds 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.
-This option should be used with caution because it leaves both clients and
-servers vulnerable. However unpatched servers and clients are likely to be
-around for some time and refusing to connect to unpatched servers or denying
-renegotion altogether may be unacceptable. So applications may be forced to
-tolerate unsafe renegotiation for the immediate future.
+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.
-The function SSL_get_secure_renegotiation_support() indicates whether the peer
-supports secure renegotiation.
+If the option B<SSL_OP_ALLOW_UNSAFE_LEGACY_RENEGOTIATION> is set then
+renegotiation B<always> succeeds.
-The deprecated SSLv2 protocol does not support secure renegotiation at all.
+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_clear_options() and SSL_clear_options() were first added in OpenSSL
0.9.8m.
-B<SSL_OP_ALLOW_UNSAFE_LEGACY_RENEGOTIATION> was 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
projects / openssl.git / blobdiff