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- =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
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