SSL_CTX_set_options(3) | OpenSSL | SSL_CTX_set_options(3)

# 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

# LIBRARY

libcrypto, -lcrypto

# 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);

# DESCRIPTION

Note: all these functions are implemented using macros.

SSL_CTX_set_options() adds the options set via bitmask in options to ctx. Options already set before are not cleared!

SSL_set_options() adds the options set via bitmask in options to ssl. Options already set before are not cleared!

SSL_CTX_clear_options() clears the options set via bitmask in options to ctx.

SSL_clear_options() clears the options set via bitmask in options to ssl.

SSL_CTX_get_options() returns the options set for ctx.

SSL_get_options() returns the options set for ssl.

SSL_get_secure_renegotiation_support() indicates whether the peer supports secure renegotiation.

# 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 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 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 ctx do not affect already created SSL objects. SSL_clear() does not affect the settings.

The following bug workaround options are available:

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.

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.

SSL_OP_NETSCAPE_REUSE_CIPHER_CHANGE_BUG | As of OpenSSL 0.9.8q and 1.0.0c, this option has no effect.

SSL_OP_SSLREF2_REUSE_CERT_TYPE_BUG | ...

SSL_OP_MICROSOFT_BIG_SSLV3_BUFFER | ...

SSL_OP_MSIE_SSLV2_RSA_PADDING | As of OpenSSL 0.9.7h and 0.9.8a, this option has no effect.

SSL_OP_SSLEAY_080_CLIENT_DH_BUG | ...

SSL_OP_TLS_D5_BUG | ...

SSL_OP_TLS_BLOCK_PADDING_BUG | ...

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.

SSL_OP_ALL | All of the above bug workarounds.

It is usually safe to use SSL_OP_ALL to enable the bug workaround options if compatibility with somewhat broken implementations is desired.

The following modifying options are available:

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.)

SSL_OP_SINGLE_DH_USE

Always create a new key when using temporary/ephemeral DH parameters (see 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 openssl_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. SSL_OP_SINGLE_DH_USE should therefore be enabled whenever temporary/ephemeral DH parameters are used.

SSL_OP_EPHEMERAL_RSA

Always use ephemeral (temporary) RSA key when doing RSA operations (see 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.

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.

SSL_OP_PKCS1_CHECK_1 | ...

SSL_OP_PKCS1_CHECK_2 | ...

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

SSL_OP_NETSCAPE_DEMO_CIPHER_CHANGE_BUG | ...

SSL_OP_NO_SSLv2 | Do not use the SSLv2 protocol.

SSL_OP_NO_SSLv3 | Do not use the SSLv3 protocol.

SSL_OP_NO_TLSv1 | Do not use the TLSv1 protocol.

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.

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.

SSL_OP_ALLOW_UNSAFE_LEGACY_RENEGOTIATION | Allow legacy insecure renegotiation between OpenSSL and unpatched clients or servers. See the SECURE RENEGOTIATION section for more details.

SSL_OP_LEGACY_SERVER_CONNECT | Allow legacy insecure renegotiation between OpenSSL and unpatched servers only: this option is currently set by default. See the SECURE RENEGOTIATION section for more details.

# 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 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 patched. A server not supporting secure renegotiation is referred to as unpatched.

The following sections describe the operations permitted by OpenSSL's secure renegotiation implementation.

## Patched client and server

Connections and renegotiation are always permitted by OpenSSL implementations.

## Unpatched client and patched OpenSSL server

The initial connection suceeds but client renegotiation is denied by the server with a no_renegotiation warning alert if TLS v1.0 is used or a fatal handshake_failure alert in SSL v3.0.

If the patched OpenSSL server attempts to renegotiate a fatal handshake_failure alert is sent. This is because the server code may be unaware of the unpatched nature of the client.

If the option SSL_OP_ALLOW_UNSAFE_LEGACY_RENEGOTIATION is set then renegotiation always succeeds.

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 no_renegotiation alert. OpenSSL versions 0.9.8m and later will regard a no_renegotiation alert as fatal and respond with a fatal handshake_failure alert. This is because the OpenSSL API currently has no provision to indicate to an application that a renegotiation attempt was refused.

## Patched OpenSSL client and unpatched server.

If the option SSL_OP_LEGACY_SERVER_CONNECT or 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 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 SSL_OP_LEGACY_SERVER_CONNECT will 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 set SSL_OP_LEGACY_SERVER_CONNECT

OpenSSL client applications that want to ensure they can not connect to unpatched servers (and thus avoid any security issues) should always clear SSL_OP_LEGACY_SERVER_CONNECT using SSL_CTX_clear_options() or SSL_clear_options().

The difference between the SSL_OP_LEGACY_SERVER_CONNECT and SSL_OP_ALLOW_UNSAFE_LEGACY_RENEGOTIATION options is that SSL_OP_LEGACY_SERVER_CONNECT enables initial connections and secure renegotiation between OpenSSL clients and unpatched servers only, while SSL_OP_ALLOW_UNSAFE_LEGACY_RENEGOTIATION allows initial connections and renegotiation between OpenSSL and unpatched clients or servers.

# RETURN VALUES

SSL_CTX_set_options() and SSL_set_options() return the new options bitmask after adding options.

SSL_CTX_clear_options() and SSL_clear_options() return the new options bitmask after clearing 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.

# SEE ALSO

ssl(3), SSL_new(3), SSL_clear(3), SSL_CTX_set_tmp_dh_callback(3), SSL_CTX_set_tmp_rsa_callback(3), openssl_dhparam(1)

# HISTORY

SSL_OP_CIPHER_SERVER_PREFERENCE and SSL_OP_NO_SESSION_RESUMPTION_ON_RENEGOTIATION have been added in OpenSSL 0.9.7.

SSL_OP_TLS_ROLLBACK_BUG has been added in OpenSSL 0.9.6 and was automatically enabled with SSL_OP_ALL. As of 0.9.7, it is no longer included in SSL_OP_ALL and must be explicitly set.

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.

SSL_OP_ALLOW_UNSAFE_LEGACY_RENEGOTIATION, SSL_OP_LEGACY_SERVER_CONNECT and the function SSL_get_secure_renegotiation_support() were first added in OpenSSL 0.9.8m.

June 5, 2011 | 1.0.1-dev