347 lines
12 KiB
Text
347 lines
12 KiB
Text
=pod
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=head1 NAME
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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
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=head1 SYNOPSIS
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#include <openssl/ssl.h>
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long SSL_CTX_set_options(SSL_CTX *ctx, long options);
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long SSL_set_options(SSL *ssl, long options);
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long SSL_CTX_clear_options(SSL_CTX *ctx, long options);
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long SSL_clear_options(SSL *ssl, long options);
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long SSL_CTX_get_options(SSL_CTX *ctx);
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long SSL_get_options(SSL *ssl);
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long SSL_get_secure_renegotiation_support(SSL *ssl);
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=head1 DESCRIPTION
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Note: all these functions are implemented using macros.
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SSL_CTX_set_options() adds the options set via bitmask in B<options> to B<ctx>.
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Options already set before are not cleared!
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SSL_set_options() adds the options set via bitmask in B<options> to B<ssl>.
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Options already set before are not cleared!
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SSL_CTX_clear_options() clears the options set via bitmask in B<options>
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to B<ctx>.
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SSL_clear_options() clears the options set via bitmask in B<options> to B<ssl>.
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SSL_CTX_get_options() returns the options set for B<ctx>.
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SSL_get_options() returns the options set for B<ssl>.
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SSL_get_secure_renegotiation_support() indicates whether the peer supports
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secure renegotiation.
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=head1 NOTES
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The behaviour of the SSL library can be changed by setting several options.
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The options are coded as bitmasks and can be combined by a logical B<or>
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operation (|).
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SSL_CTX_set_options() and SSL_set_options() affect the (external)
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protocol behaviour of the SSL library. The (internal) behaviour of
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the API can be changed by using the similar
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L<SSL_CTX_set_mode(3)|SSL_CTX_set_mode(3)> and SSL_set_mode() functions.
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During a handshake, the option settings of the SSL object are used. When
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a new SSL object is created from a context using SSL_new(), the current
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option setting is copied. Changes to B<ctx> do not affect already created
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SSL objects. SSL_clear() does not affect the settings.
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The following B<bug workaround> options are available:
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=over 4
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=item SSL_OP_MICROSOFT_SESS_ID_BUG
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www.microsoft.com - when talking SSLv2, if session-id reuse is
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performed, the session-id passed back in the server-finished message
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is different from the one decided upon.
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=item SSL_OP_NETSCAPE_CHALLENGE_BUG
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Netscape-Commerce/1.12, when talking SSLv2, accepts a 32 byte
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challenge but then appears to only use 16 bytes when generating the
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encryption keys. Using 16 bytes is ok but it should be ok to use 32.
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According to the SSLv3 spec, one should use 32 bytes for the challenge
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when operating in SSLv2/v3 compatibility mode, but as mentioned above,
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this breaks this server so 16 bytes is the way to go.
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=item SSL_OP_NETSCAPE_REUSE_CIPHER_CHANGE_BUG
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As of OpenSSL 0.9.8q and 1.0.0c, this option has no effect.
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=item SSL_OP_SSLREF2_REUSE_CERT_TYPE_BUG
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...
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=item SSL_OP_MICROSOFT_BIG_SSLV3_BUFFER
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...
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=item SSL_OP_SAFARI_ECDHE_ECDSA_BUG
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Don't prefer ECDHE-ECDSA ciphers when the client appears to be Safari on OS X.
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OS X 10.8..10.8.3 has broken support for ECDHE-ECDSA ciphers.
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=item SSL_OP_SSLEAY_080_CLIENT_DH_BUG
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...
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=item SSL_OP_TLS_D5_BUG
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...
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=item SSL_OP_TLS_BLOCK_PADDING_BUG
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...
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=item SSL_OP_DONT_INSERT_EMPTY_FRAGMENTS
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Disables a countermeasure against a SSL 3.0/TLS 1.0 protocol
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vulnerability affecting CBC ciphers, which cannot be handled by some
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broken SSL implementations. This option has no effect for connections
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using other ciphers.
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=item SSL_OP_ALL
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All of the above bug workarounds.
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=back
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It is usually safe to use B<SSL_OP_ALL> to enable the bug workaround
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options if compatibility with somewhat broken implementations is
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desired.
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The following B<modifying> options are available:
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=over 4
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=item SSL_OP_TLS_ROLLBACK_BUG
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Disable version rollback attack detection.
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During the client key exchange, the client must send the same information
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about acceptable SSL/TLS protocol levels as during the first hello. Some
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clients violate this rule by adapting to the server's answer. (Example:
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the client sends a SSLv2 hello and accepts up to SSLv3.1=TLSv1, the server
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only understands up to SSLv3. In this case the client must still use the
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same SSLv3.1=TLSv1 announcement. Some clients step down to SSLv3 with respect
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to the server's answer and violate the version rollback protection.)
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=item SSL_OP_SINGLE_DH_USE
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Always create a new key when using temporary/ephemeral DH parameters
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(see L<SSL_CTX_set_tmp_dh_callback(3)|SSL_CTX_set_tmp_dh_callback(3)>).
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This option must be used to prevent small subgroup attacks, when
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the DH parameters were not generated using "strong" primes
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(e.g. when using DSA-parameters, see L<dhparam(1)|dhparam(1)>).
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If "strong" primes were used, it is not strictly necessary to generate
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a new DH key during each handshake but it is also recommended.
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B<SSL_OP_SINGLE_DH_USE> should therefore be enabled whenever
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temporary/ephemeral DH parameters are used.
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=item SSL_OP_EPHEMERAL_RSA
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Always use ephemeral (temporary) RSA key when doing RSA operations
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(see L<SSL_CTX_set_tmp_rsa_callback(3)|SSL_CTX_set_tmp_rsa_callback(3)>).
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According to the specifications this is only done, when a RSA key
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can only be used for signature operations (namely under export ciphers
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with restricted RSA keylength). By setting this option, ephemeral
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RSA keys are always used. This option breaks compatibility with the
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SSL/TLS specifications and may lead to interoperability problems with
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clients and should therefore never be used. Ciphers with EDH (ephemeral
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Diffie-Hellman) key exchange should be used instead.
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=item SSL_OP_CIPHER_SERVER_PREFERENCE
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When choosing a cipher, use the server's preferences instead of the client
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preferences. When not set, the SSL server will always follow the clients
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preferences. When set, the SSLv3/TLSv1 server will choose following its
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own preferences. Because of the different protocol, for SSLv2 the server
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will send its list of preferences to the client and the client chooses.
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=item SSL_OP_PKCS1_CHECK_1
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...
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=item SSL_OP_PKCS1_CHECK_2
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...
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=item SSL_OP_NETSCAPE_CA_DN_BUG
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If we accept a netscape connection, demand a client cert, have a
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non-self-signed CA which does not have its CA in netscape, and the
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browser has a cert, it will crash/hang. Works for 3.x and 4.xbeta
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=item SSL_OP_NETSCAPE_DEMO_CIPHER_CHANGE_BUG
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...
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=item SSL_OP_NO_SSLv2
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Do not use the SSLv2 protocol.
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=item SSL_OP_NO_SSLv3
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Do not use the SSLv3 protocol.
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=item SSL_OP_NO_TLSv1
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Do not use the TLSv1 protocol.
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=item SSL_OP_NO_SESSION_RESUMPTION_ON_RENEGOTIATION
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When performing renegotiation as a server, always start a new session
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(i.e., session resumption requests are only accepted in the initial
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handshake). This option is not needed for clients.
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=item SSL_OP_NO_TICKET
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Normally clients and servers will, where possible, transparently make use
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of RFC4507bis tickets for stateless session resumption.
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If this option is set this functionality is disabled and tickets will
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not be used by clients or servers.
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=item SSL_OP_ALLOW_UNSAFE_LEGACY_RENEGOTIATION
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Allow legacy insecure renegotiation between OpenSSL and unpatched clients or
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servers. See the B<SECURE RENEGOTIATION> section for more details.
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=item SSL_OP_LEGACY_SERVER_CONNECT
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Allow legacy insecure renegotiation between OpenSSL and unpatched servers
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B<only>: this option is currently set by default. See the
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B<SECURE RENEGOTIATION> section for more details.
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=back
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=head1 SECURE RENEGOTIATION
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OpenSSL 0.9.8m and later always attempts to use secure renegotiation as
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described in RFC5746. This counters the prefix attack described in
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CVE-2009-3555 and elsewhere.
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The deprecated and highly broken SSLv2 protocol does not support
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renegotiation at all: its use is B<strongly> discouraged.
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This attack has far reaching consequences which application writers should be
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aware of. In the description below an implementation supporting secure
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renegotiation is referred to as I<patched>. A server not supporting secure
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renegotiation is referred to as I<unpatched>.
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The following sections describe the operations permitted by OpenSSL's secure
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renegotiation implementation.
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=head2 Patched client and server
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Connections and renegotiation are always permitted by OpenSSL implementations.
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=head2 Unpatched client and patched OpenSSL server
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The initial connection suceeds but client renegotiation is denied by the
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server with a B<no_renegotiation> warning alert if TLS v1.0 is used or a fatal
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B<handshake_failure> alert in SSL v3.0.
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If the patched OpenSSL server attempts to renegotiate a fatal
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B<handshake_failure> alert is sent. This is because the server code may be
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unaware of the unpatched nature of the client.
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If the option B<SSL_OP_ALLOW_UNSAFE_LEGACY_RENEGOTIATION> is set then
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renegotiation B<always> succeeds.
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B<NB:> a bug in OpenSSL clients earlier than 0.9.8m (all of which are
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unpatched) will result in the connection hanging if it receives a
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B<no_renegotiation> alert. OpenSSL versions 0.9.8m and later will regard
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a B<no_renegotiation> alert as fatal and respond with a fatal
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B<handshake_failure> alert. This is because the OpenSSL API currently has
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no provision to indicate to an application that a renegotiation attempt
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was refused.
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=head2 Patched OpenSSL client and unpatched server.
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If the option B<SSL_OP_LEGACY_SERVER_CONNECT> or
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B<SSL_OP_ALLOW_UNSAFE_LEGACY_RENEGOTIATION> is set then initial connections
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and renegotiation between patched OpenSSL clients and unpatched servers
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succeeds. If neither option is set then initial connections to unpatched
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servers will fail.
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The option B<SSL_OP_LEGACY_SERVER_CONNECT> is currently set by default even
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though it has security implications: otherwise it would be impossible to
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connect to unpatched servers (i.e. all of them initially) and this is clearly
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not acceptable. Renegotiation is permitted because this does not add any
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additional security issues: during an attack clients do not see any
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renegotiations anyway.
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As more servers become patched the option B<SSL_OP_LEGACY_SERVER_CONNECT> will
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B<not> be set by default in a future version of OpenSSL.
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OpenSSL client applications wishing to ensure they can connect to unpatched
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servers should always B<set> B<SSL_OP_LEGACY_SERVER_CONNECT>
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OpenSSL client applications that want to ensure they can B<not> connect to
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unpatched servers (and thus avoid any security issues) should always B<clear>
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B<SSL_OP_LEGACY_SERVER_CONNECT> using SSL_CTX_clear_options() or
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SSL_clear_options().
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The difference between the B<SSL_OP_LEGACY_SERVER_CONNECT> and
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B<SSL_OP_ALLOW_UNSAFE_LEGACY_RENEGOTIATION> options is that
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B<SSL_OP_LEGACY_SERVER_CONNECT> enables initial connections and secure
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renegotiation between OpenSSL clients and unpatched servers B<only>, while
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B<SSL_OP_ALLOW_UNSAFE_LEGACY_RENEGOTIATION> allows initial connections
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and renegotiation between OpenSSL and unpatched clients or servers.
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=head1 RETURN VALUES
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SSL_CTX_set_options() and SSL_set_options() return the new options bitmask
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after adding B<options>.
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SSL_CTX_clear_options() and SSL_clear_options() return the new options bitmask
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after clearing B<options>.
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SSL_CTX_get_options() and SSL_get_options() return the current bitmask.
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SSL_get_secure_renegotiation_support() returns 1 is the peer supports
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secure renegotiation and 0 if it does not.
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=head1 SEE ALSO
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L<ssl(3)|ssl(3)>, L<SSL_new(3)|SSL_new(3)>, L<SSL_clear(3)|SSL_clear(3)>,
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L<SSL_CTX_set_tmp_dh_callback(3)|SSL_CTX_set_tmp_dh_callback(3)>,
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L<SSL_CTX_set_tmp_rsa_callback(3)|SSL_CTX_set_tmp_rsa_callback(3)>,
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L<dhparam(1)|dhparam(1)>
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=head1 HISTORY
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B<SSL_OP_CIPHER_SERVER_PREFERENCE> and
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B<SSL_OP_NO_SESSION_RESUMPTION_ON_RENEGOTIATION> have been added in
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OpenSSL 0.9.7.
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B<SSL_OP_TLS_ROLLBACK_BUG> has been added in OpenSSL 0.9.6 and was automatically
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enabled with B<SSL_OP_ALL>. As of 0.9.7, it is no longer included in B<SSL_OP_ALL>
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and must be explicitly set.
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B<SSL_OP_DONT_INSERT_EMPTY_FRAGMENTS> has been added in OpenSSL 0.9.6e.
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Versions up to OpenSSL 0.9.6c do not include the countermeasure that
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can be disabled with this option (in OpenSSL 0.9.6d, it was always
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enabled).
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SSL_CTX_clear_options() and SSL_clear_options() were first added in OpenSSL
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0.9.8m.
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B<SSL_OP_ALLOW_UNSAFE_LEGACY_RENEGOTIATION>, B<SSL_OP_LEGACY_SERVER_CONNECT>
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and the function SSL_get_secure_renegotiation_support() were first added in
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OpenSSL 0.9.8m.
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=cut
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