#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);
SSL_set_options() adds the options set via bit mask in options to ssl. Options already set before are not cleared!
SSL_CTX_clear_options() clears the options set via bit mask in options to ctx.
SSL_clear_options() clears the options set via bit mask 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. Note, this is implemented via a macro.
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:
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:
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.)
When using session ids a copy of the session information is cached on the server and a unique id is sent to the client. When the client wishes to resume it provides the unique id so that the server can retrieve the session information from its cache.
When using stateless session tickets the server uses a session ticket encryption key to encrypt the session information. This encrypted data is sent to the client as a "ticket". When the client wishes to resume it sends the encrypted data back to the server. The server uses its key to decrypt the data and resume the session. In this way the server can operate statelessly - no session information needs to be cached locally.
The TLSv1.3 protocol only supports tickets and does not directly support session ids. However, OpenSSL allows two modes of ticket operation in TLSv1.3: stateful and stateless. Stateless tickets work the same way as in TLSv1.2 and below. Stateful tickets mimic the session id behaviour available in TLSv1.2 and below. The session information is cached on the server and the session id is wrapped up in a ticket and sent back to the client. When the client wishes to resume, it presents a ticket in the same way as for stateless tickets. The server can then extract the session id from the ticket and retrieve the session information from its cache.
By default OpenSSL will use stateless tickets. The SSL_OP_NO_TICKET option will cause stateless tickets to not be issued. In TLSv1.2 and below this means no ticket gets sent to the client at all. In TLSv1.3 a stateful ticket will be sent. This is a server-side option only.
If this option is set, Encrypt-then-MAC is disabled. Clients will not propose, and servers will not accept the extension.
The following options no longer have any effect but their identifiers are retained for compatibility purposes:
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.
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.
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.
SSL_CTX_clear_options() and SSL_clear_options() return the new options bit mask after clearing options.
SSL_CTX_get_options() and SSL_get_options() return the current bit mask.
SSL_get_secure_renegotiation_support() returns 1 is the peer supports secure renegotiation and 0 if it does not.
The SSL_OP_PRIORITIZE_CHACHA and SSL_OP_NO_RENEGOTIATION options were added in OpenSSL 1.1.1.
Licensed under the OpenSSL license (the "License"). You may not use this file except in compliance with the License. You can obtain a copy in the file LICENSE in the source distribution or at <https://www.openssl.org/source/license.html>.