On 10/20/2014 12:50 PM, Aditya Kumar wrote:
1. Will this updated client set with TLS_FALLBACK will be able to work
with un-updated Server(server using older version of OpenSSL where this
FALLBACK mode is not set)?
No, the behavior of existing OpenSSL applications will not change.
OpenSSL will signal TLS_FALLBACK_SCSV only if explicitly requested by
the application. Most applications will not want to do this.
2. Is it a good idea to hard code this option permanently in application
or should I give this as a configuration option to the user? I am asking
this as my client application may require to communicate with old
Servers not having this fix.
How do you select the protocol version? If you use SSLv23_method with
a constant set of options (which should include SSL_OP_NO_SSLv2 at
least), then there is no need to set SSL_MODE_SEND_FALLBACK_SCSV, ever.
OpenSSL will do the right thing automatically.
Unconditionally setting SSL_MODE_SEND_FALLBACK_SCSV (if by default or
after user configuration) is a time bomb—your client application will
break once the server implements TLS 1.3 (or any newer TLS version than
what is supported by the OpenSSL version you use). Extremely few
applications have to deal with SSL_MODE_SEND_FALLBACK_SCSV.
--
Florian Weimer / Red Hat Product Security
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