So the scanning tool does something simple: it negotiates a CBC cipher suite, and makes a connection with a bad MAC and a connection with bad padding and looks for any difference. This is my favorite kind of science: actually go check the real world!
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Next weird thing: the problem also happened if OpenSSL wasn't using AES-NI hardware acceleration. In practice this means it impacted 3DES (which people should have turned off for other reasons!) and older hardware.
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This also explained why FIPS software appeared in the list, because FIPS software generally can't use AES-NI.
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At this point, a lot of factors have to be combined: TLS sw would have to be coded in an uncommon way, using OpenSSL, negotiating older cipher suites, on older HW, with clients that send 0-byte records, and can be made repeat the same data over and over, with an active MITM.
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But that makes it more interesting! How do we find and prevent even these kind of rarefied cases? Automation, like the scanning tool, is clearly critical - but can we do more at the point of code?
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One thing I'm grateful for is that in s2n we kill connections on any error, and we do it in a way where s2n will completely refuse to interact with the connection after the error has happened. Just with a closed flag ... https://github.com/awslabs/s2n/blob/master/tls/s2n_connection.c#L1031 …
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s2n uses OpenSSL's libcrypto for the underlying cryptography, and the same issue in that code /could/ have caused impact within s2n were it not for that practice. Basically this check .... https://github.com/awslabs/s2n/blob/master/tls/s2n_send.c#L94 …
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Of course the impact still would have been small, because of the other factors, but I'm glad we have that check! Anyway, thanks again to the issue reporters, read their paper when it comes! and thanks for Andrew and Steven from the TLS team. That's it, unless AMA.
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