[TLS] weird ECDSA interop problem with cloudflare/nginx

mrex@sap.com (Martin Rex) Mon, 25 July 2016 19:08 UTC

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From: mrex@sap.com (Martin Rex)
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Subject: [TLS] weird ECDSA interop problem with cloudflare/nginx
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I've just run into a weird interoperability problem with an (alleged)
cloudflare/nginx TLS server and my personal Firefox settings.


Traditionally I have all TLS ciphersuites with ECDSA disabled through
about:config, but it seems that recently two new TLS ciphersuites were
added to FF, which caused complete loss of interop with regmedia.co.uk
for me with my existing configuration. (Loss of pictures&media on the
www.theregister.co.uk news site).

specifically, after the FF update, this new TLS ciphersuite:

   security.ssl3.ecdhe_ecdsa_aes_128_gcm_sha256  (0xcc, 0xa9)

was the only ECDSA cipher suite enabled in my Firefox 47.0.1, and this
kills connectivity (TLS handshake_failure alert) with regmedia.co.uk.

It looks like a bug in the cloudflare/nginx cipher suites selection
algorithm, which appears to blindly go for ECDSA, even though there is
no actual ECDSA cipher suite available which the server supports.