[TLS] RC4 Considered Harmful (Was: RC4 deprecation path)

Alyssa Rowan <akr@akr.io> Sat, 19 April 2014 20:10 UTC

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Date: Sat, 19 Apr 2014 21:10:20 +0100
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Subject: [TLS] RC4 Considered Harmful (Was: RC4 deprecation path)
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On 19/04/2014 20:28, Yoav Nir wrote:

> As long as the client is required to support such servers, I guess 
> we have to live with it.

I think the only correct deprecation path to recommend is the one
that's on the table right now: the off switch.

Warn your users if you have to. But don't negotiate RC4 without a
click-through warning.

RC4 is either on the brink of being cracked, given the serious known
weaknesses pointed out in Section 1 of the draft, or it is already
over the brink (if that's the 'cryptanalytic breakthrough' GCHQ were
talking about that they got from NSA, and that seems plausible to me,
and to several others, including Schneier).

If it's on the brink, then when it's cracked, captured traffic can
(and will) be retroactively decrypted. If it's over the brink, that's
already happening.

That window of opportunity was widened by advice given to use RC4-SHA
to avoid BEAST, which is why some servers prefer RC4 to AES-128. (That
was very bad advice, with 20:20 hindsight.)

We need to close that window now. As you've seen in this discussion,
there is only one safe way to close that window: disable RC4
completely. Any delay in disabling RC4 leaves that window open for
longer, and leaves users subject to a false sense of security about
their connections that should be protected by that little 'lock icon'.

I don't think we can in good conscience recommend any delay. That's
why the draft we have strong consensus on is crystal-clear:

   o  TLS clients MUST NOT include RC4 cipher suites in the ClientHello

   o  TLS servers MUST NOT select an RC4 cipher suite when a TLS client
      sends such a cipher suite in the ClientHello message.

   o  If the TLS client only offers RC4 cipher suites, the TLS server
      MUST terminate the handshake.  The TLS server MAY send the
      insufficient_security fatal alert in this case.

In short: RC4 is Considered Harmful. Kill it with fire.

On 19/04/2014 20:54, Kurt Roeckx wrote:

> IE on XP is known to only support 3DES, RC4, and some export 
> ciphers.

Firstly, of those options, they're all bad. 3DES is the less awful
option in my opinion (because BEAST is an active attack, but RC4 is or
will be vulnerable to a passive attack applicable in retrospect), but
none of them are recommended.

But, reminder: support for Windows XP ended 11 days ago. It's now off
life-support... it's got bigger problems than even 3DES.

- --