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Re: [TLS] Cipher suite values to indicate TLS capability

Yoav Nir <ynir@checkpoint.com> Wed, 06 June 2012 05:17 UTC

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From: Yoav Nir <ynir@checkpoint.com>
To: Chris Richardson <chris@randomnonce.org>
Date: Wed, 06 Jun 2012 08:17:24 +0300
Thread-Topic: [TLS] Cipher suite values to indicate TLS capability
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Subject: Re: [TLS] Cipher suite values to indicate TLS capability
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On Jun 6, 2012, at 4:11 AM, Chris Richardson wrote:

> On Tue, Jun 5, 2012 at 4:39 PM, Adam Langley <agl@google.com> wrote:
>> However, with the downgrade to SSLv3 we loose an important security
>> feature: ECDHE.
> ...
>> So I'd like to introduce TLS_CAPABLE_SCSV (0x00fe) in SSLv3
>> handshakes. TLS_EMPTY_RENEGOTIATION_INFO_SCSV has shown that we can
>> deploy new ciphersuites for SSLv3 and the semantics of
>> TLS_CAPABLE_SCSV would be that servers would reject any SSLv3
>> handshakes that included this ciphersuite with a fatal error.
> 
> Thinking through various scenarios... if I'm a TLS-capable server that
> does not support foward-secure cipher suites, what reason would I have
> any reason to reject an SSLv3 hello containing the TLS_CAPABLE_SCSV?

Because the client says it supports TLS, but is connecting using SSLv3. That is evidence of a downgrade attack. Regardless of what benefit the attacker intends to get from downgrading the connection to SSLv3, it should be aborted.

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