Re: [TLS] New draft: draft-ietf-tls-tls13-14.txt

David McGrew <mcgrew@cisco.com> Tue, 19 July 2016 09:53 UTC

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From: David McGrew <mcgrew@cisco.com>
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Date: Tue, 19 Jul 2016 05:47:48 -0400
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References: <CABcZeBMiLmwBeuLt=v4qdcJwe5rdsK_9R4-2TUXYC=sttmwH-g@mail.gmail.com> <D3AA5BD6.27AC0%qdang@nist.gov> <D3AAB674.709EA%kenny.paterson@rhul.ac.uk> <D3AA7549.27B09%qdang@nist.gov> <d1f35d74e93b4067bf17f587b904ebff@XCH-RTP-006.cisco.com> <D3AAD721.70A11%kenny.paterson@rhul.ac.uk> <D3AA9B01.27B9F%qdang@nist.gov> <D3AAE2B7.70A78%kenny.paterson@rhul.ac.uk> <ede4e2ffadd142f781e7a9c04081c825@XCH-RTP-006.cisco.com> <0ad33f70cbe2aabba1f16f4cac876b0f@esat.kuleuven.be> <D3AB99DD.27C8B%qdang@nist.gov> <553ea052cc05b4f7315e19c943b0c2b0@esat.kuleuven.be> <CACsn0ckFJSEabLOw60-1Pt=e3gLj1W+5yVvWRGzB=avNMQ_X+g@mail.gmail.com> <D3ABBB57.27CAC%qdang@nist.gov> <88AC1F39-6222-4D7A-AB1D-5FA4156C42C3@cisco.com> <9A043F3CF02CD34C8E74AC1594475C73F4CCD669@uxcn10-5.UoA.auckland.ac.nz>
To: Peter Gutmann <pgut001@cs.auckland.ac.nz>
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Subject: Re: [TLS] New draft: draft-ietf-tls-tls13-14.txt
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Hi Peter,

> On Jul 19, 2016, at 2:58 AM, Peter Gutmann <pgut001@cs.auckland.ac.nz>; wrote:
> 
> David McGrew <mcgrew@cisco.com>; writes:
> 
>> What is especially cool about counter mode encryption is how its real world
>> security degrades more gracefully than CBC mode encryption.
> 
> Uhh... how does CTR "degrade gracefully" compared to CBC?  

I should have said: “degrades more gracefully as the number of known plaintext blocks increases to, and beyond, the birthday bound”.  


> With CTR, any kind
> of problem with the IV/CTR leads to a catastrophic loss of security.  With CBC,
> even the worst-case IV abuse you can apply, setting it to all zeroes, just
> degrades the mode to ECB.
> 

Right, but that’s a different topic.

David

> (There have been a number of instances of CTR, or at least GCM, failures
> already, and I doubt we've seen the last of it.  It's RC4 all over again).
> 
> Peter.