Re: [kitten] WGLC on draft-ietf-kitten-aes-cts-hmac-sha2-06

Luke Howard <lukeh@padl.com> Wed, 15 April 2015 04:59 UTC

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From: Luke Howard <lukeh@padl.com>
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References: <alpine.GSO.1.10.1503301227280.22210@multics.mit.edu> <551D6C35.4080108@mit.edu> <alpine.GSO.1.10.1504081626110.22210@multics.mit.edu> <5525B044.8070509@mit.edu> <CAC2=hnfbLoRAQLwDQhL7pVYMS8kqfc1rAA6Ha1np1h1WnhT5aw@mail.gmail.com> <55271546.6020505@mit.edu>
To: Greg Hudson <ghudson@MIT.EDU>
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Subject: Re: [kitten] WGLC on draft-ietf-kitten-aes-cts-hmac-sha2-06
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> On 10 Apr 2015, at 10:11 am, Greg Hudson <ghudson@MIT.EDU> wrote:
> 
> * If the goal is to meet a checklist of Suite B requirements, using
> SHA-384 over SHA-256 internally might be necessary, but it really is
> just a meaningless checklist tick.  Moreover, the small integrity tag
> and checksum lengths could mean that the draft doesn't actually satisfy
> Suite B--I can't speak confidently either way on that point.
> 
> * If the goal is to achieve some real security strength, using truncated
> SHA-384 is not an improvement over using truncated SHA-256.

I am not a cryptographer but Greg’s arguments make sense to me. Why not always truncate the hash to 256 bits when using 256-bit AES keys?

— Luke