Re: [Cfrg] HPKE, message lengths, and IND-CCA2

Christopher Wood <caw@heapingbits.net> Fri, 23 October 2020 16:02 UTC

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Date: Fri, 23 Oct 2020 09:02:08 -0700
From: "Christopher Wood" <caw@heapingbits.net>
To: cfrg@irtf.org
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Subject: Re: [Cfrg] HPKE, message lengths, and IND-CCA2
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Thanks for raising this, John! We addressed this in the following PR:

   https://github.com/cfrg/draft-irtf-cfrg-hpke/pull/166

Best,
Chris

On Fri, Oct 23, 2020, at 1:53 AM, John Mattsson wrote:
> HI,
> 
> While the theoretical IND-CCA2 game assumes that the attacker generates 
> messages with a fixed lengths, this is not very realistic in reality 
> where messages often has variable length. It would be good to inform 
> the reader that the specified mechanism does nothing to hide the length 
> of the encrypted message, and if this is a concern, the user needs to 
> use a padding mechanism The draft could also e.g. suggest how to do 
> padding, e.g. the bit padding mechanism combined with one of the 
> padding policies from
> 
> https://tools.ietf.org/html/draft-ietf-tls-esni-08
> https://tools.ietf.org/html/rfc8467
> 
> Cheers,
> John
> 
> 
> 
> 
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