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Re: [TLS] Exporter output size

Eric Rescorla <ekr@rtfm.com> Wed, 05 October 2016 03:49 UTC

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From: Eric Rescorla <ekr@rtfm.com>
Date: Tue, 04 Oct 2016 20:48:58 -0700
Message-ID: <CABcZeBMLbDv2oOu-GFdmm2cYNbNsrocnZ5KdW4D5HBbXdAEK1g@mail.gmail.com>
To: Martin Thomson <martin.thomson@gmail.com>
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Cc: "tls@ietf.org" <tls@ietf.org>
Subject: Re: [TLS] Exporter output size
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On Tue, Oct 4, 2016 at 6:32 PM, Martin Thomson <martin.thomson@gmail.com>
wrote:

> After a bunch of discussion about the consequences of having
> insufficient output from various stages of the hash functions... Could
> we make an amendment to TLS 1.3 to force the output size of the
> exporter to be the size of the underlying hash output?  That is,
> remove the length parameter.  Or is a change to the API too
> disruptive?
>

I don't think this is a good idea. There are plenty of reasons why you
would want to
export values != hash_len (e.g., cryptographic keys). Putting a restriction
here just
pushes the problem around

-Ekr


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