Re: [TLS] Data volume limits

Watson Ladd <watsonbladd@gmail.com> Tue, 15 December 2015 22:38 UTC

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Date: Tue, 15 Dec 2015 17:38:23 -0500
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From: Watson Ladd <watsonbladd@gmail.com>
To: "Scott Fluhrer (sfluhrer)" <sfluhrer@cisco.com>
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Subject: Re: [TLS] Data volume limits
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On Tue, Dec 15, 2015 at 5:01 PM, Scott Fluhrer (sfluhrer)
<sfluhrer@cisco.com> wrote:
> Might I enquire about the cryptographical reason behind such a limit?
>
>
>
> Is this the limit on the size of a single record?  GCM does have a limit
> approximately there on the size of a single plaintext it can encrypt.  For
> TLS, it encrypts a record as a single plaintext, and so this would apply to
> extremely huge records.

The issue is the bounds in Iwata-Ohashai-Minematsu's paper, which show
a quadratic confidentiality loss after a total volume sent. This is an
exploitable issue.

>
>
>
> Or is this a limit on the total amount of traffic that can go through a
> connection over multiple records?  If this is the issue, what is the
> security concern that you would have if that limit is exceeded?
>
>
>
> Thank you.
>
>
>
> From: TLS [mailto:tls-bounces@ietf.org] On Behalf Of Eric Rescorla
> Sent: Tuesday, December 15, 2015 4:15 PM
> To: tls@ietf.org
> Subject: [TLS] Data volume limits
>
>
>
> Watson kindly prepared some text that described the limits on what's safe
>
> for AES-GCM and restricting all algorithms with TLS 1.3 to that lower
>
> limit (2^{36} bytes), even though ChaCha doesn't have the same
>
> restriction.
>
>
>
> I wanted to get people's opinions on whether that's actually what we want
>
> or whether we should (as is my instinct) allow people to use ChaCha
>
> for longer periods.
>
>
>
> -Ekr
>
>
>
>
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