Re: [TLS] Drafts for batch signing and PKCS#1 v1.5

David Benjamin <davidben@chromium.org> Wed, 31 July 2019 04:12 UTC

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From: David Benjamin <davidben@chromium.org>
Date: Wed, 31 Jul 2019 00:11:54 -0400
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To: Martin Thomson <mt@lowentropy.net>
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Subject: Re: [TLS] Drafts for batch signing and PKCS#1 v1.5
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On Tue, Jul 30, 2019 at 11:59 PM Martin Thomson <mt@lowentropy.net>; wrote:

> On Wed, Jul 31, 2019, at 13:54, Ben Schwartz wrote:
> > The batch signing idea is very cool. I'm not entirely sure I understand
> > the intended use case, though. The intro suggests that this motivated
> > by DoS defense, but presumably an attacker who controls their own TLS
> > client stack could simply omit support for these signature schemes. Do
> > you envision a future where servers can safely omit support for all the
> > non-batch signature schemes? Or are you thinking of attackers who don't
> > control the TLS client stack?
>
> The usual trick when under duress is to attempt to process some requests,
> and lowering the cost of handling those requests enables higher tolerance
> to attack and better continuity of service.  A server might choose not to
> serve clients that don't offer batching if it is stressed.
>

Yup. The signing cost of a batch-capable client is effectively zero. (I
expect it's already common to preferentially serve ECDSA-capable clients
when under load.) Also, if many clients implement this, serving load under
normal operation goes down, which is also valuable.


> > Minor question: in the tree diagrams, m2 goes to t04. Is there any
> > reason it couldn't go directly to t12? That would seem more natural to
> > me.
>
> The blinding process is explained in Section 4.3.
>

I think Ben is asking why the tree doesn't put m2 a level higher (like CT
does), instead of adding the padding nodes. That would work too. I chose
this one because I found it more straightforward, and it doesn't
particularly matter. Also it's what Roughtime did and I cribbed the
construction from Roughtime. :-)

In principle, the signer could pick any tree strategy as long as it
produced valid paths for each message. But I think it's probably better for
the draft to be opinionated, rather than risk implementers mess things up.

David