Re: [TLS] Ensuring consistent strength across certificate, ECDHE, cipher, and MAC

Eric Rescorla <ekr@rtfm.com> Wed, 23 March 2016 00:50 UTC

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From: Eric Rescorla <ekr@rtfm.com>
Date: Tue, 22 Mar 2016 17:49:15 -0700
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To: Timothy Jackson <tjackson@mobileiron.com>
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Subject: Re: [TLS] Ensuring consistent strength across certificate, ECDHE, cipher, and MAC
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On Tue, Mar 22, 2016 at 5:38 PM, Timothy Jackson <tjackson@mobileiron.com>
wrote:

> I’ve noted that many (most?) TLS implementations choose their ECDHE curves
> seemingly without regard to the cipher suite strength. Thus, they'll select
> an AES256 cipher suite (e.g. TLS_ECDHE_ECDSA_WITH_AES256_SHA384), but then
> generate an ECDHE key on the P256 curve. This seems odd to me, since the
> P256 curve obviously has a lower security strength than AES256. This seems
> important issue to resolve because most products (and even TLS libraries)
> do not allow the administrator to configure the available ECDHE curves,
> only the cipher suites. Thus, ECDHE may be invisibly undermining the
> security of your TLS connection.
>
> Is this an intentional choice by this group for some reason that I haven’t
> realized yet?
>
> How would this group feel about a proposal to address this by specifying
> in the 1.3 specification that implementations must ensure that the strength
> of the certificate must be >= strength of ECDHE/DHE >= strength of the
> cipher? Perhaps an equivalency rule for the MAC might also be in order?
> Apologies if this is already resolved somewhere in the draft RFC. I looked
> but didn’t find it.
>

Off the top of my head, this rule seems like it would have several quite
negative
side effects:

- It would require you to use Curve448 or P521 with ChaCha/Poly1305
  (256-bit key) but allow you to use Curve25519 with AES-GCM 128,
  which cuts against one of the major reasons to use ChaCha/Poly1305,
  namely that it's faster on mobile platforms.

- It would essentially preclude using RSA certificates with ECC at all,
  because the RSA modulus you need to match 256-bit ECC is impractically
  large.

It's also worth noting that AES-GCM 256 and 128 use the same authentication
tag size.

-Ekr


> For what it’s worth, I’ve noticed OpenSSL and other implementations trying
> to address this by creating a “Suite B Mode”, but that seems to address a
> specific case but leave the generic case unresolved.
>
> Cheers,
>
> Tim
>
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