Re: [TLS] Fwd: New Version Notification for draft-whyte-qsh-tls13-01.txt

Dave Garrett <davemgarrett@gmail.com> Mon, 21 September 2015 04:20 UTC

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From: Dave Garrett <davemgarrett@gmail.com>
To: William Whyte <wwhyte@securityinnovation.com>
Date: Mon, 21 Sep 2015 00:20:21 -0400
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Subject: Re: [TLS] Fwd: New Version Notification for draft-whyte-qsh-tls13-01.txt
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On Sunday, September 20, 2015 10:59:58 pm William Whyte wrote:
> Hi all,
> 
> We've updated the TLS 1.3 Quantum Safe Handshake draft to use extensions as
> suggested by DKG in Prague. All comments welcome.
> 
> There's an interesting issue here: McEliece keys, which should be
> permissible, are larger in size (about 2^20 bytes) than the maximum
> permissible extension size (2^16-1). In order to support McEliece keys it

The max _total_ payload size of all extensions is 2^16-1. There are also mandatory extensions for TLS 1.3, as well as others that will customarily be used, thus the max available space is less.

> might be worth increasing the maximum extension size to 2^24-1 for TLS 1.3.

No, I don't think the limit can be raised. The general ClientHello format has to stay frozen for interoperability with other versions, and unless I'm misreading things, the size of the length of a vector can't change. A separate message seems like what would be needed to have a larger first-flight payload. (and any new messages would need to be signaled via an extension, though it could have a 0-length payload)

Also, 16MiB sounds kinda crazy for a first flight, especially one that you can't guarantee the server will understand. Something more like a 21-bit limit sounds less nuts, but that's still 2MiB. I don't think it's a good idea to allow this to be effectively unbounded, so a reasonable (arbitrary) limit should probably be imposed.

> Is there a strong reason for keeping the maximum size at 2^24-1, other than
> saving one byte on all the relevant length fields?

Typo? Did you mean "keeping the maximum size at 2^16-1"?

A strong reason is it not being possible to change due to the need for TLS 1.3 clients to be able to connect to TLS 1.2 servers that won't understand a format change. Even if it were technically possible, I wouldn't expect all implementations to safely handle it.

> Cheers,
> 
> William
> 
> 
> 
> 
> ---------- Forwarded message ----------
> From: <internet-drafts@ietf.org>
> Date: Sun, Sep 20, 2015 at 10:32 PM
> Subject: New Version Notification for draft-whyte-qsh-tls13-01.txt
> To: Zhenfei Zhang <zzhang@securityinnovation.com>om>, William Whyte <
> wwhyte@securityinnovation.com>gt;, "John M. Schanck" <
> jschanck@securityinnovation.com>
> 
> 
> 
> A new version of I-D, draft-whyte-qsh-tls13-01.txt
> has been successfully submitted by William Whyte and posted to the
> IETF repository.
> 
> Name:           draft-whyte-qsh-tls13
> Revision:       01
> Title:          Quantum-Safe Hybrid (QSH) Ciphersuite for Transport Layer
> Security (TLS) version 1.3
> Document date:  2015-09-20
> Group:          Individual Submission
> Pages:          18
> URL:
> https://www.ietf.org/internet-drafts/draft-whyte-qsh-tls13-01.txt
> Status:         https://datatracker.ietf.org/doc/draft-whyte-qsh-tls13/
> Htmlized:       https://tools.ietf.org/html/draft-whyte-qsh-tls13-01
> Diff:           https://www.ietf.org/rfcdiff?url2=draft-whyte-qsh-tls13-01
> 
> Abstract:
>    This document describes the Quantum-Safe Hybrid ciphersuite, a new
>    cipher suite providing modular design for quantum-safe cryptography
>    to be adopted in the handshake for the Transport Layer Security (TLS)
>    protocol version 1.3.  In particular, it specifies the use of the
>    NTRUEncrypt encryption scheme in a TLS handshake.
> 
> 
> 
> 
> 
> Please note that it may take a couple of minutes from the time of submission
> until the htmlized version and diff are available at tools.ietf.org.
> 
> The IETF Secretariat
>