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

[Dave Garrett <[e-mail]>]

On Monday, September 21, 2015 12:20:21 am Dave Garrett wrote:
> 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.
[...]
> 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)

To amend my previous email:

Another option might be to implement things as an encrypted-only TLS extension. This would restrict it to 0-RTT mode only, meaning clients would need the server's semi-static config in order to use it. (retrieved from a previous connection or possibly through an out-of-bound method, yet to be specified) This would, however, put it in the EncryptedExtensions message which currently has a similar 2^16-1 limit, but this is new so it could be changed. Again, though, that might not be the best idea.

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In case you're confused as to where the limit I'm referring to comes from:
https://tools.ietf.org/html/rfc5246#section-4.3

The notation is somewhat atypical. The value/range in angle brackets is the length, in bytes, of the vector, rather than the length in total number of elements.

The ClientHello.extensions vector max size is 2^16-1:
https://tools.ietf.org/html/rfc5246#section-7.4.1.2

Each Extension.extension_data vector max size is also 2^16-1:
https://tools.ietf.org/html/rfc5246#section-7.4.1.4

And... actually, looking at this, I think that could be considered wrong. The max individual extension payload size would be 2^16-5, to account for overhead. Less, of course, taking into account other extensions in the same hello.


Dave