[noise] Negotiating transport message size (was: Re: NoiseSocket revision 1)

[Trevor Perrin]

On Fri, Aug 4, 2017 at 9:14 PM, Rhys Weatherley
<rhys.weatherley at gmail.com> wrote:
>
> I think having separate send / receive is useful.  Consider an interactive
> SSH client.  The client is sending keystrokes so it doesn't need a big
> outgoing buffer, but the incoming buffer may be larger to handle responses
> from the server.  On the server side, the receive buffer is the one that is
> smaller (think an IOT device running an SSH server for remote
> administration).
>
> In the first message:
>
> negotiation_data:
> {
>   "max_receive_size": 2048,
>   "max_send_size": 256
> }
>
> In the response from a memory-rich peer:
>
> {
>    "max_send_size": 4096,
>    "max_receive_size": 4096
> }
>
> In the response from a memory-poor peer:
>
> {
>   "max_send_size": 256,
>   "max_receive_size": 64
> }
>
> Both sides will use the minimum of the respective "me: max_send_size, peer:
> max_receive_size" pairs when choosing buffer sizes for a direction.


Is there a need to transmit max_send_size?

If not, the question is whether it's worth having separate
max_receive_size for each direction, versus a single
max_transport_size.

In your case:
 * The SSH-like client probably doesn't have a limit here, and can
just use the default of 65535.
 * Could the memory-poor peer just use the same buffer for encrypting
and decrypting, and set max_transport_size to 320 or something?


> For handshake packets, I think we need to allow packets to be larger than
> the negotiated maximum because each handshake step has a minimum size prior
> to the payload.  The negotiated maximum can limit the size of the payload in
> a handshake packet, but not the size of the mandatory Noise handshake data.

I was assuming the negotiated limit would just apply to transport
messages.  At least, for the large-message case that's probably all
that's needed.

For the small-memory case, I think you're right the handshake messages
will have a mandatory size, so there's not much room to negotiate.

Trevor