Subject: Re: [bitcoin-dev] p2p authentication and encryption BIPs

From: Lee Clagett <[email-masked]>
Date: Tue, 24 May 2016 20:22:50 -0400

On Wed, 18 May 2016 10:00:44 +0200
Jonas Schnelli via bitcoin-dev <[email-masked]>
wrote:

> Hi Lee
> 
> Thank you very much for the valuable input.
> I'm still processing your feedback....

[...]

> > Why have a fixed MAC length? I think the MAC length should be
> > inferred from the cipher + authentication mode. And the Poly1305
> > tag is 16 bytes.
> > 
> > *Unauthenticated Buffering*
> > Implementations are unlikely to (i.e. should not) process the
> > payload until authentication succeeds. Since the length field is 4
> > bytes, this means an implementation may have to buffer up to 4 GiB
> > of data _per connection_ before it can authenticate the length
> > field. If the outter length field were reduced to 2 or 3 bytes, the
> > unauthenticated buffering requirements drop to 64 KiB and 16 MiB
> > respectively. Inner messages already have their own length, so they
> > can span multiple encrypted blocks without other changes. This will
> > increase the bandwidth requirements when the size of a single
> > message exceeds 64 KiB or 16 MiB, since it will require multiple
> > authentication tags for that message. I think an additional 16
> > bytes per 16 MiB seems like a good tradeoff.
> >   
> 
> Good point.
> I have mentioned this now in the BIP but I think the BIP should allow
> message > 16 MiB.
> I leave the max. message length up to the implementation while keeping
> the 4 byte length on the protocol level.

I expect the implementation defined max size to work (SSH 2.0 does this
after all), but I want to make sure my suggestion is understood
completely.

There is a length field for the encrypted data, and length field(s)
inside of the encrypted data to indicate the length of the plaintext
Bitcoin messages. I am suggesting that the outter (encrypted) length
field be reduced, which will _not limit_ the length of Bitcoin
messages. For example, if a 1 GiB Bitcoin message needed to be sent
and the encrypted length field was 3 bytes - the sender is forced to
send a minimum of 64 MACs for this message. The tradeoff is allowing
the receiver to detect malformed data sooner and have a lower max
buffering window **against** slightly higher bandwidth and CPU
requirements due to the additional headers+MACs (the CPU requirements
should primarily be in "finalizing each Poly1305").

An alternative way to think about the suggestion is tunnelling Bitcoin
messages over TLS or SSH. TLS 1.2 has a 2-byte length field and SSH 2.0
a 4-byte length field, but neither prevents larger Bitcoin messages from
being tunnelled; the lengths are independent.

[...]

> </jonas>

Lee