Thanks,
I just couldn't find where is the message sequence number comes from.
So if it's max 1GB and it's an incremental counter that cannot be reseted
without a rekeying than it's perfectly fine :).

Thanks for the answer!

On Mon, Jun 17, 2019 at 12:20 PM Jonas Schnelli <dev@jonasschnelli.ch>
wrote:

> Hi Elichai
>
> > About the nonce being 64bit. (rfc7539 changed it to 96bit, which djb
> later calls xchacha)
> >
> > You suggest that we use the "message sequence number" as the nonce for
> Chacha20, Is this number randomly generate or is this a counter?
> > And could it be reseted without rekeying?
>
> The in BIP324 (v2 message transport protocol) proposed AEAD,
> ChaCha20Poly1305@Bitcoin [1], uses a „message sequence number“. There is
> no such thing as random nonce described in the BIP (hence the term
> „sequence number“). The message sequence number starts with 0 and the max
> traffic before a rekey must occur is 1GB. A nonce/key reuse is conceptually
> impossible (of course implementations could screw up at this point).
>
> Using XChaCha20 with the possibility of a random nonce could be done, but
> I don’t see a reason to use it in our case since the usage of a sequence
> number as nonce seems perfectly save.
>
> [1]
> https://gist.github.com/jonasschnelli/c530ea8421b8d0e80c51486325587c52#chacha20-poly1305bitcoin-cipher-suite
>


-- 
PGP: 5607C93B5F86650C