A sha256 hash is 32 bytes, but otherwise I agree with this proposal.
Genesis block hash is the logical way to identify chains, moving forward.
On Aug 9, 2015 7:12 AM, "Ross Nicoll via bitcoin-dev" <
bitcoin-dev@lists.linuxfoundation.org> wrote:

> BIP 70 currently lists two networks, main and test (inferred as testnet3)
> for payment protocol requests. This means that different testnets cannot be
> supported trivially, and the protocol cannot be used for alternative coins
> (or, lacks context to indicate which coin the request applies to, which is
> particularly dangerous in cases where coins share address prefixes).
>
> I propose adding a new optional "genesis" field as a 16 byte sequence
> containing the SHA-256 hash of the genesis block of the network the request
> belongs to, uniquely identifying chains without any requirement for a
> central registry. For backwards compatibility, the "network" field would
> contain "main" for Bitcoin main net, "test" for Bitcoin testnet3, and
> "other" for other networks apart from those two.
>
> I'd appreciate initial feedback on the idea, and if there's no major
> objections I'll raise this as a BIP.
>
> Ross
>
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