Bitcoin currently uses raw hashes extensively as UUIDs; whether the payment
protocol should be influence by that or not, I've not given thought to yet.
Some alt coins may share a blockchain, or even merely the genesis block (two
currently do; despite one of those being a scamcoin, I think the possibility
should not be dismissed). Because of this, requiring a 1:1 mapping between
genesis block and chain or coin seems non-ideal.
On Monday, May 20, 2013 11:59:39 PM Mark Friedenbach wrote:
> At the developer round-table it was asked if the payment protocol would
> alt-chains, and Gavin noted that it has a UTF-8 encoded string
> identifying the network ("main" or "test"). As someone with two
> proposals in the works which also require chain/coin identification (one
> for merged mining, one for colored coins), I am opinionated on this. I
> believe that we need a standard mechanism for identifying chains, and
> one which avoids the trap of maintaining a standard registry of
> string-to-chain mappings.
>
> Any chain can be uniquely identified by its genesis block, 122 random
> bits is more than sufficient for uniquely tagging chains/colored assets,
> and the low-order 16-bytes of the block's hash are effectively random.
> With these facts in mind, I propose that we identify chains by UUID.
>
> So as to remain reasonably compliant with RFC 4122, I recommend that we
> use Version 4 (random) UUIDs, with the random bits extracted from the
> double-SHA256 hash of the genesis block of the chain. (For colored
> coins, the colored coin definition transaction would be used instead,
> but I will address that in a separate proposal and will say just one
> thing about it: adopting this method for identifying chains/coins will
> greatly assist in adopting the payment protocol to colored coins.)
>
> The following Python code illustrates how to construct the chain
> identifier from the serialized genesis block:
>
> from hashlib import sha256
> from uuid import UUID
> def chain_uuid(serialized_genesis_block):
> h = sha256(serialized_genesis_block).digest()
> h = sha256(h).digest()
> h = h[:16]
> h = ''.join([
> h[:6],
> chr(0x40 | ord(h[6]) & 0x0f),
> h[7],
> chr(0x80 | ord(h[8]) & 0x3f),
> h[9:]
> ])
> return UUID(bytes=h)
>
> And some example chain identifiers:
>
> mainnet: UUID('6fe28c0a-b6f1-4372-81a6-a246ae63f74f')
> testnet3: UUID('43497fd7-f826-4571-88f4-a30fd9cec3ae')
> namecoin: UUID('70c7a9f0-a2fb-4d48-a635-a70d5b157c80')
>
> As for encoding the chain identifier, the simplest method is to give
> "network" the "bytes" type, but defining a "UUID" message type is also
> possible. In either case bitcoin mainnet would be the default, so the
> extra 12 bytes (vs: "main" or "test") would only be an issue for
> alt-chains or colored coins.
>
> Kind regards,
> Mark Friedenbach
>
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