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([
             chr(0x40 | ord(h[6]) & 0x0f),
             chr(0x80 | ord(h[8]) & 0x3f),
         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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