Quote from: telepatheic on May 17, 2014, 05:09:21 PM

Quote from: Peter R on May 17, 2014, 04:17:32 PM

The Blockchain (proper noun) is the "best" chain (of blocks containing digital signatures) that originates from the Genesis Block (proper noun). A proper noun refers to a "unique entity." A chain of blocks that does not originate from the Genesis Block cannot be the Blockchain by definition.

Whilst true, SPV nodes have no way of validating the genesis block so they can only work when we assume the greatest difficulty chain segment is bitcoin (regardless of which genesis block the chain originates from).

Why do you think that? SPV clients are hardcoded with the contents of the genesis block just like any other node. There is no difference between the two. If you are following a chain which doesn't begin with the Bitcoin genesis block you are not part of the Bitcoin network.

SPV clients still validate the block headers. To validate block one requires knowing the correct hash for the previous block (block zero = genesis block). Technically an SPV client could be hardcoded with just the genesis block hash, but an SPV client already must have the ability to compute the current block difficulty, perform block hashing, and validate that the block hash is smaller than the target so you don't really save anything by hardcoding the hash instead of the contents.

Part of the problem may come from shorthand language. "Longest chain" doesn't just mean the chain with the most difficulty, it means the chain of valid blocks with the most difficulty. An invalid block can't extend the chain. "Valid" for Bitcoin means a variety of checks including the prior block hash and by extension that necessitates that the chain begin from the genesis block. While you are correct that currently one could skip the genesis block validation that was never part of Satoshi's security design and it may not be true in the future (another coin could someday have more computing power). SPV are no different than full nodes in this respect. Given how trivially easy it is to verify the genesis block (and the added DOS hardening that adds) there is no reason to skip this check.