We have seen this 'longest chain' verbiage where 'valid' is notably excluded for some time. I take it as a backup or auxiliary attack strategy. Every opportunity is taken to lay the foundation participant misunderstanding however. That misinformation effort seems to have picked up again over this last month here on bitcointalk.org.
Basically, if a bloatblock (for instance) can be inserted just one time, it would fork the chain and obsolete Bitcoin core software.
So what if a given version of bitcoin software is obsoleted by evolution of the network? The "core" developers have done this themselves several times. Arguments along the line of "longest valid chain" do not resolve the issue under debate. They just move the debate to the meaning of the word "valid".
Despite some advertising claims, the security of bitcoin does not depend on mathematics alone, it depends upon running software, which intern depends on running hardware which is supported by the economic majority.
Any other way of defining "valid" necessarily requires the argument from authority, which ultimately results in a debate over who is the authority. With this, any hope of a decentralized system goes out the door.Great post!
Regarding "longest chain" versus "longest valid chain," I am using the language from Satoshi's white paper. He refers to it simply as "the longest chain" and only refers to transactions as being valid or invalid. From page 3:
- Nodes accept the block only if all transactions in it are valid and not already spent.
- Nodes express their acceptance of the block by working on creating the next block in the chain, using the hash of the accepted block as the previous hash.
- Nodes always consider the longest chain to be the correct one and will keep working on extending it.In other words, "the longest chain" means the chain with the greatest cumulative work that is composed exclusively of valid transactions.