[bitcoin-dev] Increasing the blocksize as a (generalized) softfork.

This is a draft. --joe Introduction ============ It is generally assumed that increasing the blocksize limit requires a hardfork. Instead we show that a increasing the limit can be achieved using a "generalized" softfork. Like standard softforks, generalized softforks need a mere miner majority (>50% hashpower) rather than global consensus. Standard Softforks ================== After a softfork two potential chains exist: * The new chain B3,B4,B5,... valid under the new rules and old rules. * The old chain B3',B4',B5',... valid under the old rules only. E.g. +-- B3 --- B4 --- B5 | ... -- B1 -- B2 --+ | +-- B3' -- B4' -- B5' -- B6' Assuming that >50% of the hashpower follow the new rules, the old chain is doomed to be orphaned: +-- B3 --- B4 --- B5 --- B6 --- B7 --- B8 --- ... | ... -- B1 -- B2 --+ | +-- B3' -- B4' -- B5' -- B6' (orphaned) Hardforks may result in two chains that can co-exist indefinitely: +-- B3 --- B4 --- B5 --- B6 --- B7 --- B8 --- ... | ... -- B1 -- B2 --+ | +-- B3' -- B4' -- B5' -- B6' -- B7' -- B8' -- ... Generalized Softforks ===================== A *generalized* softfork introduces a transform function f(B)=B' that maps a block B valid under the new rules to a block B' valid under the old rules. After a generalized softfork three chains may exist: * The new chain B3,B4,B5,... valid under the new rules only. * The mapped chain f(B3),f(B4),f(B5),... valid under the old rules. * The old chain B3',B4',B5',... valid under the old rules only. E.g. +-- B3 ---- B4 ---- B5 | ... -- B1 -- B2 --+-- f(B3) - f(B4) - f(B5) | +-- B3' --- B4' --- B5' --- B6' This is "generalized" softfork since defining f(B)=B (identity function) reduces to the standard softfork case above. As with standard softforks, if the majority of the hashpower follow the new rules then the old chain B3',B4',B5',... is doomed to be orphaned: +-- B3 ---- B4 ---- B5 ---- B6 ---- B7 ---- ... | ... -- B1 -- B2 --+-- f(B3) - f(B4) - f(B5) - f(B6) - f(B7) - ... | +-- B3' --- B4' --- B5' --- B6' (orphaned) Example: -------- Segregated Witness can be thought of as an example of a generalized softfork. Here the new block format consists of the combined old block and witness data. The function f() simply strips the witness data to reveal a valid block under the old rules: NewBlock := OldBlock ++ Witness f(NewBlock) = OldBlock An Arbitrary Block-size Increase Via a Generalized Softfork =========================================================== Segregated Witness only allows for a modest effective blocksize increase (although there can be other motivations for SW, but that is off-topic). Instead we engineer a generalized softfork that allows an arbitrarily increase of the blocksize limit. The proposal consists of two parts: (a) new rules for valid blocks, and (b) a transformation function f(). The new block rules are very similar to the old block rules but with some small changes. In summary the changes are: * The MAX_BLOCK_SIZE limit is raised to some new limit (e.g. 8Mb, BIP101, 2-4-8, BIP202, etc., or some other limit) * The MerkleRoot field in the header has been re-interpreted. * The CoinBaseTx must obey some additional new rules. As with old blocks, a block under the new rules consists of a block header followed by a vector of transactions [CoinBaseTx, Tx1, .., Txn], i.e. NewBlock := BlockHeader ++ NumTx ++ CoinBaseTx ++ Tx1 ++ .. ++ Txn The block header format is the same as under the old rules defined as follows: +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Ver | PrevHash | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | MerkleRoot | Time | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Bits | Nonce | +-+-+-+-+-+-+-+-+ Under the old rules MerkleRoot is the Merkle root of the hashes of all transactions included in the block, i.e. MerkleRoot = merkleRoot([hash(CoinBaseTx), hash(Tx1), .., hash(Txn)]) Under the new rules we instead define: MerkleRoot = merkleRoot([hash(CoinBaseTx)]) That is, under the new rules, MerkleRoot is the Merkle root of a singleton vector containing the CoinBaseTx hash only. In order to preserve the security properties of Bitcoin we additionally require that the CoinBaseTx somehow encodes the Merkle root of the remaining transactions [Tx1, .., Txn]. For example, this could be achieved by requiring a mandatory OP_RETURN output that encodes this information, e.g. OP_RETURN merkleRoot([hash(Tx1), .., hash(Txn)]) Alternatively the Merkle root could be encoded in the coinbase itself. This ensures that new transactions cannot be added/deleted from the block without altering the MerkleRoot field in the header. Aside from these changes and the increased MAX_BLOCK_SIZE, the new block must obey all the rules of the old block format, e.g. valid PoW, have valid block reward, contain valid transactions, etc., etc. In order to be a generalized softfork we also need to define a mapping f() from valid new blocks to valid blocks under the old rules. We can define this as follows: NewBlock := BlockHeader ++ NumTx ++ CoinBaseTx ++ Tx1 ++ .. ++ Txn f(NewBlock) := BlockHeader ++ 1 ++ CoinBaseTx That is, function f() simply truncates the block so that it contains the coinbase transaction only. After truncation, the MerkleRoot field of the block header is valid under the old rules. The proposed new rules combined with the transformation f() comprise a generalized softfork. After the fork a new chain B3,B4,B5,... will be generated under the new rules defined above, including an increased blocksize limit. This new chain can be mapped to a valid chain f(B3),f(B4),f(B5),... under the old rules. Assuming that >50% of the hashpower has adopted the new rules, the mapped chain will orphan any competing chain under the old rules, just like a standard softfork. An interesting consequence of this design is that, since all mapped blocks are empty, old clients will never see transactions confirming. This is be a strong incentive for users to update their clients. Conclusion ========== Conventional wisdoms suggests that increasing the blocksize limit requires a hardfork. We show that it can instead be achieved using a generalized softfork. Like with a standard softfork, a generalized softfork merely requires a majority (>50%) of hash power rather than global consensus. Experience has shown that the former is significantly easier to achieve. Future Work ----------- Investigate other kinds of hardforks that can instead be implemented as generalized softforks, and the security implications of such... 7943a2934d0be2f96589fdef2b2e00a2a7d8c3b782546bb37625d1669accb9b1 72f018588572ca2786168cb531d10e79b81b86d3fada92298225a0f950eed3a5