May 2018: 32 MB blocks and restored opcodes

What changed

This upgrade activated when the median time past reached Unix time 1526400000 (15 May 2018, 16:00 UTC). The first block under the new rules is height 530,356.

Consensus changes:

• Block size limit: 32 MB (32,000,000 bytes), up from 8 MB.
• Nine opcodes restored or added. Bitcoin disabled many opcodes in 2010–2011 after serious bugs. This upgrade brought back a reviewed, redesigned set:
  • Splice: OP_CAT and OP_SPLIT. OP_SPLIT is new; it replaces the old OP_SUBSTR, OP_LEFT and OP_RIGHT.
  • Bitwise: OP_AND, OP_OR, OP_XOR.
  • Arithmetic: OP_DIV, OP_MOD.
  • Conversion (new): OP_NUM2BIN and OP_BIN2NUM.

Recommended policy (not consensus) changes:

• OP_RETURN data outputs may carry up to 223 bytes in total.
• Automatic replay protection for the next upgrade, as a way to retire old node versions.

Why

Blocks on BCH were far from full, but the 8 MB limit was a fixed ceiling. Raising it to 32 MB kept plenty of headroom while node software was optimised further.

The opcode work was about Script itself. The spec says the goal was to “restore the functionality” these opcodes provided, not to copy the old versions. Each opcode was re-examined, and some were redesigned.

What it enables

• String handling in scripts. OP_CAT and OP_SPLIT let a script build and take apart byte strings. This is the base for later tricks like checking parts of a signed message.
• Real arithmetic. Division and modulo let contracts compute shares, fees and ratios.
• Number/byte conversion. OP_NUM2BIN and OP_BIN2NUM let scripts move between numbers and fixed-width bytes.
• More room for data. The 223-byte OP_RETURN allowance gave on-chain data protocols more space per transaction.

Many of the ideas Bitcoin (BTC) still debates, like OP_CAT, have run on BCH since this upgrade.