Halo 2

Halo 2 circuits are two-dimensional: they use a grid of "cells" identified by columns and rows, into which values are assigned. Constraints on those cells are grouped into "gates", which apply to every row simultaneously, and can refer to cells at relative rows. To enable both low-level relative cell references in gates, and high-level layout optimisations, circuit developers can define "regions" in which assigned cells will preserve their relative offsets.

Column Types

• Instance columns contain per-proof public values, that the prover gives to the verifier.
• Advice columns are where the prover assigns private (witness) values, that the verifier learns zero knowledge about.
• Fixed columns contain constants used by every proof that are baked into the circuit.
• Selector columns are special cases of fixed columns that can be used to selectively enable gates.

In the example circuit layout pictured, the columns are indicated with different backgrounds. The instance column in white; advice columns in red; fixed columns in light blue; and selector columns in dark blue. Regions are shown in light green, and assigned cells in dark green or black.