circom-witnessgen/README.md

Circom witness generators

This piece of software takes the computation graph files generated by circom-witnesscalc, and either interprets or compiles them to various algebra backends.

Implementation status

Compiler (in Haskell):

• parsing the graph files
• naive interpreter
• constantine backend
• zikkurat backend
• arkworks backend

Nim witness generator:

• parsing the graph files
• generating the witness
• proper error handling

Testing & correctness

I haven't yet done any proper testing, apart from "works for our purposes".

Circuit optimizations

NOTE: you have to run circom with the --O2 options, otherwise the witness format will be most probably incompatible with the the one generated by circom-witnesscalc.

Graph file format

circom-witnesscalc produces binary files encoding a computation graph.

This has the following format:

• magic header: "wtns.graph.001" (14 bytes)
• number of nodes (8 bytes little endian)
• list of nodes, in protobuf serialized format. Each prefixed by a varint length
• GraphMetaData, in protobuf
• 8 bytes offset (yes, at the very end...), pointing to the start of GraphMetaData

Node format:

• varint length prefix
• protobuf tag-byte (lower 3 bits are 0x02, upper bits are node type 1..5)
• varint length
• several record fields
  • protobuf tag-byte (lower 3 bits are 0x00, upper bits are field index 1,2,3,4)
  • value is varint word32, except for ConstantNode when it's a little-endian bytes (wrapped a few times, because protobuf is being protobuf...)