powdr-brainfuck

This repository contains different Brainfuck implementations as powdr VMs.

1. Brainfuck ISA and Compiler
2. Brainfuck interpreter in powdr-IR
3. Brainfuck interpreter in powdr-Rust

1. Brainfuck ISA and Compiler

brainfuck_template.asm contains a compact Brainfuck ISA definition with just enough registers and instructions. Instead of interpreting an input program, we compile Brainfuck programs to powdr-IR programs using this ISA, effectively compiling input programs to custom circuits.

The compiler is in bf_to_powdr.py.

We first compile the program to powdr-IR, which only has to be done once:

./bf_to_powdr.py hello_world.bf

We can now run powdr pil and get a proof:

powdr pil hello_world.asm  --prove-with plonky3-composite

2. Brainfuck interpreter in powdr-IR

brainfuck_vm.asm is a hand-written assembly interpreter that takes the Brainfuck program (with a zero at the end) and the program inputs as a single list of numbers encoded as <program length> <program> <input length> <input> and generates a ZK proof of execution, effectively being a zkVM. This is different from approach (2) which implements an ISA that Brainfuck programs can be transpiled to.

powdr pil brainfuck_vm.asm -o output -f -i "3,62,44,0,17"

This command compiles the interpreter and generates a witness for the program >, with input [17]. Note that the program is given as a list of the ASCII codes of the program characters.

You can add --prove-with plonky3 to the command above to generate a STARK proof of the execution, or --prove-with halo2 --field bn254 to generate a SNARK.

The script program_formatter.py helps to format Brainfuck programs as powdr inputs:

powdr pil brainfuck_vm.asm -i $(./program_formatter.py hello_world.bf) -o output -f

Hello World! should be printed during witness generation.

TODO:

• Verify public program commitment

3. Brainfuck interpreter in powdr-Rust

The directory brainfuck-rs has 3 crates:

• interpreter: A Brainfuck interpreter that runs a given program on given inputs and returns the outputs that would be printed on stdout. The interpreter is used by both host and powdr-guest.
• host: The main binary, able to run the interpreter natively as well as make ZK proofs using powdr.
• powdr-guest: The code to be proven, interfaces with powdr's input API and uses the same interpreter code as host.

To run just the native interpreter, run:

cargo run -r -- --program ../hello_world.bf -e

To run powdr's compilation and RISCV executor only, run:

RUST_LOG=info cargo run -r -- --program ../hello_world.bf -f

The info option also prints the trace length.

To run powdr's compilation and full witness generation, run:

RUST_LOG=info cargo run -r -- --program ../hello_world.bf -w

To run powdr's compilation, full witness and proof generation, run:

cargo run -r -- --program ../hello_world.bf --proof