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stwo-brainfuck

stwo-brainfuck is a ZK-VM for the Brainfuck language¹, based on Stwo².

Objectives

• Capacity of generating and verifying a proof for arbitrary Brainfuck programs.
• Understanding of using Stwo for building ZK-VMs
• Understanding of modern AIR (RAP) design for (C)STARK-based systems.

Design choices

Brainfuck VM

The Brainfuck language has a very loose specification, though, a general specification has been established as a minimal base. We try to follow these guidelines.

• The memory cells take values in the Mersenne31 (M31) field: $[0..2^{31} - 1)$
• Memory is fixed at 30,000 cells by default, but is configurable.
• Memory wraps on overflow/underflow.
  • It can be used for memory value mv and memory pointer mp, but it will usually panic for mp as the memory size will be much smaller than $2^{31} - 1$.
• Inputs are line-buffered (ends with the linefeed ASCII character 10).
• CLI uses Stdin and Stdout for IO.
• For library use, input can be simulated by any reader (e.g. Cursor) and output with any writer (e.g. a custom writer).

Acknowledgements

The constraints used here rely on work made by Alan Szepieniec³ and sibling article from Neptune Cash⁴. The Brainfuck compiler and interpreter have been adapted from rkdud007⁵