What's in a ZKP program?

If you've already read through our FHE compiler documentation, you'll find that ZKPs are, in many ways, much simpler. This is perhaps unsurprising; whereas FHE enables general purpose private computation, ZKPs have a narrower purpose. For example, ZKP programs don't even have return values!

This section describes the anatomy of a ZKP program, what you can and can't do, and the different types involved in ZKP programs.

ZKP program interface requirements

ZKP programs implement their logic in the fn function beneath the #[zkp_program] attribute.

The fn function you write has to satisfy the following conditions:

• Must be stand-alone (i.e. not a struct method, closure, trait method, etc).
• Must take one generic param F: FieldSpec.
• May take any number of arguments, but each type must be supported.
• May not have any return values.

Operations

Some aspects of writing ZKP programs may be surprising to you if you're not familiar with R1CS constraints at a high level. Not to worry, we'll explain how this plays into the operations available to you.

In ZKP programs, you can:

• Perform basic arithmetic operations (+, -, *)—specially only those native to R1CS. Integer division (with remainder) and field inversion are not directly possible since these operations are not native to R1CS, but can be implemented via a gadget.
• Add constraints to enforce that certain mathematical statements hold in the proof.
• Call other functions.
• Use any Rust construct (e.g. match, for i in ..., if...else) on data not derived from any argument. We walk through a number of examples in the limitations chapter.