# zkp: a toolkit for Schnorr proofs

This crate has a toolkit for Schnorr-style zero-knowledge proofs,
instantiated using the ristretto255 group.

It provides two levels of API:

* a higher-level, declarative API based around the `define_proof` macro,
  which provides an embedded DSL for specifying proof statements in
  Camenisch-Stadler-like notation:
  ```
  define_proof! {
    vrf_proof,   // Name of the module for generated implementation
    "VRF",       // Label for the proof statement
    (x),         // Secret variables
    (A, G, H),   // Public variables unique to each proof
    (B) :        // Public variables common between proofs
    A = (x * B), // Statements to prove
    G = (x * H) 
    }
  ```
  This expands into a module containing an implementation of proving,
  verification, and batch verification.  Proving uses constant-time
  implementations, and the proofs have a derived implementation of
  (memory-safe) serialization and deserialization via Serde.

* a lower-level, imperative API inspired by [Bellman][bellman], which
  provides a constraint system for Schnorr-style statements.  This
  allows programmable construction of proof statements at runtime.  The
  higher-level `define_proof` macro expands into an invocation of the
  lower-level API.
  The lower-level API is contained in the `toolbox` module.

# Examples

Examples of how to use the API can be found in the library's `tests`
directory.

Currently, the examples include:

* Specification of an "anonymous credential presentation with 10 hidden
  attributes" proof from CMZ'13.  Depending on the backend selection, the
  generated implementation is between 20 to 40 times faster than the benchmark
  numbers reported in that paper.

* A transcript-based signature and VRF construction with an auto-generated
  implementation.  This includes an example of using the online interactive
  composition [described in the Merlin blog post][merlin_blog] to provide chained
  signatures with a counterparty.

* An example of using the lower-level constraint system API.


# Use and features

To enable the `define_proof` macro, import the crate like so:
```
#[macro_use]
extern crate zkp;
```

#### Nightly features

The `nightly` feature enables nightly-specific features.  It is required
to build the documentation.

#### Backend selection

`zkp` provides the following pass-through features to select a
`curve25519-dalek` backend:

* `u32_backend`
* `u64_backend`
* `simd_backend`

#### Transcript debugging

The `debug-transcript` feature is for development and testing, and
prints a log of the data fed into the proof transcript.

#### Autogenerated benchmarks

The `define_proof` macro builds benchmarks for the generated proof
statements, but because these are generated in the client crate (where
the macro expansion happens), they need an extra step to be enabled.

**To enable generated benchmarks in your crate, do the following**:

* Add a `bench` feature to your crate's `Cargo.toml`;
* Add `#[cfg_attr(feature = "bench", feature(test))]` to your crate's
  `lib.rs` or `main.rs`, to enable Rust's nightly-only benchmark
  feature.

# WARNING

**THIS IMPLEMENTATION IS NOT YET READY FOR PRODUCTION USE**

While I expect the 1.0 version to be largely unchanged from the current
code, for now there are no stability guarantees on the proofs, so they
should not yet be deployed.

[bellman]: https://github.com/zkcrypto/bellman
[merlin_blog]: https://medium.com/@hdevalence/merlin-flexible-composable-transcripts-for-zero-knowledge-proofs-28d9fda22d9a