Zero Knowledge Authentication

This repository provides a Rust library implementing the Chaum-Pedersen protocol, enabling the creation of zero-knowledge proofs for cryptographic verification while ensuring privacy. The project implements two flavors of the Chaum-Pedersen cryptographic proofs; one using discrete logarithms and the other using elliptive curves. These mechanisms allow a prover to demonstrate knowledge of a secret corresponding to a public value without revealing the secret itself. The project also includes a gRPC server and client implementation for integration into applications that wish to utilize the protocol to register and authenticate users.

Docs

Features

Chaum-Pedersen protocol-based gRPC authentication service.
Discrete logarithm flavor using BigInt via num-bigint.
Elliptic curve flavor using Ristretto points with curve25519-dalek.
gRPC server and client defined in protobufs.
Server and client CLIs.

Project Layout

zkauth: Core library implementing the discrete_logarithm and elliptic_curve flavors of the protocol.
zkauth-protobuf: Generated protobuf types and stubs for the gRPC service.
zkauth-server: Implementation of the gRPC service, acting as the verifier in the Chaum-Pedersen protocol. Includes a CLI entrypoint used for execution of the server.
zkauth-client: Implementation of the gRPC service client, acting as the prover in the Chaum-Pedersen protocol. Includes a CLI entrypoint that used for interacting with the server as a client.
tests: A suite of functional tests that encode the expectations of the client/prover and server/verifier in an end-to-end way.

User Workflows

The authentication workflows are as follows:

Registration

Login

Chaum-Pedersen Proofs

The library supports two flavors:

Discrete Logarithm

The classic Chaum-Pedersen protocol is a cryptographic technique mainly used for proving that two discrete logarithms are equal and that they correspond to the same base without revealing the actual values. This protocol is commonly utilized in privacy-preserving cryptographic systems such as electronic voting schemes and zero-knowledge proof constructions.

Here are the steps of the Chaum-Pedersen protocol:

Setup: The prover and verifier agree on a prime $p$ and a generator $g$ of a cyclic group $G$ of order $q$, where $q$ is a large prime factor of $p-1$. The prover knows a secret $x$, which is the discrete logarithm of both $y_1 = g^x \mod p$ and $y_2 = h^x \mod p$ to the bases $g$ and $h$, respectively. Note that $h$ is another element of $G$, and the equality of logarithms $\log_g(y_1) = \log_h(y_2) = x$ is what the prover intends to prove without revealing $x$.
Commitment: The prover selects a random value $k$ from the group $G$ and computes two commitments $r_1 = g^k \mod p$ and $r_2 = h^k \mod p$. The prover then sends the commitments $r_1$ and $r_2$ to the verifier.
Challenge: The verifier sends a random challenge $c$ to the prover. This challenge is typically a random number selected from a range that ensures security, such as the order of the group $q$.
Response: Upon receiving the challenge $c$, the prover computes the response $s = k - c \cdot x \mod q$ and sends $s$ to the verifier.
Verification: The verifier checks the validity of the prover's response by ensuring that both $r_1 = g^s \cdot y_1^c \mod p$ and $r_2 = h^s \cdot y_2^c \mod p$ hold true. If both equations are satisfied, the verifier accepts the proof; otherwise, the proof is rejected.

The protocol ensures that the prover knows the discrete logarithm $x$ without revealing it. The security of the protocol relies on the difficulty of computing discrete logarithms in the group $G$.

Elliptic Curve

Adapting the Chaum-Pedersen protocol to elliptic curves involves leveraging the elliptic curve discrete logarithm problem (ECDLP) instead of the classical discrete logarithm problem in a cyclic group. The fundamental principles remain similar, but the operations are adapted to the properties and operations of elliptic curves.

Here's how the steps adapt:

Setup: Instead of agreeing on a prime $p$ and a generator $g$ of a cyclic group, the prover and verifier agree on an elliptic curve $E$ defined over a finite field and a base point $G$ on $E$ of prime order $q$. The prover knows a secret scalar $x$, which corresponds to the discrete logarithm (with respect to base point $G$) of two points $Y_1 = xG$ and $Y_2 = xH$ on the elliptic curve, where $H$ is another point on the curve. The prover intends to demonstrate that $\log_G(Y_1) = \log_H(Y_2) = x$ without revealing $x$.
Commitment: The prover picks a random scalar $k$ from the set $1, ..., q-1$ and computes two commitment points $R_1 = kG$ and $R_2 = kH$ on the elliptic curve. These commitments $R_1$ and $R_2$ are then sent to the verifier.
Challenge: The verifier generates a random challenge scalar $c$ and sends it to the prover. This challenge is again a random scalar from the set $1, ..., q-1$.
Response: Upon receiving $c$, the prover calculates the response scalar $s = k + cx \mod q$ and sends $s$ back to the verifier.
Verification: The verifier receives $s$ and validates the prover’s claims by checking if $sG = R_1 + cY_1$ and $sH = R_2 + cY_2$ on the elliptic curve, or equivalently if $R_1 = sG - cY_1$ and $R_2 = sH - cY_2$. If both equations hold, the prover's claim is accepted; otherwise, it is rejected.

Adapting the protocol to elliptic curves maintains the privacy and security characteristics of the original Chaum-Pedersen protocol while leveraging the added security benefits and efficiency of elliptic curve cryptography, which typically allows for shorter key sizes compared to traditional discrete logarithm-based systems for a comparable level of security. The main changes involve moving from multiplicative group operations to additive elliptic curve group operations and from working with integers modulo a prime to working with points on an elliptic curve.

Getting Started

Local command-line

This project is written in Rust, which should be available on your system.

Clone the repository:

git clone https://github.com/snormore/zkauth.git
cd zkauth

Run the tests:

cargo test

Run lint checks:

cargo clippy --all-features --no-deps

dev/lint

Build the libraries and binaries:

cargo build

Usage

Server

$ cd zkauth-server
$ cargo run -- --help

Usage: zkauth-server [OPTIONS]

Options:
  -v, --verbose...
          Increase logging verbosity
  -q, --quiet...
          Decrease logging verbosity
      --host <HOST>
          Specifies the IP address or name of the host to which the server is bound [default: 127.0.0.1]
  -p, --port <PORT>
          Specifies the TCP/IP port number on which the server listens for incoming client requests [env: PORT=] [default: 0]
      --config-path <CONFIG_PATH>
          Specifies the configuration file path. If not specified, a non-persistent configuration will be generated and used [env: CONFIG_PATH=]
      --config-generate
          Specifies whether to generate a new configuration file at the specified path. If true, this will exit after generating the configuration file, and not run the server. If the file already exists, it will not be overwritten unless the --config-overwrite is specified
      --config-overwrite
          Specifies whether to overwrite an existing configuration file when generating a new one
      --config-flavor <CONFIG_FLAVOR>
          Specifies the configuration flavor to use [default: discrete-logarithm] [possible values: discrete-logarithm, elliptic-curve]
      --config-prime-bits <CONFIG_PRIME_BITS>
          Specifies the number of bits to use for generating prime numbers for the public parameters [default: 256]
      --config-prime <CONFIG_PRIME>
          Specifies a prime number to use for generating the configuration
  -h, --help
          Print help
  -V, --version
          Print version

The server can be run with the following command:

zkauth-server --port 50001

The server can generate a configuration file using the following command:

zkauth-server --config-generate --config-path=config.json

You can specify the configuration flavor using the --config-flavor option, and the number of bits for the prime number using the --config-prime-bits option, or specify a prime number directly using the --config-prime option.

zkauth-server --config-generate --config-path=config.json --config-flavor=elliptic-curve

zkauth-server --config-generate --config-path=config.json --config-prime-bits=256

zkauth-server --config-generate --config-path=config.json --config-prime=42765216643065397982265462252423826320512529931694366715111734768493812630447

Client

$ cd zkauth-client
$ cargo run -- --help

Usage: zkauth-client [OPTIONS] --address <ADDRESS> --user <USER> --password <PASSWORD>

Options:
  -v, --verbose...           Increase logging verbosity
  -q, --quiet...             Decrease logging verbosity
  -a, --address <ADDRESS>    Specifies the address of the gRPC server to connect to. Example: http://127.0.0.1:50001 [env: ZKAUTH_ADDRESS=]
  -u, --user <USER>          Specifies the username to authenticate with [env: ZKAUTH_USER=]
  -p, --password <PASSWORD>  Specifies the password to authenticate with [env: ZKAUTH_PASSWORD=]
      --register             Specifies whether to execute the registration step
      --login                Specifies whether to execute the login step
  -h, --help                 Print help
  -V, --version              Print version

Execute the register and login workflows against the server:

cargo run -- --address http://localhost:50001 --user user --password password --register --login

Local docker-compose

Build and spin up the docker containers for the server and client:

docker-compose up -d

List the containers:

docker-compose ps -a

View the logs:

docker-compose logs -f

Run the client with arguments:

docker-compose run --rm client --user user --password password --register --login

If you need to rebuild the containers after changing the code, you can run

docker-compose build
docker-compose up -d

docker-compose up -d --build

AWS/EKS

Spin up an EKS cluster:

dev/eks-up

Deploy the server and client:

dev/k8s-deploy

Exec into the client container and use the zkauth-client CLI:

dev/k8s-exec-client

zkauth-client --user user --password password --register --login

Use the local CLI against the server on EKS via LB:

cd zkauth-client
cargo run -- --address http://$(kubectl get svc zkauth-server -o jsonpath='{.status.loadBalancer.ingress[0].hostname}'):5000 --user user --password password --register --login

Tear down the EKS cluster if no longer needed:

dev/eks-down

License

This project is licensed under the MIT license.

Acknowledgements

Wallet Databases with Observers by David Chaum and Torben Pryds Pedersen
Cryptography: An Introduction by Nigel Smart
Chaum-Pedersen Protocol on Wikipedia
Chaum-Pedersen Protocol question on StackExchange
Chaum-Pedersen protocol adapted to elliptic curves question on StackExchange
Chaum-Pedersen Protocol Explained for discrete logarithms and elliptic curves via ChatGPT
Similar projects:

snormore / zkauth