threshold_crypto

A pairing-based threshold cryptosystem for collaborative decryption and signatures.

The threshold_crypto crate provides constructors for encrypted message handling. It utilizes the pairing elliptic curve library to create and enable reconstruction of public and private key shares.

In a network environment, messages are signed and encrypted, and key and signature shares are distributed to network participants. A message can be decrypted and authenticated only with cooperation from at least threshold + 1 nodes.

Usage

Cargo.toml:

[dependencies]
rand = "0.4"
threshold_crypto = { version = "0.1", git = "https://github.com/poanetwork/threshold_crypto" }

main.rs:

extern crate rand;
extern crate threshold_crypto;

use threshold_crypto::SecretKey;

/// Very basic secret key usage.
fn main() {
    let sk0: SecretKey = rand::random();
    let sk1: SecretKey = rand::random();

    let pk0 = sk0.public_key();

    let msg0 = b"Real news";
    let msg1 = b"Fake news";

    assert!(pk0.verify(&sk0.sign(msg0), msg0));
    assert!(!pk0.verify(&sk1.sign(msg0), msg0)); // Wrong key.
    assert!(!pk0.verify(&sk0.sign(msg1), msg0)); // Wrong message.
}

Testing

Run tests using the following command:

$ MLOCK_SECRETS=false cargo test

The test suite runs quickly without setting the envvar MLOCK_SECRETS=false, but runs even faster when it is set. Setting this envvar also ensures that tests won't fail if we reach the testing system's locked memory limit (which we won't hit unless the system's locked memory limit is set unreasonably low, i.e. 1-2 pages or the equivalent, 4-8 kilobytes).

Examples

Run examples from the examples directory using:

$ MLOCK_SECRETS=false cargo run --example <example name>

Also see the distributed_key_generation test.

Environment Variables

MLOCK_SECRETS: Sets whether or not the Unix syscall mlock or WinAPI function VirtualLock is called on portions of memory containing secret values. This option is enabled by default (MLOCK_SECRETS=true). Disabling memory locking (MLOCK_SECRETS=false) allows secret values to be copied to disk, where they will not be zeroed on drop and may persist indefinitely. Disabling memory locking should only be done in development and testing.

Disabling memory locking is useful because it removes the possibility of tests failing due to reaching the testing system's locked memory limit. For example, if your crate uses threshold_crypto and you write a test that maintains hundreds or thousands of secrets in memory simultaneously, you run the risk of reaching your system's allowed number of locked pages, which will cause this library to fail.

Application Details

The basic usage outline is:

• choose a threshold value t
• create a key set
• distribute N secret key shares among the participants
• publish the public master key

A third party can now encrypt a message to the public master key and any set of t + 1 participants (but no fewer!) can collaborate to decrypt it. Also, any set of t + 1 participants can collaborate to sign a message, producing a signature that is verifiable with the public master key.

In this system, a signature is unique and independent of the set of participants that produced it. If S1 and S2 are signatures for the same message, produced by two different sets of t + 1 secret key share holders, both signatures will be valid AND equal. This is useful in some applications, for example a message signature can serve as a pseudorandom number unknown to anyone until t + 1 participants agree to reveal it.

In its simplest form, threshold_crypto requires a trusted dealer to produce and distribute the secret key shares. However, keys can be produced so that only the corresponding participant knows their secret in the end. This crate includes the basic tools to implement such a Distributed Key Generation scheme.

A major application for this library is within a distributed network that must tolerate up to t adversarial (malicious or faulty) nodes. Because t + 1 nodes are required to sign or reveal information, messages can be trusted by third-parties as representing the consensus of the network.

Performance

Benchmarking functionality is kept in the benches directory. You can run the benchmarks with the following command:

$ RUSTFLAGS="-C target_cpu=native" cargo bench

We use the criterion benchmarking library.

License

Licensed under either of:

• Apache License, Version 2.0, (LICENSE-APACHE or http://www.apache.org/licenses/LICENSE-2.0)
• MIT license (LICENSE-MIT or http://opensource.org/licenses/MIT)

at your option.

Contributing

See the CONTRIBUTING document for contribution, testing and pull request protocol.

Unless you explicitly state otherwise, any contribution intentionally submitted for inclusion in the work by you, as defined in the Apache-2.0 license, shall be dual licensed as above, without any additional terms or conditions.