blsbs

BLS Blind Signatures

Warning

This crate is very experimental at present, probably has serious bugs, and should not be used for anything serious. It is unpublished for a reason.

About

This crate implements a simple API for working with Chaumian Blind Signatures using BLS cryptography.

The API supports both single key signatures and multisig. This is based on SecretKeyShare and SignatureShare from blsttc, originally implemented in threshold_crypto.

This API embraces the metaphor of a Slip of paper that is enclosed in a carbon-lined Envelope. The carbon lining is important because a Signature written outside the Envelope will transfer to the Slip inside as well. The SlipPreparer puts a message on the Slip and places it in the Envelope, then sends it to another party, the BlindSigner.

The BlindSigner signs the outside of the Envelope without seeing the Slip inside, then returns it to the SlipPreparer. The SlipPreparer opens the Envelope, removes the Slip, and can then verify with the BlindSigner's public key that both the Envelope and the Slip have the BlindSigner's Signature. Any party with a copy of the Slip and BlindSigner's public key can perform this verification.

See Chaum's original paper for a fuller discussion.

The multisig API extends the above metaphor with the idea that at least m BlindSigner parties must sign the Envelope for the combined Signature to be considered valid.

Example Usage

A basic example with a single blind signer:

use blsbs::*;

fn main() {
    let official = BlindSigner::new();
    let voter = SlipPreparer::new();

    let slip: Slip = b"I vote for mickey mouse".to_vec();
    let envelope = voter.place_slip_in_envelope(&slip);

    let signed_envelope = official.sign_envelope(envelope)?;

    let slip_sig = signed_envelope.signature_for_slip(voter.blinding_factor())?;
    let result = voter.verify_slip_signature(&slip, &slip_sig, &official.public_key());

    assert!(result.is_ok());
}

see tests in src/shared.rs for examples of m-of-n usage.