nym/common/credentials/src/utils.rs

// Copyright 2021 - Nym Technologies SA <contact@nymtech.net>
// SPDX-License-Identifier: Apache-2.0

use crate::error::Error;
use coconut_interface::{
    aggregate_signature_shares, aggregate_verification_keys, hash_to_scalar, prepare_blind_sign,
    prove_credential, Attribute, BlindSignRequestBody, Credential, Parameters, Signature,
    SignatureShare, VerificationKey,
};
use url::Url;

/// Contacts all provided validators and then aggregate their verification keys.
///
/// # Arguments
///
/// * `validators`: list of validators to obtain verification keys from.
///
/// Note: list of validators must be correctly ordered by the polynomial coordinates used
/// during key generation and it is responsibility of the caller to ensure that correct
/// number of them is provided
///
/// # Examples
///
/// ```no_run
/// use url::{Url, ParseError};
/// use credentials::obtain_aggregate_verification_key;
///
/// async fn example() -> Result<(), ParseError> {
///     let validators = vec!["https://testnet-milhon-validator1.nymtech.net/api".parse()?, "https://testnet-milhon-validator2.nymtech.net/api".parse()?];
///     let aggregated_key = obtain_aggregate_verification_key(&validators).await;
///     // deal with the obtained Result
///     Ok(())
/// }
/// ```
pub async fn obtain_aggregate_verification_key(
    validators: &[Url],
) -> Result<VerificationKey, Error> {
    if validators.is_empty() {
        return Err(Error::NoValidatorsAvailable);
    }

    let mut indices = Vec::with_capacity(validators.len());
    let mut shares = Vec::with_capacity(validators.len());

    let mut client = validator_client::ApiClient::new(validators[0].clone());
    let response = client.get_coconut_verification_key().await?;

    indices.push(0);
    shares.push(response.key);

    for (id, validator_url) in validators.iter().enumerate().skip(1) {
        client.change_validator_api(validator_url.clone());
        let response = client.get_coconut_verification_key().await?;
        indices.push(id as u64);
        shares.push(response.key);
    }

    Ok(aggregate_verification_keys(&shares, Some(&indices))?)
}

async fn obtain_partial_credential(
    params: &Parameters,
    public_attributes: &[Attribute],
    private_attributes: &[Attribute],
    client: &validator_client::ApiClient,
) -> Result<Signature, Error> {
    let elgamal_keypair = coconut_interface::elgamal_keygen(params);
    let blind_sign_request = prepare_blind_sign(
        params,
        elgamal_keypair.public_key(),
        private_attributes,
        public_attributes,
    )?;

    let blind_sign_request_body = BlindSignRequestBody::new(
        blind_sign_request,
        elgamal_keypair.public_key(),
        public_attributes,
        (public_attributes.len() + private_attributes.len()) as u32,
    );

    let blinded_signature = client
        .blind_sign(&blind_sign_request_body)
        .await?
        .blinded_signature;
    Ok(blinded_signature.unblind(elgamal_keypair.private_key()))
}

pub async fn obtain_aggregate_signature(
    params: &Parameters,
    public_attributes: &[Attribute],
    private_attributes: &[Attribute],
    validators: &[Url],
) -> Result<Signature, Error> {
    if validators.is_empty() {
        return Err(Error::NoValidatorsAvailable);
    }

    let mut shares = Vec::with_capacity(validators.len());

    let mut client = validator_client::ApiClient::new(validators[0].clone());
    let first =
        obtain_partial_credential(params, public_attributes, private_attributes, &client).await?;
    shares.push(SignatureShare::new(first, 0));

    for (id, validator_url) in validators.iter().enumerate().skip(1) {
        client.change_validator_api(validator_url.clone());
        let signature =
            obtain_partial_credential(params, public_attributes, private_attributes, &client)
                .await?;
        let share = SignatureShare::new(signature, id as u64);
        shares.push(share)
    }

    Ok(aggregate_signature_shares(&shares)?)
}

// TODO: better type flow
pub fn prepare_credential_for_spending(
    params: &Parameters,
    public_attributes: Vec<Vec<u8>>,
    private_attributes: Vec<Vec<u8>>,
    signature: &Signature,
    verification_key: &VerificationKey,
) -> Result<Credential, Error> {
    let private_attributes = private_attributes
        .iter()
        .map(hash_to_scalar)
        .collect::<Vec<Attribute>>();
    let theta = prove_credential(params, verification_key, signature, &private_attributes)?;

    Ok(Credential::new(
        (public_attributes.len() + private_attributes.len()) as u32,
        theta,
        public_attributes,
        signature,
    ))
}