cmz/src/ggm.rs

// Implementation of CMZ14 credentials (GGM version, which is more
// efficient, but makes a stronger security assumption): "Algebraic MACs
// and Keyed-Verification Anonymous Credentials" (Chase, Meiklejohn,
// and Zaverucha, CCS 2014)

// The notation follows that of the paper "Hyphae: Social Secret
// Sharing" (Lovecruft and de Valence, 2017), Section 4.

#![allow(non_snake_case)]

use sha2::Sha512;

use curve25519_dalek::constants as dalek_constants;
use curve25519_dalek::ristretto::RistrettoPoint;
use curve25519_dalek::ristretto::RistrettoBasepointTable;
use curve25519_dalek::scalar::Scalar;

use lazy_static::lazy_static;

lazy_static! {
    pub static ref CMZ_A: RistrettoPoint =
        RistrettoPoint::hash_from_bytes::<Sha512>(b"CMZ Generator A");
    pub static ref CMZ_B: RistrettoPoint =
        dalek_constants::RISTRETTO_BASEPOINT_POINT;
    pub static ref CMZ_A_TABLE: RistrettoBasepointTable =
        RistrettoBasepointTable::create(&CMZ_A);
    pub static ref CMZ_B_TABLE: RistrettoBasepointTable =
        dalek_constants::RISTRETTO_BASEPOINT_TABLE;
}

#[derive(Clone,Debug)]
pub struct IssuerPrivKey {
    x0tilde: Scalar,
    x: Vec<Scalar>,
}

impl IssuerPrivKey {
    // Create an IssuerPrivKey for credentials with the given number of
    // attributes.
    pub fn new(n: u16) -> IssuerPrivKey {
        let mut rng: rand::rngs::ThreadRng = rand::thread_rng();
        let x0tilde: Scalar = Scalar::random(&mut rng);
        let mut x: Vec<Scalar> = Vec::with_capacity((n+1) as usize);

        // Set x to a vector of n+1 random Scalars
        x.resize_with((n+1) as usize, || { Scalar::random(&mut rng) });

        IssuerPrivKey { x0tilde, x }
    }
}

#[derive(Clone,Debug)]
pub struct IssuerPubKey {
    X: Vec<RistrettoPoint>,
}

impl IssuerPubKey {
    // Create an IssuerPubKey from the corresponding IssuerPrivKey
    pub fn new(privkey: &IssuerPrivKey) -> IssuerPubKey {
        let Atable : &RistrettoBasepointTable = &CMZ_A_TABLE;
        let Btable : &RistrettoBasepointTable = &CMZ_B_TABLE;
        let n_plus_one: usize = privkey.x.len();
        let mut X: Vec<RistrettoPoint> = Vec::with_capacity(n_plus_one);

        // The first element is a special case; it is
        // X[0] = x0tilde*A + x[0]*B
        X.push(&privkey.x0tilde * Atable + &privkey.x[0] * Btable);

        // The other elements (1 through n) are X[i] = x[i]*A
        for i in 1..n_plus_one {
            X.push(&privkey.x[i] * Atable);
        }
        IssuerPubKey { X }
    }
}

#[derive(Debug)]
pub struct Issuer {
    privkey: IssuerPrivKey,
    pubkey: IssuerPubKey,
}

impl Issuer {
    // Create an issuer for credentials with the given number of
    // attributes
    pub fn new(n: u16) -> Issuer {
        let privkey = IssuerPrivKey::new(n);
        let pubkey = IssuerPubKey::new(&privkey);
        Issuer { privkey, pubkey }
    }
}