zei 0.0.10

Zei: Confidential Assets
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//! Pedersen Commitments = v*G + b*H
//! 
//! Tim Ruffing:
//! Switch commitment is (v*G + b*H), where b = b' + hash(v*G + b'*H,
//! b'*J). (So this "tweaks" the commitment, in a pay-to-contract / taproot
//! style).
// After the switch, users can reveal (v*G + b'*H, b'*J), and verifiers
// check if it's computed correctly and use as if it were the ElGamal
// commitment (v*G + b*H, b*J).

#![allow(non_snake_case)]

use mohan::{
    hash::{
        blake256,
        hash_to_ristretto
    },
    dalek::{
        constants::{
            RISTRETTO_BASEPOINT_COMPRESSED,
            RISTRETTO_BASEPOINT_POINT
        },
        ristretto::RistrettoPoint,
        scalar::Scalar,
        traits::MultiscalarMul,
    }  
};
use bacteria::Transcript;


/// Represents a pair of base points for Pedersen commitments.
///
/// The Bulletproofs implementation and API is designed to support
/// pluggable bases for Pedersen commitments, so that the choice of
/// bases is not hard-coded.
///
/// The default generators are:
///
/// * `B`: the `ristretto255` basepoint;
/// * `B_blinding`: the result of `ristretto255` SHA3-512
/// hash-to-group on input `B_bytes`.
#[derive(Copy, Clone)]
pub struct PedersenGens
 {
    /// Base for the committed value
    pub B: RistrettoPoint,
    /// Base for the blinding factor
    pub B_blinding: RistrettoPoint,
    /// Elgmal extension base 
    pub J_blinding: RistrettoPoint
}

impl PedersenGens {
    /// Creates a Pedersen commitment using the value scalar and a blinding factor.
    pub fn commit(&self, value: &Scalar, blinding: &Scalar) -> RistrettoPoint {
        RistrettoPoint::multiscalar_mul(vec![value, blinding].into_iter(), &[self.B, self.B_blinding])
    }

    /// Creats a Switch Commitment Blind
    pub fn blind_switch(&self, value: &Scalar, blinding: &Scalar) -> Scalar {
        //r = r' + hash( v*H + r'*G  ,  r'*J )
        let rj = blinding * self.J_blinding;
        //xG + vH
        let sub_commit = self.commit(value, blinding);
        let mut transcript = Transcript::new(b"blind_switch");
        let c = {
            transcript.commit_point(b"left", &sub_commit.compress());
            transcript.commit_point(b"right", &rj.compress());
            transcript.challenge_scalar(b"c")
        };

        blinding + c
    }

    /// Create a Switch Commitment
    pub fn switch_commit(&self, value: &Scalar, blinding: &Scalar) -> RistrettoPoint {
       self. commit(&value, &self.blind_switch(&value, &blinding))
    }
}

impl Default for PedersenGens {
    fn default() -> Self {
        PedersenGens {
            B: RISTRETTO_BASEPOINT_POINT,
            B_blinding: hash_to_ristretto(RISTRETTO_BASEPOINT_COMPRESSED.as_bytes()),
            J_blinding: hash_to_ristretto(
                            hash_to_ristretto(
                                RISTRETTO_BASEPOINT_COMPRESSED.as_bytes()
                            ).compress().as_bytes()
                        )
        }
    }
}