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use crate::constants::BASEPOINT;
use crate::keys::{PrivateSet, PublicSet};
use crate::transcript::TranscriptProtocol;
use curve25519_dalek::ristretto::RistrettoPoint;
use curve25519_dalek::scalar::Scalar;
use merlin::Transcript;
use rand;
use sha2::Sha512;
#[derive(Debug)]
pub enum Error {
// Occurs when you try to use a method specific to
// a signer as a decoy
NotASigner,
// Occurs when you try to use a method specific to
// a decoy as a signer
NotADecoy,
}
// A member represents a member in the ring
// This includes the signer of the ring
#[derive(Clone)]
pub struct Member {
// The signer is the only member with a set of private keys
private_set: Option<PrivateSet>,
pub(crate) public_set: PublicSet,
// The signing member wil have a nonce per public key in the public/private set.
// In an sigma protocol, this nonce would signify the commit phase.
pub(crate) nonces: Option<Vec<Scalar>>,
// Each member will have a response value per public key in their set
// In an sigma protocol, this would signify the reponse phase.
pub(crate) responses: Vec<Scalar>,
}
impl Member {
// Creates a member who will be the signer of the ring
// Protocol explicitly checks if there is one signer per ring
pub fn new_signer(private_keys: Vec<Scalar>) -> Self {
let private_set = PrivateSet::new(private_keys);
let num_private_keys = private_set.len();
let nonces = generate_rand_scalars(num_private_keys);
let responses = Vec::with_capacity(num_private_keys);
let public_set = private_set.to_public_set();
Member {
nonces: Some(nonces),
public_set: public_set,
private_set: Some(private_set),
responses: responses,
}
}
// Creates a member who will be a decoy in the ring
pub fn new_decoy(public_keys: Vec<RistrettoPoint>) -> Self {
let num_public_keys = public_keys.len();
let responses = generate_rand_scalars(num_public_keys);
Self::new_decoy_with_responses(public_keys, responses)
}
// Creates a member who will be used for verification in a signature
pub(crate) fn new_decoy_with_responses(
public_keys: Vec<RistrettoPoint>,
responses: Vec<Scalar>,
) -> Self {
Member {
nonces: None,
public_set: PublicSet(public_keys),
private_set: None,
responses: responses,
}
}
// Returns true if the member has a set of private keys
pub fn is_signer(&self) -> bool {
self.private_set.is_some()
}
// Returns the number of keys the member has
pub fn num_keys(&self) -> usize {
self.public_set.len()
}
// Computes the key images if the member is a signer
pub fn compute_key_images(&self) -> Result<Vec<RistrettoPoint>, Error> {
match &self.private_set {
Some(priv_set) => Ok(priv_set.compute_key_images(&self.public_set)),
None => Err(Error::NotASigner),
}
}
// This function uses the nonces to calculate the first challenge scalar
// Effectively committing the current member; the ring will therefore
// only be completed if the current member can generate the corresponding
// responses per nonce, which can only be done if the current member possess
// the discrete log to the public keys corresponding to his position in the ring.
// returns a challenge scalar or an error if the user is not a signer
pub fn compute_challenge_commitment(&self, msg: &[u8]) -> Result<Scalar, Error> {
if !self.is_signer() {
return Err(Error::NotASigner);
}
let nonces = match &self.nonces {
Some(x) => Ok(x),
_ => Err(Error::NotASigner),
}?;
assert_eq!(nonces.len(), self.public_set.len());
let mut transcript = Transcript::new(b"mlsag");
transcript.append_message(b"msg", msg);
for (nonce, public_key) in nonces.iter().zip(self.public_set.0.iter()) {
// Add `nonce_i * basepoint` to the transcript
transcript.append_scalar_mult(b"", nonce, &BASEPOINT);
// Append `nonce_i * Hash(PublicKey_i)
transcript.append_scalar_hash_point(b"", nonce, public_key);
}
Ok(transcript.challenge_scalar(b""))
}
// This function is for the signer and will use the signers
// private set to calculate the correct response values
// returns a vector of responses or an error, if the user is not a signer
pub fn compute_signer_responses(&self, challenge: Scalar) -> Result<Vec<Scalar>, Error> {
let private_set = self.private_set.as_ref().ok_or(Error::NotASigner)?;
let nonces = self.nonces.as_ref().ok_or(Error::NotASigner)?;
let nonces_len = nonces.len();
let mut signer_responses: Vec<Scalar> = Vec::with_capacity(nonces_len);
// calculate r_i = nonce_i - c * private_key_i
for i in 0..nonces_len {
let nonce = nonces[i];
let private_key = private_set.0[i];
let response = nonce - challenge * private_key;
signer_responses.push(response);
}
Ok(signer_responses)
}
// This function is ran by all members who did not compute the challenge commitment (decoys)
// Each member that runs this function, will link themselves to the ring using the challenge
// passed to them by the newest member of the ring.
// returns a challenge scalar, to be used by the next member who wants to join the ring
pub fn compute_decoy_challenge(
&self,
challenge: &Scalar,
key_images: &[RistrettoPoint],
msg: &[u8],
) -> Result<Scalar, Error> {
if self.private_set.is_some() {
return Err(Error::NotADecoy);
}
assert_eq!(self.public_set.len(), self.responses.len());
assert_eq!(self.public_set.len(), key_images.len());
let challenge = compute_challenge_ring(
&self.public_set.0,
challenge,
key_images,
&self.responses,
msg,
);
Ok(challenge)
}
}
// A generic function to calculate the challenge for any member in the ring
// While signing, this function will be used by the decoys
// When verifying this function will be used by all members
pub fn compute_challenge_ring(
public_keys: &[RistrettoPoint],
challenge: &Scalar,
key_images: &[RistrettoPoint],
responses: &[Scalar],
msg: &[u8],
) -> Scalar {
let mut transcript = Transcript::new(b"mlsag");
transcript.append_message(b"msg", msg);
for i in 0..public_keys.len() {
let response = &responses[i];
let public_key = &public_keys[i];
let key_image = &key_images[i];
// Append `r_i * basepoint + c * PublicKey_i
transcript.append_double_scalar_mult_add(
b"",
(response, challenge),
(&BASEPOINT, public_key),
);
let hashed_pub_key =
&RistrettoPoint::hash_from_bytes::<Sha512>(public_key.compress().as_bytes());
// Append `r_i * HashToPoint(PublicKey_i) + c * KeyImage_i
transcript.append_double_scalar_mult_add(
b"",
(response, challenge),
(hashed_pub_key, key_image),
);
}
transcript.challenge_scalar(b"")
}
fn generate_rand_scalars(num: usize) -> Vec<Scalar> {
let mut rng = rand::thread_rng();
let mut scalars = Vec::<Scalar>::with_capacity(num);
for _ in 0..num {
scalars.push(Scalar::random(&mut rng));
}
scalars
}
#[cfg(test)]
mod test {
use super::*;
// Simple tests to check that when the members are instantiated
// We have the correct number of values
#[test]
fn test_new() {
let num_private_keys = 10;
let scalars = generate_rand_scalars(num_private_keys);
let signer = Member::new_signer(scalars);
// We should have a nonce per public/private key for the signer
match signer.nonces {
Some(nonces) => {
assert_eq!(nonces.len(), num_private_keys);
}
None => panic!(
"We should not have a `None` value here as we have instantiated a signing member"
),
}
// The number of private keys argument we passed in as an argument
//should equal the length of the private key set
match signer.private_set {
Some(priv_set) => {
assert_eq!(priv_set.len(), num_private_keys);
}
_ => panic!("we should not have a `None` value for the private key set"),
}
// The number of private keys argument we passed in as an argument
//should equal the length of the public key set
assert_eq!(signer.public_set.len(), num_private_keys)
}
}