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//! Implementation of Sinsemilla outside the circuit.
use group::{Curve, Wnaf};
use halo2_proofs::arithmetic::{CurveAffine, CurveExt};
use pasta_curves::pallas;
use subtle::CtOption;
mod addition;
use self::addition::IncompletePoint;
mod sinsemilla_s;
pub use sinsemilla_s::SINSEMILLA_S;
/// Number of bits of each message piece in $\mathsf{SinsemillaHashToPoint}$
pub const K: usize = 10;
/// $\frac{1}{2^K}$
pub const INV_TWO_POW_K: [u8; 32] = [
1, 0, 192, 196, 160, 229, 70, 82, 221, 165, 74, 202, 85, 7, 62, 34, 0, 0, 0, 0, 0, 0, 0, 0, 0,
0, 0, 0, 0, 0, 240, 63,
];
/// The largest integer such that $2^c \leq (r_P - 1) / 2$, where $r_P$ is the order
/// of Pallas.
pub const C: usize = 253;
// Sinsemilla Q generators
/// SWU hash-to-curve personalization for Sinsemilla $Q$ generators.
pub const Q_PERSONALIZATION: &str = "z.cash:SinsemillaQ";
// Sinsemilla S generators
/// SWU hash-to-curve personalization for Sinsemilla $S$ generators.
pub const S_PERSONALIZATION: &str = "z.cash:SinsemillaS";
pub(crate) fn lebs2ip_k(bits: &[bool]) -> u32 {
assert!(bits.len() == K);
bits.iter()
.enumerate()
.fold(0u32, |acc, (i, b)| acc + if *b { 1 << i } else { 0 })
}
/// Coordinate extractor for Pallas.
///
/// Defined in [Zcash Protocol Spec § 5.4.9.7: Coordinate Extractor for Pallas][concreteextractorpallas].
///
/// [concreteextractorpallas]: https://zips.z.cash/protocol/nu5.pdf#concreteextractorpallas
fn extract_p_bottom(point: CtOption<pallas::Point>) -> CtOption<pallas::Base> {
point.map(|p| {
p.to_affine()
.coordinates()
.map(|c| *c.x())
.unwrap_or_else(pallas::Base::zero)
})
}
/// Pads the given iterator (which MUST have length $\leq K * C$) with zero-bits to a
/// multiple of $K$ bits.
struct Pad<I: Iterator<Item = bool>> {
/// The iterator we are padding.
inner: I,
/// The measured length of the inner iterator.
///
/// This starts as a lower bound, and will be accurate once `padding_left.is_some()`.
len: usize,
/// The amount of padding that remains to be emitted.
padding_left: Option<usize>,
}
impl<I: Iterator<Item = bool>> Pad<I> {
fn new(inner: I) -> Self {
Pad {
inner,
len: 0,
padding_left: None,
}
}
}
impl<I: Iterator<Item = bool>> Iterator for Pad<I> {
type Item = bool;
fn next(&mut self) -> Option<Self::Item> {
loop {
// If we have identified the required padding, the inner iterator has ended,
// and we will never poll it again.
if let Some(n) = self.padding_left.as_mut() {
if *n == 0 {
// Either we already emitted all necessary padding, or there was no
// padding required.
break None;
} else {
// Emit the next padding bit.
*n -= 1;
break Some(false);
}
} else if let Some(ret) = self.inner.next() {
// We haven't reached the end of the inner iterator yet.
self.len += 1;
assert!(self.len <= K * C);
break Some(ret);
} else {
// Inner iterator just ended, so we now know its length.
let rem = self.len % K;
if rem > 0 {
// The inner iterator requires padding in the range [1,K).
self.padding_left = Some(K - rem);
} else {
// No padding required.
self.padding_left = Some(0);
}
}
}
}
}
/// A domain in which $\mathsf{SinsemillaHashToPoint}$ and $\mathsf{SinsemillaHash}$ can
/// be used.
#[derive(Debug, Clone)]
#[allow(non_snake_case)]
pub struct HashDomain {
Q: pallas::Point,
}
impl HashDomain {
/// Constructs a new `HashDomain` with a specific prefix string.
pub fn new(domain: &str) -> Self {
HashDomain {
Q: pallas::Point::hash_to_curve(Q_PERSONALIZATION)(domain.as_bytes()),
}
}
/// $\mathsf{SinsemillaHashToPoint}$ from [§ 5.4.1.9][concretesinsemillahash].
///
/// [concretesinsemillahash]: https://zips.z.cash/protocol/nu5.pdf#concretesinsemillahash
pub fn hash_to_point(&self, msg: impl Iterator<Item = bool>) -> CtOption<pallas::Point> {
self.hash_to_point_inner(msg).into()
}
#[allow(non_snake_case)]
fn hash_to_point_inner(&self, msg: impl Iterator<Item = bool>) -> IncompletePoint {
let padded: Vec<_> = Pad::new(msg).collect();
padded
.chunks(K)
.fold(IncompletePoint::from(self.Q), |acc, chunk| {
let (S_x, S_y) = SINSEMILLA_S[lebs2ip_k(chunk) as usize];
let S_chunk = pallas::Affine::from_xy(S_x, S_y).unwrap();
(acc + S_chunk) + acc
})
}
/// $\mathsf{SinsemillaHash}$ from [§ 5.4.1.9][concretesinsemillahash].
///
/// [concretesinsemillahash]: https://zips.z.cash/protocol/nu5.pdf#concretesinsemillahash
///
/// # Panics
///
/// This panics if the message length is greater than [`K`] * [`C`]
pub fn hash(&self, msg: impl Iterator<Item = bool>) -> CtOption<pallas::Base> {
extract_p_bottom(self.hash_to_point(msg))
}
/// Constructs a new `HashDomain` from a given `Q`.
///
/// This is only for testing use.
#[cfg(test)]
#[allow(non_snake_case)]
pub(crate) fn from_Q(Q: pallas::Point) -> Self {
HashDomain { Q }
}
/// Returns the Sinsemilla $Q$ constant for this domain.
#[cfg(any(test, feature = "test-dependencies"))]
#[cfg_attr(docsrs, doc(cfg(feature = "test-dependencies")))]
#[allow(non_snake_case)]
pub fn Q(&self) -> pallas::Point {
self.Q
}
}
/// A domain in which $\mathsf{SinsemillaCommit}$ and $\mathsf{SinsemillaShortCommit}$ can
/// be used.
#[derive(Debug)]
#[allow(non_snake_case)]
pub struct CommitDomain {
M: HashDomain,
R: pallas::Point,
}
impl CommitDomain {
/// Constructs a new `CommitDomain` with a specific prefix string.
pub fn new(domain: &str) -> Self {
let m_prefix = format!("{}-M", domain);
let r_prefix = format!("{}-r", domain);
let hasher_r = pallas::Point::hash_to_curve(&r_prefix);
CommitDomain {
M: HashDomain::new(&m_prefix),
R: hasher_r(&[]),
}
}
/// $\mathsf{SinsemillaCommit}$ from [§ 5.4.8.4][concretesinsemillacommit].
///
/// [concretesinsemillacommit]: https://zips.z.cash/protocol/nu5.pdf#concretesinsemillacommit
#[allow(non_snake_case)]
pub fn commit(
&self,
msg: impl Iterator<Item = bool>,
r: &pallas::Scalar,
) -> CtOption<pallas::Point> {
// We use complete addition for the blinding factor.
CtOption::<pallas::Point>::from(self.M.hash_to_point_inner(msg))
.map(|p| p + Wnaf::new().scalar(r).base(self.R))
}
/// $\mathsf{SinsemillaShortCommit}$ from [§ 5.4.8.4][concretesinsemillacommit].
///
/// [concretesinsemillacommit]: https://zips.z.cash/protocol/nu5.pdf#concretesinsemillacommit
pub fn short_commit(
&self,
msg: impl Iterator<Item = bool>,
r: &pallas::Scalar,
) -> CtOption<pallas::Base> {
extract_p_bottom(self.commit(msg, r))
}
/// Returns the Sinsemilla $R$ constant for this domain.
#[cfg(any(test, feature = "test-dependencies"))]
#[cfg_attr(docsrs, doc(cfg(feature = "test-dependencies")))]
#[allow(non_snake_case)]
pub fn R(&self) -> pallas::Point {
self.R
}
/// Returns the Sinsemilla $Q$ constant for this domain.
#[cfg(any(test, feature = "test-dependencies"))]
#[cfg_attr(docsrs, doc(cfg(feature = "test-dependencies")))]
#[allow(non_snake_case)]
pub fn Q(&self) -> pallas::Point {
self.M.Q
}
}
#[cfg(test)]
mod tests {
use super::{Pad, K};
use pasta_curves::{arithmetic::CurveExt, pallas};
#[test]
fn pad() {
assert_eq!(
Pad::new([].iter().cloned()).collect::<Vec<_>>(),
vec![false; 0]
);
assert_eq!(
Pad::new([true].iter().cloned()).collect::<Vec<_>>(),
vec![true, false, false, false, false, false, false, false, false, false]
);
assert_eq!(
Pad::new([true, true].iter().cloned()).collect::<Vec<_>>(),
vec![true, true, false, false, false, false, false, false, false, false]
);
assert_eq!(
Pad::new([true, true, true].iter().cloned()).collect::<Vec<_>>(),
vec![true, true, true, false, false, false, false, false, false, false]
);
assert_eq!(
Pad::new(
[true, true, false, true, false, true, false, true, false, true]
.iter()
.cloned()
)
.collect::<Vec<_>>(),
vec![true, true, false, true, false, true, false, true, false, true]
);
assert_eq!(
Pad::new(
[true, true, false, true, false, true, false, true, false, true, true]
.iter()
.cloned()
)
.collect::<Vec<_>>(),
vec![
true, true, false, true, false, true, false, true, false, true, true, false, false,
false, false, false, false, false, false, false
]
);
}
#[test]
fn sinsemilla_s() {
use super::sinsemilla_s::SINSEMILLA_S;
use group::Curve;
use pasta_curves::arithmetic::CurveAffine;
let hasher = pallas::Point::hash_to_curve(super::S_PERSONALIZATION);
for j in 0..(1u32 << K) {
let computed = {
let point = hasher(&j.to_le_bytes()).to_affine().coordinates().unwrap();
(*point.x(), *point.y())
};
let actual = SINSEMILLA_S[j as usize];
assert_eq!(computed, actual);
}
}
}