Merge pull request #21 from zcash/sinsemilla

Implement Sinsemilla primitives

Cargo.toml

@@ -19,6 +19,7 @@ publish = false
 rustdoc-args = [ "--html-in-header", "katex-header.html" ]
 
 [dependencies]
+group = "0.9"
 halo2 = { git = "https://github.com/zcash/halo2.git", branch = "main" }
 nonempty = "0.6"
 

src/primitives.rs

@@ -5,3 +5,4 @@
 //     - EphemeralSecretKey
 
 pub mod redpallas;
+pub(crate) mod sinsemilla;

src/primitives/sinsemilla.rs

@@ -0,0 +1,177 @@
+//! The Sinsemilla hash function and commitment scheme.
+
+use group::{Curve, Group};
+use halo2::{
+    arithmetic::{CurveAffine, CurveExt},
+    pasta::pallas,
+};
+
+const GROUP_HASH_Q: &str = "z.cash:SinsemillaQ";
+const GROUP_HASH_S: &str = "z.cash:SinsemillaS";
+
+const K: usize = 10;
+const C: usize = 253;
+
+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 })
+}
+
+struct Pad<I: Iterator<Item = bool>> {
+    inner: I,
+    len: usize,
+    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 let Some(n) = self.padding_left.as_mut() {
+                if *n == 0 {
+                    break None;
+                } else {
+                    *n -= 1;
+                    break Some(false);
+                }
+            } else if let Some(ret) = self.inner.next() {
+                self.len += 1;
+                assert!(self.len <= K * C);
+                break Some(ret);
+            } else {
+                // Inner iterator just ended.
+                let rem = self.len % K;
+                if rem > 0 {
+                    self.padding_left = Some(K - rem);
+                } else {
+                    // No padding required.
+                    self.padding_left = Some(0);
+                }
+            }
+        }
+    }
+}
+
+/// Hash extractor for Pallas, from [§ 5.4.8.7].
+///
+/// [§ 5.4.8.7]: https://zips.z.cash/protocol/nu5.pdf#concreteextractorpallas
+fn extract(point: &pallas::Point) -> pallas::Base {
+    // TODO: Should we return the actual bits in a Vec, or allow the caller to use
+    // PrimeField::to_le_bits on the returned pallas::Base?
+    if let Some((x, _)) = point.to_affine().get_xy().into() {
+        x
+    } else {
+        pallas::Base::zero()
+    }
+}
+
+#[allow(non_snake_case)]
+fn Q(domain_prefix: &str) -> pallas::Point {
+    pallas::Point::hash_to_curve(GROUP_HASH_Q)(domain_prefix.as_bytes())
+}
+
+/// `SinsemillaHashToPoint` from [§ 5.4.1.9].
+///
+/// [§ 5.4.1.9]: https://zips.z.cash/protocol/nu5.pdf#concretesinsemillahash
+#[allow(non_snake_case)]
+pub(crate) fn hash_to_point(domain_prefix: &str, msg: impl Iterator<Item = bool>) -> pallas::Point {
+    let padded: Vec<_> = Pad::new(msg).collect();
+
+    let hasher_S = pallas::Point::hash_to_curve(GROUP_HASH_S);
+    let S = |chunk: &[bool]| hasher_S(&lebs2ip_k(chunk).to_le_bytes());
+
+    padded
+        .chunks(K)
+        .fold(Q(domain_prefix), |acc, chunk| acc.double() + S(chunk))
+}
+
+/// `SinsemillaHash` from [§ 5.4.1.9].
+///
+/// [§ 5.4.1.9]: https://zips.z.cash/protocol/nu5.pdf#concretesinsemillahash
+pub(crate) fn hash(domain_prefix: &str, msg: impl Iterator<Item = bool>) -> pallas::Base {
+    extract(&hash_to_point(domain_prefix, msg))
+}
+
+/// `SinsemillaCommit` from [§ 5.4.7.4].
+///
+/// [§ 5.4.7.4]: https://zips.z.cash/protocol/nu5.pdf#concretesinsemillacommit
+#[allow(non_snake_case)]
+pub(crate) fn commit(
+    domain_prefix: &str,
+    msg: impl Iterator<Item = bool>,
+    r: &pallas::Scalar,
+) -> pallas::Point {
+    let m_prefix = domain_prefix.to_owned() + "-M";
+    let r_prefix = domain_prefix.to_owned() + "-r";
+
+    let hasher_r = pallas::Point::hash_to_curve(&r_prefix);
+
+    hash_to_point(&m_prefix, msg) + hasher_r(&[]) * r
+}
+
+/// `SinsemillaShortCommit` from [§ 5.4.7.4].
+///
+/// [§ 5.4.7.4]: https://zips.z.cash/protocol/nu5.pdf#concretesinsemillacommit
+pub(crate) fn short_commit(
+    domain_prefix: &str,
+    msg: impl Iterator<Item = bool>,
+    r: &pallas::Scalar,
+) -> pallas::Base {
+    extract(&commit(domain_prefix, msg, r))
+}
+
+#[cfg(test)]
+mod tests {
+    use super::Pad;
+
+    #[test]
+    fn pad() {
+        assert_eq!(Pad::new([].iter().cloned()).collect::<Vec<_>>(), vec![]);
+        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
+            ]
+        );
+    }
+}