mirror of https://github.com/zcash/orchard.git
98 lines
2.8 KiB
Rust
98 lines
2.8 KiB
Rust
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//! The Sinsemilla hash function and commitment scheme.
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use std::iter;
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use group::{Curve, Group};
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use halo2::{
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arithmetic::{CurveAffine, CurveExt},
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pasta::pallas,
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};
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const GROUP_HASH_Q: &str = "z.cash:SinsemillaQ";
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const GROUP_HASH_S: &str = "z.cash:SinsemillaS";
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const K: usize = 10;
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const C: usize = 253;
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fn lebs2ip_32(bits: &[bool]) -> u32 {
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bits.iter()
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.enumerate()
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.fold(0u32, |acc, (i, b)| acc + if *b { 1 << i } else { 0 })
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}
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/// Hash extractor for Pallas, from [§ 5.4.8.7].
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///
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/// [§ 5.4.8.7]: https://zips.z.cash/protocol/orchard.pdf#concreteextractorpallas
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fn extract(point: &pallas::Point) -> pallas::Base {
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// TODO: Should we return the actual bits in a Vec, or allow the caller to use
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// PrimeField::to_le_bits on the returned pallas::Base?
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if let Some((x, _)) = point.to_affine().get_xy().into() {
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x
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} else {
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pallas::Base::zero()
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}
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}
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#[allow(non_snake_case)]
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fn Q(domain_prefix: &str) -> pallas::Point {
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pallas::Point::hash_to_curve(GROUP_HASH_Q)(domain_prefix.as_bytes())
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}
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/// `SinsemillaHashToPoint` from [§ 5.4.1.9].
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///
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/// [§ 5.4.1.9]: https://zips.z.cash/protocol/orchard.pdf#concretesinsemillahash
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#[allow(non_snake_case)]
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pub(crate) fn hash_to_point(
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domain_prefix: &str,
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msg: impl Iterator<Item = bool> + ExactSizeIterator,
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) -> pallas::Point {
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assert!(msg.len() <= K * C);
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let pad = msg.len() % K;
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let padded: Vec<_> = msg.chain(iter::repeat(false).take(pad)).collect();
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let hasher_S = pallas::Point::hash_to_curve(GROUP_HASH_S);
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let S = |chunk: &[bool]| hasher_S(&lebs2ip_32(chunk).to_le_bytes());
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padded
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.chunks(K)
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.fold(Q(domain_prefix), |acc, chunk| acc.double() + S(chunk))
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}
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/// `SinsemillaHash` from [§ 5.4.1.9].
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///
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/// [§ 5.4.1.9]: https://zips.z.cash/protocol/orchard.pdf#concretesinsemillahash
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pub(crate) fn hash(
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domain_prefix: &str,
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msg: impl Iterator<Item = bool> + ExactSizeIterator,
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) -> pallas::Base {
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extract(&hash_to_point(domain_prefix, msg))
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}
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/// `SinsemillaCommit` from [§ 5.4.7.4].
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///
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/// [§ 5.4.7.4]: https://zips.z.cash/protocol/orchard.pdf#concretesinsemillacommit
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#[allow(non_snake_case)]
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pub(crate) fn commit(
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domain_prefix: &str,
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msg: impl Iterator<Item = bool> + ExactSizeIterator,
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r: &pallas::Scalar,
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) -> pallas::Point {
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let m_prefix = domain_prefix.to_owned() + "-M";
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let r_prefix = domain_prefix.to_owned() + "-r";
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let hasher_r = pallas::Point::hash_to_curve(&r_prefix);
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hash_to_point(&m_prefix, msg) + hasher_r(&[]) * r
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}
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/// `SinsemillaShortCommit` from [§ 5.4.7.4].
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///
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/// [§ 5.4.7.4]: https://zips.z.cash/protocol/orchard.pdf#concretesinsemillacommit
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pub(crate) fn short_commit(
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domain_prefix: &str,
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msg: impl Iterator<Item = bool> + ExactSizeIterator,
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r: &pallas::Scalar,
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) -> pallas::Base {
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extract(&commit(domain_prefix, msg, r))
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}
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