Rework API based on split-sponge refactor in our fork.

This uses the API changes from:
- https://github.com/arkworks-rs/sponge/issues/29
- arkworks-rs/sponge#30
to avoid working through the Arkworks sponge interface, and do hashing using
the permutation directly.

Cargo.toml

@@ -9,6 +9,7 @@ license = "MIT OR Apache-2.0"
 # See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
 
 [dependencies]
+once_cell = "1.8"
 ark-ff = "0.3"
-ark-sponge = { git = "https://github.com/arkworks-rs/sponge", rev = "51d6fc9aac1fa69f44a04839202b5de828584ed8" }
+ark-sponge = { git = "https://github.com/penumbra-zone/sponge", branch = "split-sponge" }
 ark-ed-on-bls12-377 = "0.3"

src/hash.rs

@@ -0,0 +1,57 @@
+use crate::{Fq, State};
+
+/// Hash a single [`Fq`] element with the provided `domain_separator`.
+pub fn hash_1(domain_separator: &Fq, value: Fq) -> Fq {
+    let mut state = State::from(crate::RATE_1_PARAMS.clone());
+
+    // Use the domain separator as the sponge's capacity element
+    state[0] = domain_separator.clone();
+    state[1] = value;
+
+    state.permute();
+    state[1]
+}
+
+/// Hash two [`Fq`] elements with the provided `domain_separator`.
+pub fn hash_2(domain_separator: &Fq, value: (Fq, Fq)) -> Fq {
+    let mut state = State::from(crate::RATE_2_PARAMS.clone());
+
+    // Use the domain separator as the sponge's capacity element
+    state[0] = domain_separator.clone();
+    state[1] = value.0;
+    state[2] = value.1;
+
+    state.permute();
+    state[1]
+}
+
+/// Hash four [`Fq`] elements with the provided `domain_separator`.
+pub fn hash_4(domain_separator: &Fq, value: (Fq, Fq, Fq, Fq)) -> Fq {
+    let mut state = State::from(crate::RATE_4_PARAMS.clone());
+
+    // Use the domain separator as the sponge's capacity element
+    state[0] = domain_separator.clone();
+    state[1] = value.0;
+    state[2] = value.1;
+    state[3] = value.2;
+    state[4] = value.3;
+
+    state.permute();
+    state[1]
+}
+
+/// Hash five [`Fq`] elements with the provided `domain_separator`.
+pub fn hash_5(domain_separator: &Fq, value: (Fq, Fq, Fq, Fq, Fq)) -> Fq {
+    let mut state = State::from(crate::RATE_5_PARAMS.clone());
+
+    // Use the domain separator as the sponge's capacity element
+    state[0] = domain_separator.clone();
+    state[1] = value.0;
+    state[2] = value.1;
+    state[3] = value.2;
+    state[4] = value.3;
+    state[5] = value.4;
+
+    state.permute();
+    state[1]
+}

src/lib.rs

@@ -1,45 +1,20 @@
-mod sponge;
+//! An instantiation of Poseidon for the BLS12-377 scalar field.
 
-pub mod params;
+use once_cell::sync::Lazy;
 
-// Since we depend on a git version of ark-sponge, re-exporting it here means
-// our deps can access it without having to keep git revisions in sync.
-//
-// Going forward, this re-export should be removed and the functionality our
-// deps need from direct use of ark-sponge should be folded into this crate.
-// However, it's faster to iterate on required functionality without imposing
-// hard compartmentalization boundaries from the start.
-pub use ark_sponge;
+mod hash;
+mod params;
 
-#[cfg(test)]
-mod tests {
-    use super::*;
+pub use hash::{hash_1, hash_2, hash_4, hash_5};
 
-    #[test]
-    fn it_works() {
-        use ark_ed_on_bls12_377::Fq; // lazy import, fix
-        use ark_ff::{One, Zero};
-        use ark_sponge::{
-            poseidon::PoseidonSponge, CryptographicSponge, DuplexSpongeMode,
-            FieldBasedCryptographicSponge,
-        };
+/// Parameters for the rate-1 instance of Poseidon.
+pub const RATE_1_PARAMS: Lazy<Parameters<Fq>> = Lazy::new(params::rate_1);
+/// Parameters for the rate-2 instance of Poseidon.
+pub const RATE_2_PARAMS: Lazy<Parameters<Fq>> = Lazy::new(params::rate_2);
+/// Parameters for the rate-4 instance of Poseidon.
+pub const RATE_4_PARAMS: Lazy<Parameters<Fq>> = Lazy::new(params::rate_4);
+/// Parameters for the rate-5 instance of Poseidon.
+pub const RATE_5_PARAMS: Lazy<Parameters<Fq>> = Lazy::new(params::rate_5);
 
-        // Current API has a `new()` method as part of the `CryptographicSponge`
-        // trait, but this method doesn't allow setting the initial state
-        // manually.  Instead, the fields can be set manually.
-        // Slightly inconvenient that we have to initialize the mode.
-        let mut sponge = PoseidonSponge {
-            parameters: params::rate_2(),
-            state: vec![Fq::zero(); 3],
-            mode: DuplexSpongeMode::Absorbing {
-                next_absorb_index: 0,
-            },
-        };
-
-        sponge.absorb(&Fq::one());
-        sponge.absorb(&Fq::one());
-
-        let output = sponge.squeeze_native_field_elements(1);
-        dbg!(output);
-    }
-}
+pub use ark_ed_on_bls12_377::Fq;
+pub use ark_sponge::poseidon::{Parameters, State};