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Reduce Poseidon hash rounds, following 2019/458. #14

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Reduce Poseidon hash rounds, following 2019/458. #14

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12 changes: 7 additions & 5 deletions src/circuit/poseidon_hash.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -262,6 +262,7 @@ fn poseidon_mimc_round<E: PoseidonEngine<SBox = QuinticSBox<E> >, CS>(
let r_f = params.r_f();
let r_p = params.r_p();
let t = params.t();
let pre_full_rounds = r_f - r_f / 2;

fn add_round_constants<E: PoseidonEngine, CS>(
params: &E::Params,
Expand All @@ -286,7 +287,7 @@ fn poseidon_mimc_round<E: PoseidonEngine<SBox = QuinticSBox<E> >, CS>(

// do releated applications of MDS and then round constants and s-boxes

for full_round in 0..(r_f-1) {
for full_round in 0..(pre_full_rounds - 1) {
let s_box_applied = E::SBox::apply_sbox(
cs.namespace(|| format!("apply s-box for full round {}", full_round)),
&state[..]
Expand Down Expand Up @@ -316,7 +317,7 @@ fn poseidon_mimc_round<E: PoseidonEngine<SBox = QuinticSBox<E> >, CS>(
// transformation and add first round constants before going through partial rounds
{
let s_box_applied = E::SBox::apply_sbox(
cs.namespace(|| format!("apply s-box for full round {}", r_f-1)),
cs.namespace(|| format!("apply s-box for full round {}", pre_full_rounds-1)),
&state[..]
)?;

Expand Down Expand Up @@ -376,13 +377,13 @@ fn poseidon_mimc_round<E: PoseidonEngine<SBox = QuinticSBox<E> >, CS>(
linear_transformation_results.push(linear_applied);
}

add_round_constants::<E, CS>(params, &mut linear_transformation_results[..], r_f, true);
add_round_constants::<E, CS>(params, &mut linear_transformation_results[..], pre_full_rounds, true);
state = linear_transformation_results;

round += 1;
}

for full_round in r_f..(2*r_f - 1) {
for full_round in pre_full_rounds..(r_f - 1) {
let s_box_applied = E::SBox::apply_sbox(
cs.namespace(|| format!("apply s-box for full round {}", full_round)),
&state[..]
Expand All @@ -403,9 +404,10 @@ fn poseidon_mimc_round<E: PoseidonEngine<SBox = QuinticSBox<E> >, CS>(
}

// for a final round we only apply s-box
let full_round = r_f - 1;

let state = E::SBox::apply_sbox(
cs.namespace(|| format!("apply s-box for full round {}", 2*r_f - 1)),
cs.namespace(|| format!("apply s-box for full round {}", full_round)),
&state[..]
)?;

Expand Down
2 changes: 1 addition & 1 deletion src/poseidon/bn256.rs
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Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -20,7 +20,7 @@ impl Bn256PoseidonParams {
pub fn new<H: GroupHasher>() -> Self {
let t = 6u32;
let r_f = 8u32;
let r_p = 84u32;
let r_p = 57u32;
let security_level = 126u32;

Self::new_for_params::<H>(t, r_f, r_p, security_level)
Expand Down
8 changes: 4 additions & 4 deletions src/poseidon/mod.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -171,8 +171,9 @@ pub fn poseidon_mimc<E: PoseidonEngine>(

let r_f = params.r_f();
let r_p = params.r_p();
let pre_full_rounds = r_f - r_f / 2;

for full_round in 0..r_f {
for full_round in 0..pre_full_rounds {
let round_constants = params.full_round_key(full_round);
for (el, c) in state.iter_mut().zip(round_constants.iter()) {
el.add_assign(c);
Expand Down Expand Up @@ -211,8 +212,7 @@ pub fn poseidon_mimc<E: PoseidonEngine>(

// reference implementation says that last round does not have matrix muptiplication step,
// that is true due to ease of inversion
for full_round in r_f..(2*r_f - 1) {
// for full_round in r_f..(2*r_f) {
for full_round in pre_full_rounds..(r_f - 1) {
let round_constants = params.full_round_key(full_round);
for (el, c) in state.iter_mut().zip(round_constants.iter()) {
el.add_assign(c);
Expand All @@ -231,7 +231,7 @@ pub fn poseidon_mimc<E: PoseidonEngine>(
}

{
let full_round = 2*r_f - 1;
let full_round = r_f - 1;
let round_constants = params.full_round_key(full_round);
for (el, c) in state.iter_mut().zip(round_constants.iter()) {
el.add_assign(c);
Expand Down