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feat: Added efficient field comparisons for bn254 #4042

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Jan 16, 2024
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3 changes: 1 addition & 2 deletions noir_stdlib/src/eddsa.nr
Original file line number Diff line number Diff line change
@@ -1,7 +1,6 @@
use crate::hash::poseidon;
use crate::ec::consts::te::baby_jubjub;
use crate::ec::tecurve::affine::Point as TEPoint;
use crate::field::bn254::lt as field_lt;

// Returns true if signature is valid
pub fn eddsa_poseidon_verify(
Expand All @@ -22,7 +21,7 @@ pub fn eddsa_poseidon_verify(
let signature_r8 = TEPoint::new(signature_r8_x, signature_r8_y);
assert(bjj.curve.contains(signature_r8));
// Ensure S < Subgroup Order
assert(field_lt(signature_s, bjj.suborder));
assert(signature_s.lt(bjj.suborder));
// Calculate the h = H(R, A, msg)
let hash: Field = poseidon::bn254::hash_5([signature_r8_x, signature_r8_y, pub_key_x, pub_key_y, message]);
// Calculate second part of the right side: right2 = h*8*A
Expand Down
31 changes: 31 additions & 0 deletions noir_stdlib/src/field.nr
Original file line number Diff line number Diff line change
@@ -1,4 +1,5 @@
mod bn254;
use bn254::lt as bn254_lt;

impl Field {
pub fn to_le_bits(self: Self, bit_size: u32) -> [u1] {
Expand Down Expand Up @@ -76,6 +77,15 @@ impl Field {
pub fn sgn0(self) -> u1 {
self as u1
}

pub fn lt(self, another: Field) -> bool {
if crate::compat::is_bn254() {
bn254_lt(self, another)
} else {
lt_fallback(self, another)
}
}
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}

#[builtin(modulus_num_bits)]
Expand Down Expand Up @@ -107,3 +117,24 @@ pub fn bytes32_to_field(bytes32: [u8; 32]) -> Field {
// Abuse that a % p + b % p = (a + b) % p and that low < p
low + high * v
}

fn lt_fallback(x: Field, y: Field) -> bool {
let num_bytes = (modulus_num_bits() as u32 + 7) / 8;
let x_bytes = x.to_le_bytes(num_bytes);
let y_bytes = y.to_le_bytes(num_bytes);
let mut x_is_lt = false;
let mut done = false;
for i in 0..num_bytes {
if (!done) {
let x_byte = x_bytes[num_bytes - 1 - i] as u8;
let y_byte = y_bytes[num_bytes - 1 - i] as u8;
let bytes_match = x_byte == y_byte;
if !bytes_match {
x_is_lt = x_byte < y_byte;
done = true;
}
}
}
x_is_lt
}

5 changes: 0 additions & 5 deletions noir_stdlib/src/field/bn254.nr
Original file line number Diff line number Diff line change
Expand Up @@ -18,7 +18,6 @@ unconstrained fn decompose_unsafe(x: Field) -> (Field, Field) {
(low, high)
}

#[field(bn254)]
pub fn decompose(x: Field) -> (Field, Field) {
let (xlo, xhi) = decompose_unsafe(x);
let borrow = lt_unsafe(PLO, xlo, 16);
Expand Down Expand Up @@ -59,7 +58,6 @@ unconstrained fn lte_unsafe(x: Field, y: Field, num_bytes: u32) -> bool {
lt_unsafe(x, y, num_bytes) | (x == y)
}

#[field(bn254)]
pub fn assert_gt(a: Field, b: Field) {
let (alo, ahi) = decompose(a);
let (blo, bhi) = decompose(b);
Expand All @@ -73,12 +71,10 @@ pub fn assert_gt(a: Field, b: Field) {
rhi.assert_max_bit_size(128);
}

#[field(bn254)]
pub fn assert_lt(a: Field, b: Field) {
assert_gt(b, a);
}

#[field(bn254)]
pub fn gt(a: Field, b: Field) -> bool {
if a == b {
false
Expand All @@ -91,7 +87,6 @@ pub fn gt(a: Field, b: Field) -> bool {
}
}

#[field(bn254)]
pub fn lt(a: Field, b: Field) -> bool {
gt(b, a)
}
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