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Build to_2s_complement by individual elements. (#264)

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This change is a no-op on the result; it is a precursor to having
bounds that can vary on each element in the input array.
This commit is contained in:
Ryan Orendorff
2023-06-26 19:42:51 -07:00
committed by GitHub
parent 4fbac668d0
commit c7052bbfda
1 changed files with 53 additions and 31 deletions
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84
logproof/src/linear_relation.rs
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@@ -833,13 +833,64 @@ impl LogProof {
(r_2, r_1)
}
/**
* Takes an individual values in a field `Zq`, treats the value as signed [q's
* complement](https://en.wikipedia.org/wiki/Method_of_complements) and
* converts the value to binary 2's complement. This is then appended to an
* provided BitVec.
*
* `value` is the element in Zq and `b` is the number of bits needed
* to represent the signed value.
*
* This modifies bitvec in place.
*
*/
fn to_2s_complement_single<Q, const N: usize>(
value: &Fp<MontBackend<Q, N>, N>,
log_b: u64,
bitvec: &mut BitVec,
) where
Q: MontConfig<N>,
{
// Get the value out of Montgomery form.
let value = MontBackend::into_bigint(*value);
let mod_div_2 = Fp::<MontBackend<Q, N>, N>::field_modulus_div_2();
let modulus = Fp::<MontBackend<Q, N>, N>::field_modulus();
let is_negative = value > mod_div_2;
// Compute the q's complement of value
let mut as_neg: BigInt<N> = modulus;
as_neg.sub_with_borrow(&value);
// The smaller of value and it's q's complement is the absolute
// value.
let mut abs_value = BigInt::min(value, as_neg);
// To make a positive number negative in 2's complement,
// subtract 1 and flip the bits. So, here we sub 1 from abs if
// original value was negative.
let big_negative = BigInt::from(is_negative as u8);
abs_value.sub_with_borrow(&big_negative);
for i in 0..(log_b - 1) {
let bit = abs_value.get_bit(i as usize);
// Invert the bit if the original value was negative
bitvec.push(bit ^ is_negative);
}
// Now push the sign bit
bitvec.push(is_negative);
}
/**
* Takes a slice of values in a field `Zq`, treats the values as signed [q's
* complement](https://en.wikipedia.org/wiki/Method_of_complements)
* and converts the value to binary 2's complement.
*
* `value` is the element in Zq and `b` is the number of bits needed
* to represent the
* to represent the signed value.
*/
fn to_2s_complement<Q, const N: usize>(
values: &[Fp<MontBackend<Q, N>, N>],
@@ -852,36 +903,7 @@ impl LogProof {
// This code should not feature timing side-channels.
for value in values.iter() {
// Get the value out of Montgomery form.
let value = MontBackend::into_bigint(*value);
let mod_div_2 = Fp::<MontBackend<Q, N>, N>::field_modulus_div_2();
let modulus = Fp::<MontBackend<Q, N>, N>::field_modulus();
let is_negative = value > mod_div_2;
// Compute the q's complement of value
let mut as_neg: BigInt<N> = modulus;
as_neg.sub_with_borrow(&value);
// The smaller of value and it's q's complement is the absolute
// value.
let mut abs_value = BigInt::min(value, as_neg);
// To make a positive number negative in 2's complement,
// subtract 1 and flip the bits. So, here we sub 1 from abs if
// original value was negative.
let big_negative = BigInt::from(is_negative as u8);
abs_value.sub_with_borrow(&big_negative);
for i in 0..(log_b - 1) {
let bit = abs_value.get_bit(i as usize);
// Invert the bit if the original value was negative
bitvec.push(bit ^ is_negative);
}
// Now push the sign bit
bitvec.push(is_negative);
LogProof::to_2s_complement_single(value, log_b, &mut bitvec);
}
bitvec