Implement uniform sampling of Jubjub scalars
Co-authored-by: Sean Bowe <ewillbefull@gmail.com>
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@ -1,7 +1,10 @@
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use pairing::{Field, PrimeField, SqrtField, PrimeFieldRepr, PrimeFieldDecodingError, LegendreSymbol};
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use byteorder::{ByteOrder, LittleEndian};
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use pairing::{BitIterator, Field, PrimeField, SqrtField, PrimeFieldRepr, PrimeFieldDecodingError, LegendreSymbol};
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use pairing::LegendreSymbol::*;
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use pairing::{adc, sbb, mac_with_carry};
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use super::ToUniform;
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// s = 6554484396890773809930967563523245729705921265872317281365359162392183254199
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const MODULUS: FsRepr = FsRepr([0xd0970e5ed6f72cb7, 0xa6682093ccc81082, 0x6673b0101343b00, 0xe7db4ea6533afa9]);
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@ -548,6 +551,31 @@ impl Fs {
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(self.0).0[3] = r7;
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self.reduce();
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}
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fn mul_bits<S: AsRef<[u64]>>(&self, bits: BitIterator<S>) -> Self {
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let mut res = Self::zero();
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for bit in bits {
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res.double();
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if bit {
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res.add_assign(self)
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}
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}
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res
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}
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}
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impl ToUniform for Fs {
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/// Convert a little endian byte string into a uniform
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/// field element. The number is reduced mod s. The caller
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/// is responsible for ensuring the input is 64 bytes of
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/// Random Oracle output.
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fn to_uniform(digest: &[u8]) -> Self {
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assert_eq!(digest.len(), 64);
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let mut repr: [u64; 8] = [0; 8];
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LittleEndian::read_u64_into(digest, &mut repr);
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Self::one().mul_bits(BitIterator::new(repr))
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}
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}
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impl SqrtField for Fs {
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@ -85,12 +85,16 @@ pub enum FixedGenerators {
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Max = 6
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}
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pub trait ToUniform {
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fn to_uniform(digest: &[u8]) -> Self;
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}
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/// This is an extension to the pairing Engine trait which
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/// offers a scalar field for the embedded curve (Jubjub)
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/// and some pre-computed parameters.
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pub trait JubjubEngine: Engine {
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/// The scalar field of the Jubjub curve
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type Fs: PrimeField + SqrtField;
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type Fs: PrimeField + SqrtField + ToUniform;
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/// The parameters of Jubjub and the Sapling protocol
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type Params: JubjubParams<Self>;
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}
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