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ff_derive: Generate modulus representation with correct endianness

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Now that PrimeField::ReprEndianness exists, users can obtain a
known-endianness representation from the output of PrimeField::char
(which is a PrimeField::Repr, and should return a representation with
the same endianness as PrimeField::into_repr).
This commit is contained in:
Jack Grigg
2020-05-02 18:25:26 +12:00
parent 15e229509a
commit 9114c367f4
3 changed files with 42 additions and 29 deletions
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34
ff/ff_derive/src/lib.rs
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@@ -38,6 +38,23 @@ impl ReprEndianness {
}
}
fn modulus_repr(&self, modulus: &BigUint, bytes: usize) -> Vec<u8> {
match self {
ReprEndianness::Big => {
let buf = modulus.to_bytes_be();
iter::repeat(0)
.take(bytes - buf.len())
.chain(buf.into_iter())
.collect()
}
ReprEndianness::Little => {
let mut buf = modulus.to_bytes_le();
buf.extend(iter::repeat(0).take(bytes - buf.len()));
buf
}
}
}
fn from_repr(&self, name: &syn::Ident, limbs: usize) -> proc_macro2::TokenStream {
let read_repr = match self {
ReprEndianness::Big => quote! {
@@ -159,8 +176,14 @@ pub fn prime_field(input: proc_macro::TokenStream) -> proc_macro::TokenStream {
let mut gen = proc_macro2::TokenStream::new();
let (constants_impl, sqrt_impl) =
prime_field_constants_and_sqrt(&ast.ident, &repr_ident, &modulus, limbs, generator);
let (constants_impl, sqrt_impl) = prime_field_constants_and_sqrt(
&ast.ident,
&repr_ident,
&modulus,
&endianness,
limbs,
generator,
);
gen.extend(constants_impl);
gen.extend(prime_field_repr_impl(&repr_ident, &endianness, limbs * 8));
@@ -466,6 +489,7 @@ fn prime_field_constants_and_sqrt(
name: &syn::Ident,
repr: &syn::Ident,
modulus: &BigUint,
endianness: &ReprEndianness,
limbs: usize,
generator: BigUint,
) -> (proc_macro2::TokenStream, proc_macro2::TokenStream) {
@@ -576,11 +600,7 @@ fn prime_field_constants_and_sqrt(
let r2 = biguint_to_u64_vec((&r * &r) % modulus, limbs);
let r = biguint_to_u64_vec(r, limbs);
let modulus_repr = {
let mut buf = modulus.to_bytes_le();
buf.extend(iter::repeat(0).take((limbs * 8) - buf.len()));
buf
};
let modulus_repr = endianness.modulus_repr(modulus, limbs * 8);
let modulus = biguint_to_real_u64_vec(modulus.clone(), limbs);
// Compute -m^-1 mod 2**64 by exponentiating by totient(2**64) - 1