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Constant-time field inversion in ff_derive using Field::pow_vartime

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This is around 2.5-3x slower than the non-constant-time inversion. We
can regain some of this speed later by dynamically generating addition
chains.
This commit is contained in:
Jack Grigg 2019-12-19 08:22:06 -06:00
parent 26ef9c9842
commit 56999d0f73
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GPG Key ID: 9E8255172BBF9898
1 changed files with 30 additions and 55 deletions
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85
ff/ff_derive/src/lib.rs
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@ -51,7 +51,7 @@ pub fn prime_field(input: proc_macro::TokenStream) -> proc_macro::TokenStream {
gen.extend(constants_impl);
gen.extend(prime_field_repr_impl(&repr_ident, limbs));
gen.extend(prime_field_impl(&ast.ident, &repr_ident, limbs));
gen.extend(prime_field_impl(&ast.ident, &repr_ident, &modulus, limbs));
gen.extend(sqrt_impl);
// Return the generated impl
@ -540,6 +540,7 @@ fn prime_field_constants_and_sqrt(
fn prime_field_impl(
name: &syn::Ident,
repr: &syn::Ident,
modulus: &BigUint,
limbs: usize,
) -> proc_macro2::TokenStream {
// Returns r{n} as an ident.
@ -742,8 +743,35 @@ fn prime_field_impl(
gen
}
/// Generates an implementation of multiplicative inversion within the target prime
/// field.
fn inv_impl(
a: proc_macro2::TokenStream,
name: &syn::Ident,
modulus: &BigUint,
limbs: usize,
) -> proc_macro2::TokenStream {
let mod_minus_2 = biguint_to_u64_vec(modulus - BigUint::from(2u64), limbs);
// TODO: Improve on this by computing an addition chain for mod_minus_two
quote! {
use ::subtle::ConstantTimeEq;
// By Euler's theorem, if `a` is coprime to `p` (i.e. `gcd(a, p) = 1`), then:
// a^-1 ≡ a^(phi(p) - 1) mod p
//
// `ff_derive` requires that `p` is prime; in this case, `phi(p) = p - 1`, and
// thus:
// a^-1 ≡ a^(p - 2) mod p
let inv = #a.pow_vartime(#mod_minus_2);
::subtle::CtOption::new(inv, !#a.ct_eq(&#name::zero()))
}
}
let squaring_impl = sqr_impl(quote! {self}, limbs);
let multiply_impl = mul_impl(quote! {self}, quote! {other}, limbs);
let invert_impl = inv_impl(quote! {self}, name, modulus, limbs);
let montgomery_impl = mont_impl(limbs);
// (self.0).0[0].ct_eq(&(other.0).0[0]) & (self.0).0[1].ct_eq(&(other.0).0[1]) & ...
@ -1056,61 +1084,8 @@ fn prime_field_impl(
ret
}
/// WARNING: THIS IS NOT ACTUALLY CONSTANT TIME YET!
/// TODO: Make this constant-time.
fn invert(&self) -> ::subtle::CtOption<Self> {
if self.is_zero() {
::subtle::CtOption::new(#name::zero(), ::subtle::Choice::from(0))
} else {
// Guajardo Kumar Paar Pelzl
// Efficient Software-Implementation of Finite Fields with Applications to Cryptography
// Algorithm 16 (BEA for Inversion in Fp)
let one = #repr::from(1);
let mut u = self.0;
let mut v = MODULUS;
let mut b = #name(R2); // Avoids unnecessary reduction step.
let mut c = Self::zero();
while u != one && v != one {
while u.is_even() {
u.div2();
if b.0.is_even() {
b.0.div2();
} else {
b.0.add_nocarry(&MODULUS);
b.0.div2();
}
}
while v.is_even() {
v.div2();
if c.0.is_even() {
c.0.div2();
} else {
c.0.add_nocarry(&MODULUS);
c.0.div2();
}
}
if v < u {
u.sub_noborrow(&v);
b.sub_assign(&c);
} else {
v.sub_noborrow(&u);
c.sub_assign(&b);
}
}
if u == one {
::subtle::CtOption::new(b, ::subtle::Choice::from(1))
} else {
::subtle::CtOption::new(c, ::subtle::Choice::from(1))
}
}
#invert_impl
}
#[inline(always)]