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Auto merge of #47 - ebfull:stable-wnaf, r=ebfull
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Introduce a more typesafe wNAF API, and remove the unstable-wnaf feature

Closes #27.
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bmerge committed Sep 28, 2017
2 parents 68b57b2 + 6708878 commit 2540ab3
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Showing 5 changed files with 236 additions and 54 deletions.
3 changes: 1 addition & 2 deletions Cargo.toml
Original file line number Diff line number Diff line change
Expand Up @@ -15,7 +15,6 @@ byteorder = "1.1.0"
clippy = { version = "0.0.151", optional = true }

[features]
unstable-wnaf = []
unstable-features = ["unstable-wnaf"]
unstable-features = []
u128-support = []
default = ["u128-support"]
36 changes: 15 additions & 21 deletions src/bls12_381/ec.rs
Original file line number Diff line number Diff line change
Expand Up @@ -518,7 +518,7 @@ macro_rules! curve_impl {
(*self).into()
}

fn recommended_wnaf_for_scalar(scalar: <Self::Scalar as PrimeField>::Repr) -> Option<usize> {
fn recommended_wnaf_for_scalar(scalar: <Self::Scalar as PrimeField>::Repr) -> usize {
Self::empirical_recommended_wnaf_for_scalar(scalar)
}

Expand Down Expand Up @@ -859,20 +859,17 @@ pub mod g1 {
}

impl G1 {
fn empirical_recommended_wnaf_for_scalar(scalar: FrRepr) -> Option<usize>
fn empirical_recommended_wnaf_for_scalar(scalar: FrRepr) -> usize
{
const RECOMMENDATIONS: [usize; 3] = [12, 34, 130];

let mut ret = None;
let num_bits = scalar.num_bits() as usize;

for (i, r) in RECOMMENDATIONS.iter().enumerate() {
if *r >= num_bits {
ret = Some(i + 2)
}
if num_bits >= 130 {
4
} else if num_bits >= 34 {
3
} else {
2
}

ret
}

fn empirical_recommended_wnaf_for_num_scalars(num_scalars: usize) -> usize
Expand Down Expand Up @@ -1398,20 +1395,17 @@ pub mod g2 {
}

impl G2 {
fn empirical_recommended_wnaf_for_scalar(scalar: FrRepr) -> Option<usize>
fn empirical_recommended_wnaf_for_scalar(scalar: FrRepr) -> usize
{
const RECOMMENDATIONS: [usize; 3] = [13, 37, 103];

let mut ret = None;
let num_bits = scalar.num_bits() as usize;

for (i, r) in RECOMMENDATIONS.iter().enumerate() {
if *r >= num_bits {
ret = Some(i + 2)
}
if num_bits >= 103 {
4
} else if num_bits >= 37 {
3
} else {
2
}

ret
}

fn empirical_recommended_wnaf_for_num_scalars(num_scalars: usize) -> usize
Expand Down
12 changes: 6 additions & 6 deletions src/lib.rs
Original file line number Diff line number Diff line change
Expand Up @@ -12,6 +12,7 @@
#![cfg_attr(feature = "clippy", allow(inline_always))]
#![cfg_attr(feature = "clippy", allow(too_many_arguments))]
#![cfg_attr(feature = "clippy", allow(unreadable_literal))]
#![cfg_attr(feature = "clippy", allow(new_without_default_derive))]

// Force public structures to implement Debug
#![deny(missing_debug_implementations)]
Expand All @@ -24,8 +25,8 @@ pub mod tests;

pub mod bls12_381;

#[cfg(feature = "unstable-wnaf")]
pub mod wnaf;
mod wnaf;
pub use self::wnaf::Wnaf;

use std::fmt;
use std::error::Error;
Expand Down Expand Up @@ -144,10 +145,9 @@ pub trait CurveProjective: PartialEq +
/// Converts this element into its affine representation.
fn into_affine(&self) -> Self::Affine;

/// Recommends a wNAF window table size given a scalar. Returns `None` if normal
/// scalar multiplication is encouraged. If `Some` is returned, it will be between
/// 2 and 22, inclusive.
fn recommended_wnaf_for_scalar(scalar: <Self::Scalar as PrimeField>::Repr) -> Option<usize>;
/// Recommends a wNAF window table size given a scalar. Always returns a number
/// between 2 and 22, inclusive.
fn recommended_wnaf_for_scalar(scalar: <Self::Scalar as PrimeField>::Repr) -> usize;

/// Recommends a wNAF window size given the number of scalars you intend to multiply
/// a base by. Always returns a number between 2 and 22, inclusive.
Expand Down
106 changes: 95 additions & 11 deletions src/tests/curve.rs
Original file line number Diff line number Diff line change
@@ -1,4 +1,4 @@
use rand::{SeedableRng, XorShiftRng, Rand};
use rand::{SeedableRng, XorShiftRng, Rand, Rng};

use ::{CurveProjective, CurveAffine, Field, EncodedPoint};

Expand Down Expand Up @@ -62,31 +62,115 @@ pub fn curve_tests<G: CurveProjective>()
random_encoding_tests::<G::Affine>();
}

#[cfg(not(feature = "unstable-wnaf"))]
fn random_wnaf_tests<G: CurveProjective>() { }

#[cfg(feature = "unstable-wnaf")]
fn random_wnaf_tests<G: CurveProjective>() {
use ::wnaf::*;
use ::PrimeField;

let mut rng = XorShiftRng::from_seed([0x5dbe6259, 0x8d313d76, 0x3237db17, 0xe5bc0654]);

let mut table = vec![];
let mut wnaf = vec![];
{
let mut table = vec![];
let mut wnaf = vec![];

for w in 2..14 {
for _ in 0..100 {
let g = G::rand(&mut rng);
let s = G::Scalar::rand(&mut rng).into_repr();
let mut g1 = g;
g1.mul_assign(s);

wnaf_table(&mut table, g, w);
wnaf_form(&mut wnaf, s, w);
let g2 = wnaf_exp(&table, &wnaf);

assert_eq!(g1, g2);
}
}
}

{
fn only_compiles_if_send<S: Send>(_: &S) { }

for w in 2..14 {
for _ in 0..100 {
let g = G::rand(&mut rng);
let s = G::Scalar::rand(&mut rng).into_repr();
let mut g1 = g;
g1.mul_assign(s);

wnaf_table(&mut table, g, w);
wnaf_form(&mut wnaf, s, w);
let g2 = wnaf_exp(&table, &wnaf);
let g2 = {
let mut wnaf = Wnaf::new();
wnaf.base(g, 1).scalar(s)
};
let g3 = {
let mut wnaf = Wnaf::new();
wnaf.scalar(s).base(g)
};
let g4 = {
let mut wnaf = Wnaf::new();
let mut shared = wnaf.base(g, 1).shared();

only_compiles_if_send(&shared);

shared.scalar(s)
};
let g5 = {
let mut wnaf = Wnaf::new();
let mut shared = wnaf.scalar(s).shared();

only_compiles_if_send(&shared);

shared.base(g)
};

let g6 = {
let mut wnaf = Wnaf::new();
{
// Populate the vectors.
wnaf.base(rng.gen(), 1).scalar(rng.gen());
}
wnaf.base(g, 1).scalar(s)
};
let g7 = {
let mut wnaf = Wnaf::new();
{
// Populate the vectors.
wnaf.base(rng.gen(), 1).scalar(rng.gen());
}
wnaf.scalar(s).base(g)
};
let g8 = {
let mut wnaf = Wnaf::new();
{
// Populate the vectors.
wnaf.base(rng.gen(), 1).scalar(rng.gen());
}
let mut shared = wnaf.base(g, 1).shared();

only_compiles_if_send(&shared);

shared.scalar(s)
};
let g9 = {
let mut wnaf = Wnaf::new();
{
// Populate the vectors.
wnaf.base(rng.gen(), 1).scalar(rng.gen());
}
let mut shared = wnaf.scalar(s).shared();

only_compiles_if_send(&shared);

shared.base(g)
};

assert_eq!(g1, g2);
assert_eq!(g1, g3);
assert_eq!(g1, g4);
assert_eq!(g1, g5);
assert_eq!(g1, g6);
assert_eq!(g1, g7);
assert_eq!(g1, g8);
assert_eq!(g1, g9);
}
}
}
Expand Down
133 changes: 119 additions & 14 deletions src/wnaf.rs
Original file line number Diff line number Diff line change
@@ -1,13 +1,8 @@
use super::{CurveProjective, PrimeFieldRepr};
use super::{CurveProjective, PrimeFieldRepr, PrimeField};

/// Replaces the contents of `table` with a w-NAF window table for the given window size.
///
/// This function will panic if provided a window size below two, or above 22.
pub fn wnaf_table<G: CurveProjective>(table: &mut Vec<G>, mut base: G, window: usize)
pub(crate) fn wnaf_table<G: CurveProjective>(table: &mut Vec<G>, mut base: G, window: usize)
{
assert!(window < 23);
assert!(window > 1);

table.truncate(0);
table.reserve(1 << (window-1));

Expand All @@ -21,13 +16,8 @@ pub fn wnaf_table<G: CurveProjective>(table: &mut Vec<G>, mut base: G, window: u
}

/// Replaces the contents of `wnaf` with the w-NAF representation of a scalar.
///
/// This function will panic if provided a window size below two, or above 22.
pub fn wnaf_form<S: PrimeFieldRepr>(wnaf: &mut Vec<i64>, mut c: S, window: usize)
pub(crate) fn wnaf_form<S: PrimeFieldRepr>(wnaf: &mut Vec<i64>, mut c: S, window: usize)
{
assert!(window < 23);
assert!(window > 1);

wnaf.truncate(0);

while !c.is_zero() {
Expand Down Expand Up @@ -58,7 +48,7 @@ pub fn wnaf_form<S: PrimeFieldRepr>(wnaf: &mut Vec<i64>, mut c: S, window: usize
///
/// This function must be provided a `table` and `wnaf` that were constructed with
/// the same window size; otherwise, it may panic or produce invalid results.
pub fn wnaf_exp<G: CurveProjective>(table: &[G], wnaf: &[i64]) -> G
pub(crate) fn wnaf_exp<G: CurveProjective>(table: &[G], wnaf: &[i64]) -> G
{
let mut result = G::zero();

Expand All @@ -82,3 +72,118 @@ pub fn wnaf_exp<G: CurveProjective>(table: &[G], wnaf: &[i64]) -> G

result
}

/// A "w-ary non-adjacent form" exponentiation context.
#[derive(Debug)]
pub struct Wnaf<W, B, S> {
base: B,
scalar: S,
window_size: W
}

impl<G: CurveProjective> Wnaf<(), Vec<G>, Vec<i64>> {
/// Construct a new wNAF context without allocating.
pub fn new() -> Self {
Wnaf {
base: vec![],
scalar: vec![],
window_size: ()
}
}

/// Given a base and a number of scalars, compute a window table and return a `Wnaf` object that
/// can perform exponentiations with `.scalar(..)`.
pub fn base(
&mut self,
base: G,
num_scalars: usize
) -> Wnaf<usize, &[G], &mut Vec<i64>>
{
// Compute the appropriate window size based on the number of scalars.
let window_size = G::recommended_wnaf_for_num_scalars(num_scalars);

// Compute a wNAF table for the provided base and window size.
wnaf_table(&mut self.base, base, window_size);

// Return a Wnaf object that immutably borrows the computed base storage location,
// but mutably borrows the scalar storage location.
Wnaf {
base: &self.base,
scalar: &mut self.scalar,
window_size: window_size
}
}

/// Given a scalar, compute its wNAF representation and return a `Wnaf` object that can perform
/// exponentiations with `.base(..)`.
pub fn scalar(
&mut self,
scalar: <<G as CurveProjective>::Scalar as PrimeField>::Repr
) -> Wnaf<usize, &mut Vec<G>, &[i64]>
{
// Compute the appropriate window size for the scalar.
let window_size = G::recommended_wnaf_for_scalar(scalar);

// Compute the wNAF form of the scalar.
wnaf_form(&mut self.scalar, scalar, window_size);

// Return a Wnaf object that mutably borrows the base storage location, but
// immutably borrows the computed wNAF form scalar location.
Wnaf {
base: &mut self.base,
scalar: &self.scalar,
window_size: window_size
}
}
}

impl<'a, G: CurveProjective> Wnaf<usize, &'a [G], &'a mut Vec<i64>> {
/// Constructs new space for the scalar representation while borrowing
/// the computed window table, for sending the window table across threads.
pub fn shared(&self) -> Wnaf<usize, &'a [G], Vec<i64>> {
Wnaf {
base: self.base,
scalar: vec![],
window_size: self.window_size
}
}
}

impl<'a, G: CurveProjective> Wnaf<usize, &'a mut Vec<G>, &'a [i64]> {
/// Constructs new space for the window table while borrowing
/// the computed scalar representation, for sending the scalar representation
/// across threads.
pub fn shared(&self) -> Wnaf<usize, Vec<G>, &'a [i64]> {
Wnaf {
base: vec![],
scalar: self.scalar,
window_size: self.window_size
}
}
}

impl<B, S: AsRef<[i64]>> Wnaf<usize, B, S> {
/// Performs exponentiation given a base.
pub fn base<G: CurveProjective>(
&mut self,
base: G
) -> G
where B: AsMut<Vec<G>>
{
wnaf_table(self.base.as_mut(), base, self.window_size);
wnaf_exp(self.base.as_mut(), self.scalar.as_ref())
}
}

impl<B, S: AsMut<Vec<i64>>> Wnaf<usize, B, S> {
/// Performs exponentiation given a scalar.
pub fn scalar<G: CurveProjective>(
&mut self,
scalar: <<G as CurveProjective>::Scalar as PrimeField>::Repr
) -> G
where B: AsRef<[G]>
{
wnaf_form(self.scalar.as_mut(), scalar, self.window_size);
wnaf_exp(self.base.as_ref(), self.scalar.as_mut())
}
}

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