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limbs/bigint: Make obvious LeakyLimb->Limb conversions explicit.
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Take a step towards making `Limb` an opaque type.

Because of inference issues, places where we'd like to write
`Limb::from(<literal>)` can't be written that way until `Limb`
(actually `Word`) becomes a newtype. There are only a few of these,
so defer them until the actual newtype change happens.
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@briansmith
briansmith committed Dec 8, 2024
1 parent c1c8a44 commit 8086c79
Showing 1 changed file with 49 additions and 20 deletions.
69 changes: 49 additions & 20 deletions src/limb.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -182,7 +182,7 @@ pub fn parse_big_endian_and_pad_consttime(
// TODO: Improve this.
input.read_all(error::Unspecified, |input| {
for i in 0..num_encoded_limbs {
let mut limb: Limb = 0;
let mut limb = 0;
for _ in 0..bytes_in_current_limb {
let b: Limb = input.read_byte()?.into();
limb = (limb << 8) | b;
Expand Down Expand Up @@ -356,16 +356,17 @@ prefixed_extern! {
#[cfg(test)]
mod tests {
use super::*;
use alloc::vec::Vec;

const MAX: Limb = Limb::MAX;
const MAX: LeakyLimb = LeakyLimb::MAX;

fn leak_in_test(a: LimbMask) -> bool {
a.leak()
}

#[test]
fn test_limbs_are_even() {
static EVENS: &[&[Limb]] = &[
static EVENS: &[&[LeakyLimb]] = &[
&[],
&[0],
&[2],
Expand All @@ -377,9 +378,10 @@ mod tests {
&[0, 0, 0, 0, MAX],
];
for even in EVENS {
let even = &Vec::from_iter(even.iter().copied().map(Limb::from));
assert!(leak_in_test(limbs_are_even_constant_time(even)));
}
static ODDS: &[&[Limb]] = &[
static ODDS: &[&[LeakyLimb]] = &[
&[1],
&[3],
&[1, 0],
Expand All @@ -390,11 +392,14 @@ mod tests {
&[1, 0, 0, 0, MAX],
];
for odd in ODDS {
let odd = &Vec::from_iter(odd.iter().copied().map(Limb::from));
assert!(!leak_in_test(limbs_are_even_constant_time(odd)));
}
}

static ZEROES: &[&[Limb]] = &[
const ZERO: LeakyLimb = 0;

static ZEROES: &[&[LeakyLimb]] = &[
&[],
&[0],
&[0, 0],
Expand All @@ -406,7 +411,7 @@ mod tests {
&[0, 0, 0, 0, 0, 0, 0, 0, 0],
];

static NONZEROES: &[&[Limb]] = &[
static NONZEROES: &[&[LeakyLimb]] = &[
&[1],
&[0, 1],
&[1, 1],
Expand All @@ -419,22 +424,32 @@ mod tests {
#[test]
fn test_limbs_are_zero() {
for zero in ZEROES {
let zero = &Vec::from_iter(zero.iter().copied().map(Limb::from));
assert!(leak_in_test(limbs_are_zero_constant_time(zero)));
}
for nonzero in NONZEROES {
let nonzero = &Vec::from_iter(nonzero.iter().copied().map(Limb::from));
assert!(!leak_in_test(limbs_are_zero_constant_time(nonzero)));
}
}

#[test]
fn test_limbs_equal_limb() {
for zero in ZEROES {
assert!(leak_in_test(limbs_equal_limb_constant_time(zero, 0)));
let zero = &Vec::from_iter(zero.iter().copied().map(Limb::from));
assert!(leak_in_test(limbs_equal_limb_constant_time(
zero,
Limb::from(ZERO)
)));
}
for nonzero in NONZEROES {
assert!(!leak_in_test(limbs_equal_limb_constant_time(nonzero, 0)));
let nonzero = &Vec::from_iter(nonzero.iter().copied().map(Limb::from));
assert!(!leak_in_test(limbs_equal_limb_constant_time(
nonzero,
Limb::from(ZERO)
)));
}
static EQUAL: &[(&[Limb], Limb)] = &[
static EQUAL: &[(&[LeakyLimb], LeakyLimb)] = &[
(&[1], 1),
(&[MAX], MAX),
(&[1, 0], 1),
Expand All @@ -443,9 +458,13 @@ mod tests {
(&[0b100, 0], 0b100),
];
for &(a, b) in EQUAL {
assert!(leak_in_test(limbs_equal_limb_constant_time(a, b)));
let a = &Vec::from_iter(a.iter().copied().map(Limb::from));
assert!(leak_in_test(limbs_equal_limb_constant_time(
a,
Limb::from(b)
)));
}
static UNEQUAL: &[(&[Limb], Limb)] = &[
static UNEQUAL: &[(&[LeakyLimb], LeakyLimb)] = &[
(&[0], 1),
(&[2], 1),
(&[3], 1),
Expand All @@ -457,14 +476,15 @@ mod tests {
(&[MAX, 1], MAX),
];
for &(a, b) in UNEQUAL {
let a = &Vec::from_iter(a.iter().copied().map(Limb::from));
assert!(!leak_in_test(limbs_equal_limb_constant_time(a, b)));
}
}

#[test]
#[cfg(feature = "alloc")]
fn test_limbs_less_than_limb_constant_time() {
static LESSER: &[(&[Limb], Limb)] = &[
static LESSER: &[(&[LeakyLimb], LeakyLimb)] = &[
(&[0], 1),
(&[0, 0], 1),
(&[1, 0], 2),
Expand All @@ -474,16 +494,18 @@ mod tests {
(&[MAX - 1, 0], MAX),
];
for &(a, b) in LESSER {
let a = &Vec::from_iter(a.iter().copied().map(Limb::from));
let b = Limb::from(b);
assert!(leak_in_test(limbs_less_than_limb_constant_time(a, b)));
}
static EQUAL: &[(&[Limb], Limb)] = &[
static EQUAL: &[(&[LeakyLimb], LeakyLimb)] = &[
(&[0], 0),
(&[0, 0, 0, 0], 0),
(&[1], 1),
(&[1, 0, 0, 0, 0, 0, 0], 1),
(&[MAX], MAX),
];
static GREATER: &[(&[Limb], Limb)] = &[
static GREATER: &[(&[LeakyLimb], LeakyLimb)] = &[
(&[1], 0),
(&[2, 0], 1),
(&[3, 0, 0, 0], 1),
Expand All @@ -493,6 +515,8 @@ mod tests {
(&[MAX], MAX - 1),
];
for &(a, b) in EQUAL.iter().chain(GREATER.iter()) {
let a = &Vec::from_iter(a.iter().copied().map(Limb::from));
let b = Limb::from(b);
assert!(!leak_in_test(limbs_less_than_limb_constant_time(a, b)));
}
}
Expand All @@ -504,23 +528,23 @@ mod tests {
{
// Empty input.
let inp = untrusted::Input::from(&[]);
let mut result = [0; LIMBS];
let mut result = [0; LIMBS].map(From::<LeakyLimb>::from);
assert!(parse_big_endian_and_pad_consttime(inp, &mut result).is_err());
}

// The input is longer than will fit in the given number of limbs.
{
let inp = [1, 2, 3, 4, 5, 6, 7, 8, 9];
let inp = untrusted::Input::from(&inp);
let mut result = [0; 8 / LIMB_BYTES];
let mut result = [0; 8 / LIMB_BYTES].map(From::<LeakyLimb>::from);
assert!(parse_big_endian_and_pad_consttime(inp, &mut result[..]).is_err());
}

// Less than a full limb.
{
let inp = [0xfe];
let inp = untrusted::Input::from(&inp);
let mut result = [0; LIMBS];
let mut result = [0; LIMBS].map(From::<LeakyLimb>::from);
assert_eq!(
Ok(()),
parse_big_endian_and_pad_consttime(inp, &mut result[..])
Expand All @@ -532,7 +556,7 @@ mod tests {
{
let inp = [0xbe, 0xef, 0xf0, 0x0d];
let inp = untrusted::Input::from(&inp);
let mut result = [0; LIMBS];
let mut result = [0; LIMBS].map(From::<LeakyLimb>::from);
assert_eq!(Ok(()), parse_big_endian_and_pad_consttime(inp, &mut result));
assert_eq!(&[0xbeeff00d, 0, 0, 0], &result);
}
Expand All @@ -556,6 +580,8 @@ mod tests {
0x1122_3344_5566_7788,
];

let limbs = limbs.map(From::<LeakyLimb>::from);

let expected = [
0x11, 0x22, 0x33, 0x44, 0x55, 0x66, 0x77, 0x88, 0x99, 0xaa, 0xbb, 0xcc, 0xdd, 0xee,
0xff, 0x00, 0x01, 0x12, 0x23, 0x34, 0x45, 0x56, 0x67, 0x78, 0x89, 0x90, 0x0a, 0xab,
Expand Down Expand Up @@ -584,15 +610,17 @@ mod tests {
0x99aa_bbcc_ddee_ff00,
];

let limbs = limbs.map(From::<LeakyLimb>::from);

let mut out = [0xabu8; 32];

big_endian_from_limbs(&limbs[..], &mut out);
}

#[test]
fn test_limbs_minimal_bits() {
const ALL_ONES: Limb = Limb::MAX;
static CASES: &[(&[Limb], usize)] = &[
const ALL_ONES: LeakyLimb = LeakyLimb::MAX;
static CASES: &[(&[LeakyLimb], usize)] = &[
(&[], 0),
(&[0], 0),
(&[ALL_ONES], LIMB_BITS),
Expand All @@ -609,6 +637,7 @@ mod tests {
(&[ALL_ONES, ALL_ONES >> 1], LIMB_BITS + (LIMB_BITS) - 1),
];
for (limbs, bits) in CASES {
let limbs = &Vec::from_iter(limbs.iter().copied().map(Limb::from));
assert_eq!(limbs_minimal_bits(limbs).as_bits(), *bits);
}
}
Expand Down

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