diff --git a/src/lib.rs b/src/lib.rs index 27d05ee..96e8e82 100644 --- a/src/lib.rs +++ b/src/lib.rs @@ -1,7 +1,7 @@ // -*- mode: rust; -*- // // This file is part of subtle, part of the dalek cryptography project. -// Copyright (c) 2016-2018 isis lovecruft, Henry de Valence +// Copyright (c) 2016-2022 isis lovecruft, Henry de Valence // See LICENSE for licensing information. // // Authors: @@ -87,6 +87,10 @@ #[macro_use] extern crate std; +#[cfg(test)] +extern crate rand; + +use core::cmp::Ordering; use core::ops::{BitAnd, BitAndAssign, BitOr, BitOrAssign, BitXor, BitXorAssign, Neg, Not}; use core::option::Option; @@ -111,6 +115,11 @@ use core::option::Option; pub struct Choice(u8); impl Choice { + /// Create an instance in `const` context. + pub const fn of_bool(of: bool) -> Self { + Self(of as u8) + } + /// Unwrap the `Choice` wrapper to reveal the underlying `u8`. /// /// # Note @@ -236,7 +245,7 @@ impl From for Choice { } } -/// An `Eq`-like trait that produces a `Choice` instead of a `bool`. +/// An [`Eq`]-like trait that produces a `Choice` instead of a `bool`. /// /// # Example /// @@ -257,7 +266,6 @@ pub trait ConstantTimeEq { /// /// * `Choice(1u8)` if `self == other`; /// * `Choice(0u8)` if `self != other`. - #[inline] fn ct_eq(&self, other: &Self) -> Choice; } @@ -380,7 +388,6 @@ pub trait ConditionallySelectable: Copy { /// assert_eq!(z, y); /// # } /// ``` - #[inline] fn conditional_select(a: &Self, b: &Self, choice: Choice) -> Self; /// Conditionally assign `other` to `self`, according to `choice`. @@ -434,7 +441,7 @@ pub trait ConditionallySelectable: Copy { #[inline] fn conditional_swap(a: &mut Self, b: &mut Self, choice: Choice) { let t: Self = *a; - a.conditional_assign(&b, choice); + a.conditional_assign(b, choice); b.conditional_assign(&t, choice); } } @@ -530,7 +537,6 @@ pub trait ConditionallyNegatable { /// unchanged. /// /// This function should execute in constant time. - #[inline] fn conditional_negate(&mut self, choice: Choice); } @@ -599,10 +605,7 @@ impl CtOption { /// exposed. #[inline] pub fn new(value: T, is_some: Choice) -> CtOption { - CtOption { - value: value, - is_some: is_some, - } + CtOption { value, is_some } } /// This returns the underlying value but panics if it @@ -801,7 +804,7 @@ macro_rules! generate_unsigned_integer_greater { Choice::from((bit & 1) as u8) } } - } + }; } generate_unsigned_integer_greater!(u8, 8); @@ -813,7 +816,7 @@ generate_unsigned_integer_greater!(u128, 128); /// A type which can be compared in some manner and be determined to be less /// than another of the same type. -pub trait ConstantTimeLess: ConstantTimeEq + ConstantTimeGreater { +pub trait ConstantTimeLess: ConstantTimeGreater { /// Determine whether `self < other`. /// /// The bitwise-NOT of the return value of this function should be usable to @@ -852,7 +855,7 @@ pub trait ConstantTimeLess: ConstantTimeEq + ConstantTimeGreater { /// ``` #[inline] fn ct_lt(&self, other: &Self) -> Choice { - !self.ct_gt(other) & !self.ct_eq(other) + other.ct_gt(self) } } @@ -862,3 +865,124 @@ impl ConstantTimeLess for u32 {} impl ConstantTimeLess for u64 {} #[cfg(feature = "i128")] impl ConstantTimeLess for u128 {} + +/// A [`PartialOrd`][core::cmp::PartialOrd]-like trait for constant-time comparisons. +/// +/// This trait is automatically implemented for types supporting the "equals", "less", and +/// "greater" comparisons. +/// +/// # Example +/// +/// ``` +/// use std::cmp::Ordering; +/// use subtle::{ConstantTimePartialOrd, CtOption}; +/// let x: u8 = 5; +/// let y: u8 = 13; +/// +/// assert_eq!(x.ct_partial_cmp(&x).unwrap(), Ordering::Equal); +/// assert_eq!(x.ct_partial_cmp(&y).unwrap(), Ordering::Less); +/// assert_eq!(y.ct_partial_cmp(&x).unwrap(), Ordering::Greater); +/// ``` +pub trait ConstantTimePartialOrd { + /// This method returns an ordering between `self` and `other`, if it exists. + /// + /// This method should execute in constant time. + fn ct_partial_cmp(&self, other: &Self) -> CtOption; +} + +impl ConditionallySelectable for Ordering { + /// Delegate to [`i8::conditional_select()`], since [`Ordering`] is `#[repr(i8)]`. + #[inline] + fn conditional_select(a: &Self, b: &Self, choice: Choice) -> Self { + let delegated: i8 = i8::conditional_select(&(*a as i8), &(*b as i8), choice); + // TODO: is there a canonical way to construct a repr(i8) enum like Ordering from an i8 + // without this unsafe call to transmute? + unsafe { core::mem::transmute::(delegated) } + } +} + +impl ConstantTimePartialOrd for T { + /// We do not assume a total ordering for `T`, so we have to individually check "less than", + /// "equal", and "greater". This also respects non-default implementations of `ct_lt()`. + fn ct_partial_cmp(&self, other: &Self) -> CtOption { + let is_eq = self.ct_eq(other); + let is_lt = self.ct_lt(other); + let is_gt = self.ct_gt(other); + + let mut ret = CtOption { + value: Ordering::Equal, + is_some: Choice::of_bool(false), + }; + ret.conditional_assign( + &CtOption { + value: Ordering::Equal, + is_some: Choice::of_bool(true), + }, + is_eq, + ); + ret.conditional_assign( + &CtOption { + value: Ordering::Less, + is_some: Choice::of_bool(true), + }, + is_lt, + ); + ret.conditional_assign( + &CtOption { + value: Ordering::Greater, + is_some: Choice::of_bool(true), + }, + is_gt, + ); + ret + } +} + +/// An [`Ord`][core::cmp::Ord]-like trait for constant-time comparisons. +/// +/// This trait can be automatically implemented for types supporting the "equals" and "greater" +/// comparisons. +/// +/// # Example +/// +/// ``` +/// use std::cmp::Ordering; +/// use subtle::ConstantTimeOrd; +/// let x: u8 = 5; +/// let y: u8 = 13; +/// +/// assert_eq!(x.ct_cmp(&x), Ordering::Equal); +/// assert_eq!(x.ct_cmp(&y), Ordering::Less); +/// assert_eq!(y.ct_cmp(&x), Ordering::Greater); +/// ``` +pub trait ConstantTimeOrd: ConstantTimeEq + ConstantTimeGreater { + /// This method returns an ordering between `self` and other`. + /// + /// Although this method should never need to be overridden, it is exposed as a default method + /// here to force types to explicitly implement this trait. This ensures that types which are + /// *only* partially orderable do not pick up an incorrect `ConstantTimeOrd` impl just by + /// implementing the pairwise comparison operations. Contrast this with + /// [`ConstantTimePartialOrd`], which is automatically implemented for all types implementing + /// [`ConstantTimeGreater`], [`ConstantTimeLess`], and [`ConstantTimeEq`]. + /// + /// Here we assume a total ordering for `T`, so we need to check only "equal" and "greater", and + /// can assume "less" if both `ct_eq()` and `ct_gt()` are false. + /// + /// This method should execute in constant time. + fn ct_cmp(&self, other: &Self) -> Ordering { + let is_gt = self.ct_gt(other); + let is_eq = self.ct_eq(other); + + let mut ret = Ordering::Less; + ret.conditional_assign(&Ordering::Equal, is_eq); + ret.conditional_assign(&Ordering::Greater, is_gt); + ret + } +} + +impl ConstantTimeOrd for u8 {} +impl ConstantTimeOrd for u16 {} +impl ConstantTimeOrd for u32 {} +impl ConstantTimeOrd for u64 {} +#[cfg(feature = "i128")] +impl ConstantTimeOrd for u128 {}