diff --git a/src/lib.rs b/src/lib.rs index 27d05ee..cd16103 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`. @@ -530,7 +537,6 @@ pub trait ConditionallyNegatable { /// unchanged. /// /// This function should execute in constant time. - #[inline] fn conditional_negate(&mut self, choice: Choice); } @@ -801,7 +807,7 @@ macro_rules! generate_unsigned_integer_greater { Choice::from((bit & 1) as u8) } } - } + }; } generate_unsigned_integer_greater!(u8, 8); @@ -813,7 +819,7 @@ 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 +858,7 @@ 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 +868,144 @@ 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 ConstantTimeEq for Ordering { + /// Use our `#[repr(i8)]` to get a `ct_eq()` implementation without relying on any `match`es. + /// + /// This also means `CtOption` implements `ConstantTimeEq`. + #[inline] + fn ct_eq(&self, other: &Self) -> Choice { + let a = *self as i8; + let b = *other as i8; + a.ct_eq(&b) + } +} + +/// Select among `N + 1` results given `N` logical values, of which at most one should be true. +/// +/// This method requires a whole set of logical checks to be performed before evaluating their +/// result, and uses a lookup table to avoid branching in a `match` expression. +/// +///``` +/// use subtle::index_mutually_exclusive_logical_results; +/// +/// let r = [0xA, 0xB, 0xC]; +/// +/// let a = index_mutually_exclusive_logical_results(&r, [0.into(), 0.into()]); +/// assert_eq!(*a, 0xA); +/// let b = index_mutually_exclusive_logical_results(&r, [1.into(), 0.into()]); +/// assert_eq!(*b, 0xB); +/// let c = index_mutually_exclusive_logical_results(&r, [0.into(), 1.into()]); +/// assert_eq!(*c, 0xC); +///``` +pub fn index_mutually_exclusive_logical_results( + results: &[T], + logicals: [Choice; N], +) -> &T { + assert_eq!(results.len(), N + 1); + let combined_result: u8 = logicals.iter().enumerate().fold(0u8, |x, (i, choice)| { + x + ((i as u8) + 1) * choice.unwrap_u8() + }); + results + .get(combined_result as usize) + .expect("multiple inconsistent mutually exclusive logical operations returned true") +} + +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); + + static PARTIAL_ORDERS: [CtOption; 4] = [ + CtOption { + value: Ordering::Equal, + is_some: Choice::of_bool(false), + }, + CtOption { + value: Ordering::Equal, + is_some: Choice::of_bool(true), + }, + CtOption { + value: Ordering::Less, + is_some: Choice::of_bool(true), + }, + CtOption { + value: Ordering::Greater, + is_some: Choice::of_bool(true), + }, + ]; + *index_mutually_exclusive_logical_results(&PARTIAL_ORDERS, [is_eq, is_lt, is_gt]) + } +} + +/// 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. + /// + /// 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); + + static ORDERS: [Ordering; 3] = [Ordering::Less, Ordering::Greater, Ordering::Equal]; + *index_mutually_exclusive_logical_results(&ORDERS, [is_gt, is_eq]) + } +} + +impl ConstantTimeOrd for u8 {} +impl ConstantTimeOrd for u16 {} +impl ConstantTimeOrd for u32 {} +impl ConstantTimeOrd for u64 {} +#[cfg(feature = "i128")] +impl ConstantTimeOrd for u128 {}