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// -*- mode: rust; -*-
//
// This file is part of subtle, part of the dalek cryptography project.
// Copyright (c) 2016-2018 isis lovecruft, Henry de Valence
// See LICENSE for licensing information.
//
// Authors:
// - isis agora lovecruft <isis@patternsinthevoid.net>
// - Henry de Valence <hdevalence@hdevalence.ca>
#![no_std]
#![deny(missing_docs)]
//! # subtle [](https://crates.io/crates/subtle) [](https://doc.dalek.rs/subtle) [](https://travis-ci.org/dalek-cryptography/subtle)
//!
//! **Pure-Rust traits and utilities for constant-time cryptographic implementations.**
//!
//! It consists of a `Choice` type, and a collection of traits using `Choice`
//! instead of `bool` which are intended to execute in constant-time. The `Choice`
//! type is a wrapper around a `u8` that holds a `0` or `1`.
//!
//! ```toml
//! subtle = "2.5"
//! ```
//!
//! This crate represents a “best-effort” attempt, since side-channels
//! are ultimately a property of a deployed cryptographic system
//! including the hardware it runs on, not just of software.
//!
//! The traits are implemented using bitwise operations, and should execute in
//! constant time provided that a) the bitwise operations are constant-time and
//! b) the bitwise operations are not recognized as a conditional assignment and
//! optimized back into a branch.
//!
//! For a compiler to recognize that bitwise operations represent a conditional
//! assignment, it needs to know that the value used to generate the bitmasks is
//! really a boolean `i1` rather than an `i8` byte value. In an attempt to
//! prevent this refinement, the crate tries to hide the value of a `Choice`'s
//! inner `u8` by passing it through a volatile read. For more information, see
//! the _About_ section below.
//!
//! Rust versions from 1.66 or higher support a new best-effort optimization
//! barrier ([`core::hint::black_box`]). To use the new optimization barrier,
//! enable the `core_hint_black_box` feature.
//!
//! Rust versions from 1.51 or higher have const generics support. You may enable
//! `const-generics` feautre to have `subtle` traits implemented for arrays `[T; N]`.
//!
//! Versions prior to `2.2` recommended use of the `nightly` feature to enable an
//! optimization barrier; this is not required in versions `2.2` and above.
//!
//! Note: the `subtle` crate contains `debug_assert`s to check invariants during
//! debug builds. These invariant checks involve secret-dependent branches, and
//! are not present when compiled in release mode. This crate is intended to be
//! used in release mode.
//!
//! ## Documentation
//!
//! Documentation is available [here][docs].
//!
//! ## Minimum Supported Rust Version
//!
//! Rust **1.41** or higher.
//!
//! Minimum supported Rust version can be changed in the future, but it will be done with a minor version bump.
//!
//! ## About
//!
//! This library aims to be the Rust equivalent of Go’s `crypto/subtle` module.
//!
//! Old versions of the optimization barrier in `impl From<u8> for Choice` were
//! based on Tim Maclean's [work on `rust-timing-shield`][rust-timing-shield],
//! which attempts to provide a more comprehensive approach for preventing
//! software side-channels in Rust code.
//!
//! From version `2.2`, it was based on Diane Hosfelt and Amber Sprenkels' work on
//! "Secret Types in Rust". Version `2.5` adds the `core_hint_black_box` feature,
//! which uses the original method through the [`core::hint::black_box`] function
//! from the Rust standard library.
//!
//! `subtle` is authored by isis agora lovecruft and Henry de Valence.
//!
//! ## Warning
//!
//! This code is a low-level library, intended for specific use-cases implementing
//! cryptographic protocols. It represents a best-effort attempt to protect
//! against some software side-channels. Because side-channel resistance is not a
//! property of software alone, but of software together with hardware, any such
//! effort is fundamentally limited.
//!
//! **USE AT YOUR OWN RISK**
//!
#[cfg(feature = "std")]
#[macro_use]
extern crate std;
use core::cmp;
use core::ops::{BitAnd, BitAndAssign, BitOr, BitOrAssign, BitXor, BitXorAssign, Neg, Not};
use core::option::Option;
/// The `Choice` struct represents a choice for use in conditional assignment.
///
/// It is a wrapper around a `u8`, which should have the value either `1` (true)
/// or `0` (false).
///
/// The conversion from `u8` to `Choice` passes the value through an optimization
/// barrier, as a best-effort attempt to prevent the compiler from inferring that
/// the `Choice` value is a boolean. This strategy is based on Tim Maclean's
/// [work on `rust-timing-shield`][rust-timing-shield], which attempts to provide
/// a more comprehensive approach for preventing software side-channels in Rust
/// code.
///
/// The `Choice` struct implements operators for AND, OR, XOR, and NOT, to allow
/// combining `Choice` values. These operations do not short-circuit.
///
/// [rust-timing-shield]:
#[derive(Copy, Clone, Debug)]
pub struct Choice(u8);
impl Choice {
/// Unwrap the `Choice` wrapper to reveal the underlying `u8`.
///
/// # Note
///
/// This function only exists as an **escape hatch** for the rare case
/// where it's not possible to use one of the `subtle`-provided
/// trait impls.
///
/// **To convert a `Choice` to a `bool`, use the `From` implementation instead.**
#[inline]
pub fn unwrap_u8(&self) -> u8 {
self.0
}
}
impl From<Choice> for bool {
/// Convert the `Choice` wrapper into a `bool`, depending on whether
/// the underlying `u8` was a `0` or a `1`.
///
/// # Note
///
/// This function exists to avoid having higher-level cryptographic protocol
/// implementations duplicating this pattern.
///
/// The intended use case for this conversion is at the _end_ of a
/// higher-level primitive implementation: for example, in checking a keyed
/// MAC, where the verification should happen in constant-time (and thus use
/// a `Choice`) but it is safe to return a `bool` at the end of the
/// verification.
#[inline]
fn from(source: Choice) -> bool {
debug_assert!((source.0 == 0u8) | (source.0 == 1u8));
source.0 != 0
}
}
impl BitAnd for Choice {
type Output = Choice;
#[inline]
fn bitand(self, rhs: Choice) -> Choice {
(self.0 & rhs.0).into()
}
}
impl BitAndAssign for Choice {
#[inline]
fn bitand_assign(&mut self, rhs: Choice) {
*self = *self & rhs;
}
}
impl BitOr for Choice {
type Output = Choice;
#[inline]
fn bitor(self, rhs: Choice) -> Choice {
(self.0 | rhs.0).into()
}
}
impl BitOrAssign for Choice {
#[inline]
fn bitor_assign(&mut self, rhs: Choice) {
*self = *self | rhs;
}
}
impl BitXor for Choice {
type Output = Choice;
#[inline]
fn bitxor(self, rhs: Choice) -> Choice {
(self.0 ^ rhs.0).into()
}
}
impl BitXorAssign for Choice {
#[inline]
fn bitxor_assign(&mut self, rhs: Choice) {
*self = *self ^ rhs;
}
}
impl Not for Choice {
type Output = Choice;
#[inline]
fn not(self) -> Choice {
(1u8 & (!self.0)).into()
}
}
/// This function is a best-effort attempt to prevent the compiler from knowing
/// anything about the value of the returned `u8`, other than its type.
///
/// Because we want to support stable Rust, we don't have access to inline
/// assembly or test::black_box, so we use the fact that volatile values will
/// never be elided to register values.
///
/// Note: Rust's notion of "volatile" is subject to change over time. While this
/// code may break in a non-destructive way in the future, “constant-time” code
/// is a continually moving target, and this is better than doing nothing.
#[cfg(not(feature = "core_hint_black_box"))]
#[inline(never)]
fn black_box(input: u8) -> u8 {
debug_assert!((input == 0u8) | (input == 1u8));
unsafe {
// Optimization barrier
//
// Unsafe is ok, because:
// - &input is not NULL;
// - size of input is not zero;
// - u8 is neither Sync, nor Send;
// - u8 is Copy, so input is always live;
// - u8 type is always properly aligned.
core::ptr::read_volatile(&input as *const u8)
}
}
#[cfg(feature = "core_hint_black_box")]
#[inline(never)]
fn black_box(input: u8) -> u8 {
debug_assert!((input == 0u8) | (input == 1u8));
core::hint::black_box(input)
}
impl From<u8> for Choice {
#[inline]
fn from(input: u8) -> Choice {
// Our goal is to prevent the compiler from inferring that the value held inside the
// resulting `Choice` struct is really an `i1` instead of an `i8`.
Choice(black_box(input))
}
}
/// An `Eq`-like trait that produces a `Choice` instead of a `bool`.
///
/// # Example
///
/// ```
/// use subtle::ConstantTimeEq;
/// let x: u8 = 5;
/// let y: u8 = 13;
///
/// assert_eq!(x.ct_eq(&y).unwrap_u8(), 0);
/// assert_eq!(x.ct_eq(&x).unwrap_u8(), 1);
/// ```
pub trait ConstantTimeEq {
/// Determine if two items are equal.
///
/// The `ct_eq` function should execute in constant time.
///
/// # Returns
///
/// * `Choice(1u8)` if `self == other`;
/// * `Choice(0u8)` if `self != other`.
#[inline]
fn ct_eq(&self, other: &Self) -> Choice;
/// Determine if two items are NOT equal.
///
/// The `ct_ne` function should execute in constant time.
///
/// # Returns
///
/// * `Choice(0u8)` if `self == other`;
/// * `Choice(1u8)` if `self != other`.
#[inline]
fn ct_ne(&self, other: &Self) -> Choice {
!self.ct_eq(other)
}
}
impl<T: ConstantTimeEq> ConstantTimeEq for [T] {
/// Check whether two slices of `ConstantTimeEq` types are equal.
///
/// # Note
///
/// This function short-circuits if the lengths of the input slices
/// are different. Otherwise, it should execute in time independent
/// of the slice contents.
///
/// Since arrays coerce to slices, this function works with fixed-size arrays:
///
/// ```
/// # use subtle::ConstantTimeEq;
/// #
/// let a: [u8; 8] = [0,1,2,3,4,5,6,7];
/// let b: [u8; 8] = [0,1,2,3,0,1,2,3];
///
/// let a_eq_a = a.ct_eq(&a);
/// let a_eq_b = a.ct_eq(&b);
///
/// assert_eq!(a_eq_a.unwrap_u8(), 1);
/// assert_eq!(a_eq_b.unwrap_u8(), 0);
/// ```
#[inline]
fn ct_eq(&self, _rhs: &[T]) -> Choice {
let len = self.len();
// Short-circuit on the *lengths* of the slices, not their
// contents.
if len != _rhs.len() {
return Choice::from(0);
}
// This loop shouldn't be shortcircuitable, since the compiler
// shouldn't be able to reason about the value of the `u8`
// unwrapped from the `ct_eq` result.
let mut x = 1u8;
for (ai, bi) in self.iter().zip(_rhs.iter()) {
x &= ai.ct_eq(bi).unwrap_u8();
}
x.into()
}
}
impl ConstantTimeEq for Choice {
#[inline]
fn ct_eq(&self, rhs: &Choice) -> Choice {
!(*self ^ *rhs)
}
}
/// Given the bit-width `$bit_width` and the corresponding primitive
/// unsigned and signed types `$t_u` and `$t_i` respectively, generate
/// an `ConstantTimeEq` implementation.
macro_rules! generate_integer_equal {
($t_u:ty, $t_i:ty, $bit_width:expr) => {
impl ConstantTimeEq for $t_u {
#[inline]
fn ct_eq(&self, other: &$t_u) -> Choice {
// x == 0 if and only if self == other
let x: $t_u = self ^ other;
// If x == 0, then x and -x are both equal to zero;
// otherwise, one or both will have its high bit set.
let y: $t_u = (x | x.wrapping_neg()) >> ($bit_width - 1);
// Result is the opposite of the high bit (now shifted to low).
((y ^ (1 as $t_u)) as u8).into()
}
}
impl ConstantTimeEq for $t_i {
#[inline]
fn ct_eq(&self, other: &$t_i) -> Choice {
// Bitcast to unsigned and call that implementation.
(*self as $t_u).ct_eq(&(*other as $t_u))
}
}
};
}
generate_integer_equal!(u8, i8, 8);
generate_integer_equal!(u16, i16, 16);
generate_integer_equal!(u32, i32, 32);
generate_integer_equal!(u64, i64, 64);
#[cfg(feature = "i128")]
generate_integer_equal!(u128, i128, 128);
generate_integer_equal!(usize, isize, ::core::mem::size_of::<usize>() * 8);
/// `Ordering` is `#[repr(i8)]` making it possible to leverage `i8::ct_eq`.
impl ConstantTimeEq for cmp::Ordering {
#[inline]
fn ct_eq(&self, other: &Self) -> Choice {
(*self as i8).ct_eq(&(*other as i8))
}
}
/// A type which can be conditionally selected in constant time.
///
/// This trait also provides generic implementations of conditional
/// assignment and conditional swaps.
pub trait ConditionallySelectable: Copy {
/// Select `a` or `b` according to `choice`.
///
/// # Returns
///
/// * `a` if `choice == Choice(0)`;
/// * `b` if `choice == Choice(1)`.
///
/// This function should execute in constant time.
///
/// # Example
///
/// ```
/// use subtle::ConditionallySelectable;
/// #
/// # fn main() {
/// let x: u8 = 13;
/// let y: u8 = 42;
///
/// let z = u8::conditional_select(&x, &y, 0.into());
/// assert_eq!(z, x);
/// let z = u8::conditional_select(&x, &y, 1.into());
/// assert_eq!(z, y);
/// # }
/// ```
#[inline]
fn conditional_select(a: &Self, b: &Self, choice: Choice) -> Self;
/// Conditionally assign `other` to `self`, according to `choice`.
///
/// This function should execute in constant time.
///
/// # Example
///
/// ```
/// use subtle::ConditionallySelectable;
/// #
/// # fn main() {
/// let mut x: u8 = 13;
/// let mut y: u8 = 42;
///
/// x.conditional_assign(&y, 0.into());
/// assert_eq!(x, 13);
/// x.conditional_assign(&y, 1.into());
/// assert_eq!(x, 42);
/// # }
/// ```
#[inline]
fn conditional_assign(&mut self, other: &Self, choice: Choice) {
*self = Self::conditional_select(self, other, choice);
}
/// Conditionally swap `self` and `other` if `choice == 1`; otherwise,
/// reassign both unto themselves.
///
/// This function should execute in constant time.
///
/// # Example
///
/// ```
/// use subtle::ConditionallySelectable;
/// #
/// # fn main() {
/// let mut x: u8 = 13;
/// let mut y: u8 = 42;
///
/// u8::conditional_swap(&mut x, &mut y, 0.into());
/// assert_eq!(x, 13);
/// assert_eq!(y, 42);
/// u8::conditional_swap(&mut x, &mut y, 1.into());
/// assert_eq!(x, 42);
/// assert_eq!(y, 13);
/// # }
/// ```
#[inline]
fn conditional_swap(a: &mut Self, b: &mut Self, choice: Choice) {
let t: Self = *a;
a.conditional_assign(&b, choice);
b.conditional_assign(&t, choice);
}
}
macro_rules! to_signed_int {
(u8) => {
i8
};
(u16) => {
i16
};
(u32) => {
i32
};
(u64) => {
i64
};
(u128) => {
i128
};
(i8) => {
i8
};
(i16) => {
i16
};
(i32) => {
i32
};
(i64) => {
i64
};
(i128) => {
i128
};
}
macro_rules! generate_integer_conditional_select {
($($t:tt)*) => ($(
impl ConditionallySelectable for $t {
#[inline]
fn conditional_select(a: &Self, b: &Self, choice: Choice) -> Self {
// if choice = 0, mask = (-0) = 0000...0000
// if choice = 1, mask = (-1) = 1111...1111
let mask = -(choice.unwrap_u8() as to_signed_int!($t)) as $t;
a ^ (mask & (a ^ b))
}
#[inline]
fn conditional_assign(&mut self, other: &Self, choice: Choice) {
// if choice = 0, mask = (-0) = 0000...0000
// if choice = 1, mask = (-1) = 1111...1111
let mask = -(choice.unwrap_u8() as to_signed_int!($t)) as $t;
*self ^= mask & (*self ^ *other);
}
#[inline]
fn conditional_swap(a: &mut Self, b: &mut Self, choice: Choice) {
// if choice = 0, mask = (-0) = 0000...0000
// if choice = 1, mask = (-1) = 1111...1111
let mask = -(choice.unwrap_u8() as to_signed_int!($t)) as $t;
let t = mask & (*a ^ *b);
*a ^= t;
*b ^= t;
}
}
)*)
}
generate_integer_conditional_select!( u8 i8);
generate_integer_conditional_select!( u16 i16);
generate_integer_conditional_select!( u32 i32);
generate_integer_conditional_select!( u64 i64);
#[cfg(feature = "i128")]
generate_integer_conditional_select!(u128 i128);
/// `Ordering` is `#[repr(i8)]` where:
///
/// - `Less` => -1
/// - `Equal` => 0
/// - `Greater` => 1
///
/// Given this, it's possible to operate on orderings as if they're integers,
/// which allows leveraging conditional masking for predication.
impl ConditionallySelectable for cmp::Ordering {
fn conditional_select(a: &Self, b: &Self, choice: Choice) -> Self {
let a = *a as i8;
let b = *b as i8;
let ret = i8::conditional_select(&a, &b, choice);
// SAFETY: `Ordering` is `#[repr(i8)]` and `ret` has been assigned to
// a value which was originally a valid `Ordering` then cast to `i8`
unsafe { *((&ret as *const _) as *const cmp::Ordering) }
}
}
impl ConditionallySelectable for Choice {
#[inline]
fn conditional_select(a: &Self, b: &Self, choice: Choice) -> Self {
Choice(u8::conditional_select(&a.0, &b.0, choice))
}
}
#[cfg(feature = "const-generics")]
impl<T, const N: usize> ConditionallySelectable for [T; N]
where
T: ConditionallySelectable,
{
#[inline]
fn conditional_select(a: &Self, b: &Self, choice: Choice) -> Self {
let mut output = *a;
output.conditional_assign(b, choice);
output
}
fn conditional_assign(&mut self, other: &Self, choice: Choice) {
for (a_i, b_i) in self.iter_mut().zip(other) {
a_i.conditional_assign(b_i, choice)
}
}
}
/// A type which can be conditionally negated in constant time.
///
/// # Note
///
/// A generic implementation of `ConditionallyNegatable` is provided
/// for types `T` which are `ConditionallySelectable` and have `Neg`
/// implemented on `&T`.
pub trait ConditionallyNegatable {
/// Negate `self` if `choice == Choice(1)`; otherwise, leave it
/// unchanged.
///
/// This function should execute in constant time.
#[inline]
fn conditional_negate(&mut self, choice: Choice);
}
impl<T> ConditionallyNegatable for T
where
T: ConditionallySelectable,
for<'a> &'a T: Neg<Output = T>,
{
#[inline]
fn conditional_negate(&mut self, choice: Choice) {
// Need to cast to eliminate mutability
let self_neg: T = -(self as &T);
self.conditional_assign(&self_neg, choice);
}
}
/// The `CtOption<T>` type represents an optional value similar to the
/// [`Option<T>`](core::option::Option) type but is intended for
/// use in constant time APIs.
///
/// Any given `CtOption<T>` is either `Some` or `None`, but unlike
/// `Option<T>` these variants are not exposed. The
/// [`is_some()`](CtOption::is_some) method is used to determine if
/// the value is `Some`, and [`unwrap_or()`](CtOption::unwrap_or) and
/// [`unwrap_or_else()`](CtOption::unwrap_or_else) methods are
/// provided to access the underlying value. The value can also be
/// obtained with [`unwrap()`](CtOption::unwrap) but this will panic
/// if it is `None`.
///
/// Functions that are intended to be constant time may not produce
/// valid results for all inputs, such as square root and inversion
/// operations in finite field arithmetic. Returning an `Option<T>`
/// from these functions makes it difficult for the caller to reason
/// about the result in constant time, and returning an incorrect
/// value burdens the caller and increases the chance of bugs.
#[derive(Clone, Copy, Debug)]
pub struct CtOption<T> {
value: T,
is_some: Choice,
}
impl<T> From<CtOption<T>> for Option<T> {
/// Convert the `CtOption<T>` wrapper into an `Option<T>`, depending on whether
/// the underlying `is_some` `Choice` was a `0` or a `1` once unwrapped.
///
/// # Note
///
/// This function exists to avoid ending up with ugly, verbose and/or bad handled
/// conversions from the `CtOption<T>` wraps to an `Option<T>` or `Result<T, E>`.
/// This implementation doesn't intend to be constant-time nor try to protect the
/// leakage of the `T` since the `Option<T>` will do it anyways.
fn from(source: CtOption<T>) -> Option<T> {
if source.is_some().unwrap_u8() == 1u8 {
Option::Some(source.value)
} else {
None
}
}
}
impl<T> CtOption<T> {
/// This method is used to construct a new `CtOption<T>` and takes
/// a value of type `T`, and a `Choice` that determines whether
/// the optional value should be `Some` or not. If `is_some` is
/// false, the value will still be stored but its value is never
/// exposed.
#[inline]
pub fn new(value: T, is_some: Choice) -> CtOption<T> {
CtOption {
value: value,
is_some: is_some,
}
}
/// Returns the contained value, consuming the `self` value.
///
/// # Panics
///
/// Panics if the value is none with a custom panic message provided by
/// `msg`.
pub fn expect(self, msg: &str) -> T {
assert_eq!(self.is_some.unwrap_u8(), 1, "{}", msg);
self.value
}
/// This returns the underlying value but panics if it
/// is not `Some`.
#[inline]
pub fn unwrap(self) -> T {
assert_eq!(self.is_some.unwrap_u8(), 1);
self.value
}
/// This returns the underlying value if it is `Some`
/// or the provided value otherwise.
#[inline]
pub fn unwrap_or(self, def: T) -> T
where
T: ConditionallySelectable,
{
T::conditional_select(&def, &self.value, self.is_some)
}
/// This returns the underlying value if it is `Some`
/// or the value produced by the provided closure otherwise.
///
/// This operates in constant time, because the provided closure
/// is always called.
#[inline]
pub fn unwrap_or_else<F>(self, f: F) -> T
where
T: ConditionallySelectable,
F: FnOnce() -> T,
{
T::conditional_select(&f(), &self.value, self.is_some)
}
/// Returns a true `Choice` if this value is `Some`.
#[inline]
pub fn is_some(&self) -> Choice {
self.is_some
}
/// Returns a true `Choice` if this value is `None`.
#[inline]
pub fn is_none(&self) -> Choice {
!self.is_some
}
/// Returns a `None` value if the option is `None`, otherwise
/// returns a `CtOption` enclosing the value of the provided closure.
/// The closure is given the enclosed value or, if the option is
/// `None`, it is provided a dummy value computed using
/// `Default::default()`.
///
/// This operates in constant time, because the provided closure
/// is always called.
#[inline]
pub fn map<U, F>(self, f: F) -> CtOption<U>
where
T: Default + ConditionallySelectable,
F: FnOnce(T) -> U,
{
CtOption::new(
f(T::conditional_select(
&T::default(),
&self.value,
self.is_some,
)),
self.is_some,
)
}
/// Returns a `None` value if the option is `None`, otherwise
/// returns the result of the provided closure. The closure is
/// given the enclosed value or, if the option is `None`, it
/// is provided a dummy value computed using `Default::default()`.
///
/// This operates in constant time, because the provided closure
/// is always called.
#[inline]
pub fn and_then<U, F>(self, f: F) -> CtOption<U>
where
T: Default + ConditionallySelectable,
F: FnOnce(T) -> CtOption<U>,
{
let mut tmp = f(T::conditional_select(
&T::default(),
&self.value,
self.is_some,
));
tmp.is_some &= self.is_some;
tmp
}
/// Returns `self` if it contains a value, and otherwise returns the result of
/// calling `f`. The provided function `f` is always called.
#[inline]
pub fn or_else<F>(self, f: F) -> CtOption<T>
where
T: ConditionallySelectable,
F: FnOnce() -> CtOption<T>,
{
let is_none = self.is_none();
let f = f();
Self::conditional_select(&self, &f, is_none)
}
}
impl<T: ConditionallySelectable> ConditionallySelectable for CtOption<T> {
fn conditional_select(a: &Self, b: &Self, choice: Choice) -> Self {
CtOption::new(
T::conditional_select(&a.value, &b.value, choice),
Choice::conditional_select(&a.is_some, &b.is_some, choice),
)
}
}
impl<T: ConstantTimeEq> ConstantTimeEq for CtOption<T> {
/// Two `CtOption<T>`s are equal if they are both `Some` and
/// their values are equal, or both `None`.
#[inline]
fn ct_eq(&self, rhs: &CtOption<T>) -> Choice {
let a = self.is_some();
let b = rhs.is_some();
(a & b & self.value.ct_eq(&rhs.value)) | (!a & !b)
}
}
/// A type which can be compared in some manner and be determined to be greater
/// than another of the same type.
pub trait ConstantTimeGreater {
/// Determine whether `self > other`.
///
/// The bitwise-NOT of the return value of this function should be usable to
/// determine if `self <= other`.
///
/// This function should execute in constant time.
///
/// # Returns
///
/// A `Choice` with a set bit if `self > other`, and with no set bits
/// otherwise.
///
/// # Example
///
/// ```
/// use subtle::ConstantTimeGreater;
///
/// let x: u8 = 13;
/// let y: u8 = 42;
///
/// let x_gt_y = x.ct_gt(&y);
///
/// assert_eq!(x_gt_y.unwrap_u8(), 0);
///
/// let y_gt_x = y.ct_gt(&x);
///
/// assert_eq!(y_gt_x.unwrap_u8(), 1);
///
/// let x_gt_x = x.ct_gt(&x);
///
/// assert_eq!(x_gt_x.unwrap_u8(), 0);
/// ```
fn ct_gt(&self, other: &Self) -> Choice;
}
macro_rules! generate_unsigned_integer_greater {
($t_u: ty, $bit_width: expr) => {
impl ConstantTimeGreater for $t_u {
/// Returns Choice::from(1) iff x > y, and Choice::from(0) iff x <= y.
///
/// # Note
///
/// This algoritm would also work for signed integers if we first
/// flip the top bit, e.g. `let x: u8 = x ^ 0x80`, etc.
#[inline]
fn ct_gt(&self, other: &$t_u) -> Choice {
let gtb = self & !other; // All the bits in self that are greater than their corresponding bits in other.
let mut ltb = !self & other; // All the bits in self that are less than their corresponding bits in other.
let mut pow = 1;
// Less-than operator is okay here because it's dependent on the bit-width.
while pow < $bit_width {
ltb |= ltb >> pow; // Bit-smear the highest set bit to the right.
pow += pow;
}
let mut bit = gtb & !ltb; // Select the highest set bit.
let mut pow = 1;
while pow < $bit_width {
bit |= bit >> pow; // Shift it to the right until we end up with either 0 or 1.
pow += pow;
}
// XXX We should possibly do the above flattening to 0 or 1 in the
// Choice constructor rather than making it a debug error?
Choice::from((bit & 1) as u8)
}
}
};
}
generate_unsigned_integer_greater!(u8, 8);
generate_unsigned_integer_greater!(u16, 16);
generate_unsigned_integer_greater!(u32, 32);
generate_unsigned_integer_greater!(u64, 64);
#[cfg(feature = "i128")]
generate_unsigned_integer_greater!(u128, 128);
impl ConstantTimeGreater for cmp::Ordering {
#[inline]
fn ct_gt(&self, other: &Self) -> Choice {
// No impl of `ConstantTimeGreater` for `i8`, so use `u8`
let a = (*self as i8) + 1;
let b = (*other as i8) + 1;
(a as u8).ct_gt(&(b as u8))
}
}
/// 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 {
/// Determine whether `self < other`.
///
/// The bitwise-NOT of the return value of this function should be usable to
/// determine if `self >= other`.
///
/// A default implementation is provided and implemented for the unsigned
/// integer types.
///
/// This function should execute in constant time.
///
/// # Returns
///
/// A `Choice` with a set bit if `self < other`, and with no set bits
/// otherwise.
///
/// # Example
///
/// ```
/// use subtle::ConstantTimeLess;
///
/// let x: u8 = 13;
/// let y: u8 = 42;
///
/// let x_lt_y = x.ct_lt(&y);
///
/// assert_eq!(x_lt_y.unwrap_u8(), 1);
///
/// let y_lt_x = y.ct_lt(&x);
///
/// assert_eq!(y_lt_x.unwrap_u8(), 0);
///
/// let x_lt_x = x.ct_lt(&x);
///
/// assert_eq!(x_lt_x.unwrap_u8(), 0);
/// ```
#[inline]
fn ct_lt(&self, other: &Self) -> Choice {
!self.ct_gt(other) & !self.ct_eq(other)
}
}
impl ConstantTimeLess for u8 {}
impl ConstantTimeLess for u16 {}
impl ConstantTimeLess for u32 {}
impl ConstantTimeLess for u64 {}
#[cfg(feature = "i128")]
impl ConstantTimeLess for u128 {}
impl ConstantTimeLess for cmp::Ordering {
#[inline]
fn ct_lt(&self, other: &Self) -> Choice {
// No impl of `ConstantTimeLess` for `i8`, so use `u8`
let a = (*self as i8) + 1;
let b = (*other as i8) + 1;
(a as u8).ct_lt(&(b as u8))
}
}