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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]
#![cfg_attr(feature = "nightly", feature(asm))]
#![cfg_attr(feature = "nightly", feature(external_doc))]
#![cfg_attr(feature = "nightly", doc(include = "../README.md"))]
#![cfg_attr(feature = "nightly", deny(missing_docs))]
#![doc(html_logo_url = "https://doc.dalek.rs/assets/dalek-logo-clear.png")]
//! Note that docs will only build on nightly Rust until
//! [RFC 1990 stabilizes](https://github.com/rust-lang/rust/issues/44732).
#[cfg(feature = "std")]
#[macro_use]
extern crate std;
use core::ops::{BitAnd, BitAndAssign, BitOr, BitOrAssign, BitXor, BitXorAssign, Neg, Not};
/// 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).
///
/// With the `nightly` feature enabled, 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]: https://www.chosenplaintext.ca/open-source/rust-timing-shield/security
#[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.
#[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.
///
/// Uses inline asm when available, otherwise it's a no-op.
#[cfg(all(
feature = "nightly",
not(any(target_arch = "asmjs", target_arch = "wasm32"))
))]
fn black_box(input: u8) -> u8 {
debug_assert!(input == 0u8 || input == 1u8);
// Pretend to access a register containing the input. We "volatile" here
// because some optimisers treat assembly templates without output operands
// as "volatile" while others do not.
unsafe { asm!("" :: "r"(&input) :: "volatile") }
input
}
#[cfg(any(
target_arch = "asmjs",
target_arch = "wasm32",
not(feature = "nightly")
))]
#[inline(never)]
fn black_box(input: u8) -> u8 {
debug_assert!(input == 0u8 || input == 1u8);
// We don't have access to inline assembly or test::black_box or ...
//
// Bailing out, hopefully the compiler doesn't use the fact that `input` is 0 or 1.
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;
}
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()
}
}
/// 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 {
// First construct x such that self == other iff all bits of x are 1
let mut x: $t_u = !(self ^ other);
// Now compute the and of all bits of x.
//
// e.g. for a u8, do:
//
// x &= x >> 4;
// x &= x >> 2;
// x &= x >> 1;
//
let mut shift: usize = $bit_width / 2;
while shift >= 1 {
x &= x >> shift;
shift /= 2;
}
(x 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);
/// 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
///
/// ```
/// # extern crate subtle;
/// 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
///
/// ```
/// # extern crate subtle;
/// 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
///
/// ```
/// # extern crate subtle;
/// 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);
/// 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);
}
}