subtle-1.0.0/src/lib.rs

// -*- 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);

/// Select one of two inputs according to a `Choice` in constant time.
///
/// # Examples
///
/// ```
/// # use subtle;
/// use subtle::ConditionallySelectable;
/// use subtle::Choice;
/// let a: i32 = 5;
/// let b: i32 = 13;
///
/// assert_eq!(i32::conditional_select(&a, &b, Choice::from(0)), a);
/// assert_eq!(i32::conditional_select(&a, &b, Choice::from(1)), b);
/// ```
pub trait ConditionallySelectable {
    /// 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.
    #[inline]
    fn conditional_select(a: &Self, b: &Self, choice: Choice) -> Self;
}

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))
            }
         }
    )*)
}

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
/// which are `ConditionallyNegatable + Neg`.
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: ConditionallyAssignable,
    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);
    }
}

/// A type which can be conditionally assigned in constant time.
pub trait ConditionallyAssignable {
    /// Conditionally assign `other` to `self`, according to `choice`.
    ///
    /// This function should execute in constant time.
    ///
    /// # Examples
    ///
    /// ```
    /// # extern crate subtle;
    /// use subtle::ConditionallyAssignable;
    /// #
    /// # fn main() {
    /// let mut x: u8 = 13;
    /// let 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);
}

impl<T> ConditionallyAssignable for T
where
    T: ConditionallySelectable,
{
    #[inline]
    fn conditional_assign(&mut self, other: &Self, choice: Choice) {
        *self = T::conditional_select(self, other, choice);
    }
}

/// A type which is conditionally swappable in constant time.
pub trait ConditionallySwappable {
    /// Conditionally swap `self` and `other` if `choice == 1`; otherwise,
    /// reassign both unto themselves.
    ///
    /// # Note
    ///
    /// This trait is generically implemented for any type which implements
    /// `ConditionallyAssignable + Copy`.
    #[inline]
    fn conditional_swap(&mut self, other: &mut Self, choice: Choice);
}

impl<T> ConditionallySwappable for T
where
    T: ConditionallyAssignable + Copy,
{
    #[inline]
    fn conditional_swap(&mut self, other: &mut T, choice: Choice) {
        let temp: T = *self;
        self.conditional_assign(&other, choice);
        other.conditional_assign(&temp, choice);
    }
}