1
  2
  3
  4
  5
  6
  7
  8
  9
 10
 11
 12
 13
 14
 15
 16
 17
 18
 19
 20
 21
 22
 23
 24
 25
 26
 27
 28
 29
 30
 31
 32
 33
 34
 35
 36
 37
 38
 39
 40
 41
 42
 43
 44
 45
 46
 47
 48
 49
 50
 51
 52
 53
 54
 55
 56
 57
 58
 59
 60
 61
 62
 63
 64
 65
 66
 67
 68
 69
 70
 71
 72
 73
 74
 75
 76
 77
 78
 79
 80
 81
 82
 83
 84
 85
 86
 87
 88
 89
 90
 91
 92
 93
 94
 95
 96
 97
 98
 99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
// -*- 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);
    }
}