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
|
//! [`Uint`] bitwise right shift operations.
use super::Uint;
use crate::{limb::HI_BIT, CtChoice, Limb, Word};
use core::ops::{Shr, ShrAssign};
impl<const LIMBS: usize> Uint<LIMBS> {
/// Computes `self >> 1` in constant-time, returning [`CtChoice::TRUE`] if the overflowing bit
/// was set, and [`CtChoice::FALSE`] otherwise.
pub(crate) const fn shr_1(&self) -> (Self, CtChoice) {
let mut shifted_bits = [0; LIMBS];
let mut i = 0;
while i < LIMBS {
shifted_bits[i] = self.limbs[i].0 >> 1;
i += 1;
}
let mut carry_bits = [0; LIMBS];
let mut i = 0;
while i < LIMBS {
carry_bits[i] = self.limbs[i].0 << HI_BIT;
i += 1;
}
let mut limbs = [Limb(0); LIMBS];
let mut i = 0;
while i < (LIMBS - 1) {
limbs[i] = Limb(shifted_bits[i] | carry_bits[i + 1]);
i += 1;
}
limbs[LIMBS - 1] = Limb(shifted_bits[LIMBS - 1]);
debug_assert!(carry_bits[LIMBS - 1] == 0 || carry_bits[LIMBS - 1] == (1 << HI_BIT));
(
Uint::new(limbs),
CtChoice::from_lsb(carry_bits[0] >> HI_BIT),
)
}
/// Computes `self >> n`.
///
/// NOTE: this operation is variable time with respect to `n` *ONLY*.
///
/// When used with a fixed `n`, this function is constant-time with respect
/// to `self`.
#[inline(always)]
pub const fn shr_vartime(&self, shift: usize) -> Self {
let full_shifts = shift / Limb::BITS;
let small_shift = shift & (Limb::BITS - 1);
let mut limbs = [Limb::ZERO; LIMBS];
if shift > Limb::BITS * LIMBS {
return Self { limbs };
}
let n = LIMBS - full_shifts;
let mut i = 0;
if small_shift == 0 {
while i < n {
limbs[i] = Limb(self.limbs[i + full_shifts].0);
i += 1;
}
} else {
while i < n {
let mut lo = self.limbs[i + full_shifts].0 >> small_shift;
if i < (LIMBS - 1) - full_shifts {
lo |= self.limbs[i + full_shifts + 1].0 << (Limb::BITS - small_shift);
}
limbs[i] = Limb(lo);
i += 1;
}
}
Self { limbs }
}
/// Computes a right shift on a wide input as `(lo, hi)`.
///
/// NOTE: this operation is variable time with respect to `n` *ONLY*.
///
/// When used with a fixed `n`, this function is constant-time with respect
/// to `self`.
#[inline(always)]
pub const fn shr_vartime_wide(lower_upper: (Self, Self), n: usize) -> (Self, Self) {
let (mut lower, upper) = lower_upper;
let new_upper = upper.shr_vartime(n);
lower = lower.shr_vartime(n);
if n >= Self::BITS {
lower = lower.bitor(&upper.shr_vartime(n - Self::BITS));
} else {
lower = lower.bitor(&upper.shl_vartime(Self::BITS - n));
}
(lower, new_upper)
}
/// Computes `self << n`.
/// Returns zero if `n >= Self::BITS`.
pub const fn shr(&self, shift: usize) -> Self {
let overflow = CtChoice::from_usize_lt(shift, Self::BITS).not();
let shift = shift % Self::BITS;
let mut result = *self;
let mut i = 0;
while i < Self::LOG2_BITS {
let bit = CtChoice::from_lsb((shift as Word >> i) & 1);
result = Uint::ct_select(&result, &result.shr_vartime(1 << i), bit);
i += 1;
}
Uint::ct_select(&result, &Self::ZERO, overflow)
}
}
impl<const LIMBS: usize> Shr<usize> for Uint<LIMBS> {
type Output = Uint<LIMBS>;
/// NOTE: this operation is variable time with respect to `rhs` *ONLY*.
///
/// When used with a fixed `rhs`, this function is constant-time with respect
/// to `self`.
fn shr(self, rhs: usize) -> Uint<LIMBS> {
Uint::<LIMBS>::shr(&self, rhs)
}
}
impl<const LIMBS: usize> Shr<usize> for &Uint<LIMBS> {
type Output = Uint<LIMBS>;
/// NOTE: this operation is variable time with respect to `rhs` *ONLY*.
///
/// When used with a fixed `rhs`, this function is constant-time with respect
/// to `self`.
fn shr(self, rhs: usize) -> Uint<LIMBS> {
self.shr(rhs)
}
}
impl<const LIMBS: usize> ShrAssign<usize> for Uint<LIMBS> {
fn shr_assign(&mut self, rhs: usize) {
*self = self.shr(rhs);
}
}
#[cfg(test)]
mod tests {
use crate::{Uint, U128, U256};
const N: U256 =
U256::from_be_hex("FFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFEBAAEDCE6AF48A03BBFD25E8CD0364141");
const N_2: U256 =
U256::from_be_hex("7FFFFFFFFFFFFFFFFFFFFFFFFFFFFFFF5D576E7357A4501DDFE92F46681B20A0");
#[test]
fn shr1() {
assert_eq!(N >> 1, N_2);
}
#[test]
fn shr_wide_1_1_128() {
assert_eq!(
Uint::shr_vartime_wide((U128::ONE, U128::ONE), 128),
(U128::ONE, U128::ZERO)
);
}
#[test]
fn shr_wide_0_max_1() {
assert_eq!(
Uint::shr_vartime_wide((U128::ZERO, U128::MAX), 1),
(U128::ONE << 127, U128::MAX >> 1)
);
}
#[test]
fn shr_wide_max_max_256() {
assert_eq!(
Uint::shr_vartime_wide((U128::MAX, U128::MAX), 256),
(U128::ZERO, U128::ZERO)
);
}
}
|
