crypto_bigint/
checked.rs

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1//! Checked arithmetic.
2
3use subtle::{Choice, ConditionallySelectable, ConstantTimeEq, CtOption};
4
5#[cfg(feature = "serde")]
6use serdect::serde::{Deserialize, Deserializer, Serialize, Serializer};
7
8/// Provides intentionally-checked arithmetic on `T`.
9///
10/// Internally this leverages the [`CtOption`] type from the [`subtle`] crate
11/// in order to handle overflows in constant time.
12#[derive(Copy, Clone, Debug)]
13pub struct Checked<T>(pub CtOption<T>);
14
15impl<T> Checked<T> {
16    /// Create a new checked arithmetic wrapper for the given value.
17    pub fn new(val: T) -> Self {
18        Self(CtOption::new(val, Choice::from(1)))
19    }
20}
21
22impl<T> Default for Checked<T>
23where
24    T: Default,
25{
26    fn default() -> Self {
27        Self::new(T::default())
28    }
29}
30
31impl<T: ConditionallySelectable> ConditionallySelectable for Checked<T> {
32    #[inline]
33    fn conditional_select(a: &Self, b: &Self, choice: Choice) -> Self {
34        Self(CtOption::conditional_select(&a.0, &b.0, choice))
35    }
36}
37
38impl<T: ConstantTimeEq> ConstantTimeEq for Checked<T> {
39    #[inline]
40    fn ct_eq(&self, rhs: &Self) -> Choice {
41        self.0.ct_eq(&rhs.0)
42    }
43}
44
45impl<T> From<Checked<T>> for CtOption<T> {
46    fn from(checked: Checked<T>) -> CtOption<T> {
47        checked.0
48    }
49}
50
51impl<T> From<CtOption<T>> for Checked<T> {
52    fn from(ct_option: CtOption<T>) -> Checked<T> {
53        Checked(ct_option)
54    }
55}
56
57impl<T> From<Checked<T>> for Option<T> {
58    fn from(checked: Checked<T>) -> Option<T> {
59        checked.0.into()
60    }
61}
62
63#[cfg(feature = "serde")]
64#[cfg_attr(docsrs, doc(cfg(feature = "serde")))]
65impl<'de, T: Default + Deserialize<'de>> Deserialize<'de> for Checked<T> {
66    fn deserialize<D>(deserializer: D) -> Result<Self, D::Error>
67    where
68        D: Deserializer<'de>,
69    {
70        let value = Option::<T>::deserialize(deserializer)?;
71        let choice = Choice::from(value.is_some() as u8);
72        Ok(Self(CtOption::new(value.unwrap_or_default(), choice)))
73    }
74}
75
76#[cfg(feature = "serde")]
77#[cfg_attr(docsrs, doc(cfg(feature = "serde")))]
78impl<'de, T: Copy + Serialize> Serialize for Checked<T> {
79    fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
80    where
81        S: Serializer,
82    {
83        Option::<T>::from(self.0).serialize(serializer)
84    }
85}
86
87#[cfg(all(test, feature = "serde"))]
88mod tests {
89    use crate::{Checked, U64};
90    use subtle::{Choice, ConstantTimeEq, CtOption};
91
92    #[test]
93    fn serde() {
94        let test = Checked::new(U64::from_u64(0x0011223344556677));
95
96        let serialized = bincode::serialize(&test).unwrap();
97        let deserialized: Checked<U64> = bincode::deserialize(&serialized).unwrap();
98
99        assert!(bool::from(test.ct_eq(&deserialized)));
100
101        let test = Checked::new(U64::ZERO) - Checked::new(U64::ONE);
102        assert!(bool::from(
103            test.ct_eq(&Checked(CtOption::new(U64::ZERO, Choice::from(0))))
104        ));
105
106        let serialized = bincode::serialize(&test).unwrap();
107        let deserialized: Checked<U64> = bincode::deserialize(&serialized).unwrap();
108
109        assert!(bool::from(test.ct_eq(&deserialized)));
110    }
111
112    #[test]
113    fn serde_owned() {
114        let test = Checked::new(U64::from_u64(0x0011223344556677));
115
116        let serialized = bincode::serialize(&test).unwrap();
117        let deserialized: Checked<U64> = bincode::deserialize_from(serialized.as_slice()).unwrap();
118
119        assert!(bool::from(test.ct_eq(&deserialized)));
120
121        let test = Checked::new(U64::ZERO) - Checked::new(U64::ONE);
122        assert!(bool::from(
123            test.ct_eq(&Checked(CtOption::new(U64::ZERO, Choice::from(0))))
124        ));
125
126        let serialized = bincode::serialize(&test).unwrap();
127        let deserialized: Checked<U64> = bincode::deserialize_from(serialized.as_slice()).unwrap();
128
129        assert!(bool::from(test.ct_eq(&deserialized)));
130    }
131}
crypto_bigint/checked.rs

crypto_bigint/
checked.rs
