crypto_bigint/uint/boxed/
neg_mod.rs

1//! [`BoxedUint`] modular negation operations.
2
3use crate::{BoxedUint, CtAssign, Limb, NegMod, NonZero};
4
5impl BoxedUint {
6    /// Computes `-a mod p`.
7    /// Assumes `self` is in `[0, p)`.
8    #[must_use]
9    pub fn neg_mod(&self, p: &NonZero<Self>) -> Self {
10        debug_assert_eq!(self.bits_precision(), p.bits_precision());
11        let is_zero = self.is_zero();
12        let mut ret = p.borrowing_sub(self, Limb::ZERO).0;
13
14        for i in 0..self.nlimbs() {
15            // Set ret to 0 if the original value was 0, in which
16            // case ret would be p.
17            ret.limbs[i].ct_assign(&Limb::ZERO, is_zero);
18        }
19
20        ret
21    }
22
23    /// Computes `-a mod p` for the special modulus
24    /// `p = MAX+1-c` where `c` is small enough to fit in a single [`Limb`].
25    #[must_use]
26    pub fn neg_mod_special(&self, c: Limb) -> Self {
27        Self::zero_with_precision(self.bits_precision()).sub_mod_special(self, c)
28    }
29}
30
31impl NegMod for BoxedUint {
32    type Output = Self;
33
34    fn neg_mod(&self, p: &NonZero<Self>) -> Self {
35        debug_assert!(self < p.as_ref());
36        self.neg_mod(p)
37    }
38}
39
40#[cfg(test)]
41mod tests {
42    use crate::BoxedUint;
43    use hex_literal::hex;
44
45    #[test]
46    fn neg_mod_random() {
47        let x = BoxedUint::from_be_slice(
48            &hex!("8d16e171674b4e6d8529edba4593802bf30b8cb161dd30aa8e550d41380007c2"),
49            256,
50        )
51        .unwrap();
52        let p = BoxedUint::from_be_slice(
53            &hex!("928334a4e4be0843ec225a4c9c61df34bdc7a81513e4b6f76f2bfa3148e2e1b5"),
54            256,
55        )
56        .unwrap()
57        .to_nz()
58        .unwrap();
59
60        let actual = x.neg_mod(&p);
61        let expected = BoxedUint::from_be_slice(
62            &hex!("056c53337d72b9d666f86c9256ce5f08cabc1b63b207864ce0d6ecf010e2d9f3"),
63            256,
64        )
65        .unwrap();
66
67        assert_eq!(expected, actual);
68    }
69
70    #[test]
71    fn neg_mod_zero() {
72        let x = BoxedUint::from_be_slice(
73            &hex!("0000000000000000000000000000000000000000000000000000000000000000"),
74            256,
75        )
76        .unwrap();
77        let p = BoxedUint::from_be_slice(
78            &hex!("928334a4e4be0843ec225a4c9c61df34bdc7a81513e4b6f76f2bfa3148e2e1b5"),
79            256,
80        )
81        .unwrap()
82        .to_nz()
83        .unwrap();
84
85        let actual = x.neg_mod(&p);
86        let expected = BoxedUint::from_be_slice(
87            &hex!("0000000000000000000000000000000000000000000000000000000000000000"),
88            256,
89        )
90        .unwrap();
91
92        assert_eq!(expected, actual);
93    }
94}
crypto_bigint/uint/boxed/neg_mod.rs

crypto_bigint/uint/boxed/
neg_mod.rs
