crypto_bigint/modular/monty_form/
sub.rs

1//! Subtractions between integers in Montgomery form with a modulus set at runtime.
2
3use super::MontyForm;
4use crate::modular::sub::sub_montgomery_form;
5use core::ops::{Sub, SubAssign};
6
7impl<const LIMBS: usize> MontyForm<LIMBS> {
8    /// Subtracts `rhs`.
9    pub const fn sub(&self, rhs: &Self) -> Self {
10        Self {
11            montgomery_form: sub_montgomery_form(
12                &self.montgomery_form,
13                &rhs.montgomery_form,
14                &self.params.modulus,
15            ),
16            params: self.params,
17        }
18    }
19}
20
21impl<const LIMBS: usize> Sub<&MontyForm<LIMBS>> for &MontyForm<LIMBS> {
22    type Output = MontyForm<LIMBS>;
23    fn sub(self, rhs: &MontyForm<LIMBS>) -> MontyForm<LIMBS> {
24        debug_assert_eq!(self.params, rhs.params);
25        self.sub(rhs)
26    }
27}
28
29impl<const LIMBS: usize> Sub<MontyForm<LIMBS>> for &MontyForm<LIMBS> {
30    type Output = MontyForm<LIMBS>;
31    #[allow(clippy::op_ref)]
32    fn sub(self, rhs: MontyForm<LIMBS>) -> MontyForm<LIMBS> {
33        self - &rhs
34    }
35}
36
37impl<const LIMBS: usize> Sub<&MontyForm<LIMBS>> for MontyForm<LIMBS> {
38    type Output = MontyForm<LIMBS>;
39    #[allow(clippy::op_ref)]
40    fn sub(self, rhs: &MontyForm<LIMBS>) -> MontyForm<LIMBS> {
41        &self - rhs
42    }
43}
44
45impl<const LIMBS: usize> Sub<MontyForm<LIMBS>> for MontyForm<LIMBS> {
46    type Output = MontyForm<LIMBS>;
47    fn sub(self, rhs: MontyForm<LIMBS>) -> MontyForm<LIMBS> {
48        &self - &rhs
49    }
50}
51
52impl<const LIMBS: usize> SubAssign<&MontyForm<LIMBS>> for MontyForm<LIMBS> {
53    fn sub_assign(&mut self, rhs: &MontyForm<LIMBS>) {
54        *self = *self - rhs;
55    }
56}
57
58impl<const LIMBS: usize> SubAssign<MontyForm<LIMBS>> for MontyForm<LIMBS> {
59    fn sub_assign(&mut self, rhs: MontyForm<LIMBS>) {
60        *self -= &rhs;
61    }
62}
63
64#[cfg(test)]
65mod tests {
66    use crate::{
67        modular::{MontyForm, MontyParams},
68        Odd, U256,
69    };
70
71    #[test]
72    fn sub_overflow() {
73        let params = MontyParams::new_vartime(Odd::<U256>::from_be_hex(
74            "ffffffff00000000ffffffffffffffffbce6faada7179e84f3b9cac2fc632551",
75        ));
76
77        let x =
78            U256::from_be_hex("44acf6b7e36c1342c2c5897204fe09504e1e2efb1a900377dbc4e7a6a133ec56");
79        let mut x_mod = MontyForm::new(&x, params);
80
81        let y =
82            U256::from_be_hex("d5777c45019673125ad240f83094d4252d829516fac8601ed01979ec1ec1a251");
83        let y_mod = MontyForm::new(&y, params);
84
85        x_mod -= &y_mod;
86
87        let expected =
88            U256::from_be_hex("6f357a71e1d5a03167f34879d469352add829491c6df41ddff65387d7ed56f56");
89
90        assert_eq!(expected, x_mod.retrieve());
91    }
92}
crypto_bigint/modular/monty_form/sub.rs

crypto_bigint/modular/monty_form/
sub.rs
