vendor/crypto-bigint/src/uint/neg_mod.rs

   1 //! [`UInt`] negation modulus operations.
   2
   3 use crate::{Limb, NegMod, UInt};
   4
   5 impl<const LIMBS: usize> UInt<LIMBS> {
   6     /// Computes `-a mod p` in constant time.
   7     /// Assumes `self` is in `[0, p)`.
   8     pub const fn neg_mod(&self, p: &Self) -> Self {
   9         let z = self.ct_is_nonzero();
  10         let mut ret = p.sbb(self, Limb::ZERO).0;
  11         let mut i = 0;
  12         while i < LIMBS {
  13             // Set ret to 0 if the original value was 0, in which
  14             // case ret would be p.
  15             ret.limbs[i].0 &= z;
  16             i += 1;
  17         }
  18         ret
  19     }
  20
  21     /// Computes `-a mod p` in constant time for the special modulus
  22     /// `p = MAX+1-c` where `c` is small enough to fit in a single [`Limb`].
  23     pub const fn neg_mod_special(&self, c: Limb) -> Self {
  24         Self::ZERO.sub_mod_special(self, c)
  25     }
  26 }
  27
  28 impl<const LIMBS: usize> NegMod for UInt<LIMBS> {
  29     type Output = Self;
  30
  31     fn neg_mod(&self, p: &Self) -> Self {
  32         debug_assert!(self < p);
  33         self.neg_mod(p)
  34     }
  35 }
  36
  37 #[cfg(test)]
  38 mod tests {
  39     use crate::U256;
  40
  41     #[test]
  42     fn neg_mod_random() {
  43         let x =
  44             U256::from_be_hex("8d16e171674b4e6d8529edba4593802bf30b8cb161dd30aa8e550d41380007c2");
  45         let p =
  46             U256::from_be_hex("928334a4e4be0843ec225a4c9c61df34bdc7a81513e4b6f76f2bfa3148e2e1b5");
  47
  48         let actual = x.neg_mod(&p);
  49         let expected =
  50             U256::from_be_hex("056c53337d72b9d666f86c9256ce5f08cabc1b63b207864ce0d6ecf010e2d9f3");
  51
  52         assert_eq!(expected, actual);
  53     }
  54
  55     #[test]
  56     fn neg_mod_zero() {
  57         let x =
  58             U256::from_be_hex("0000000000000000000000000000000000000000000000000000000000000000");
  59         let p =
  60             U256::from_be_hex("928334a4e4be0843ec225a4c9c61df34bdc7a81513e4b6f76f2bfa3148e2e1b5");
  61
  62         let actual = x.neg_mod(&p);
  63         let expected =
  64             U256::from_be_hex("0000000000000000000000000000000000000000000000000000000000000000");
  65
  66         assert_eq!(expected, actual);
  67     }
  68 }