1 files changed, 55 insertions, 0 deletions
diff --git a/vendor/crypto-bigint/src/uint/modular/reduction.rs b/vendor/crypto-bigint/src/uint/modular/reduction.rs
new file mode 100644
index 0000000..b206ae3
--- /dev/null
+++ b/vendor/crypto-bigint/src/uint/modular/reduction.rs
@@ -0,0 +1,55 @@
+use crate::{Limb, Uint, WideWord, Word};
+
+/// Returns `(hi, lo)` such that `hi * R + lo = x * y + z + w`.
+#[inline(always)]
+const fn muladdcarry(x: Word, y: Word, z: Word, w: Word) -> (Word, Word) {
+    let res = (x as WideWord)
+        .wrapping_mul(y as WideWord)
+        .wrapping_add(z as WideWord)
+        .wrapping_add(w as WideWord);
+    ((res >> Word::BITS) as Word, res as Word)
+}
+
+/// Algorithm 14.32 in Handbook of Applied Cryptography <https://cacr.uwaterloo.ca/hac/about/chap14.pdf>
+pub const fn montgomery_reduction<const LIMBS: usize>(
+    lower_upper: &(Uint<LIMBS>, Uint<LIMBS>),
+    modulus: &Uint<LIMBS>,
+    mod_neg_inv: Limb,
+) -> Uint<LIMBS> {
+    let (mut lower, mut upper) = *lower_upper;
+
+    let mut meta_carry = Limb(0);
+    let mut new_sum;
+
+    let mut i = 0;
+    while i < LIMBS {
+        let u = lower.limbs[i].0.wrapping_mul(mod_neg_inv.0);
+
+        let (mut carry, _) = muladdcarry(u, modulus.limbs[0].0, lower.limbs[i].0, 0);
+        let mut new_limb;
+
+        let mut j = 1;
+        while j < (LIMBS - i) {
+            (carry, new_limb) = muladdcarry(u, modulus.limbs[j].0, lower.limbs[i + j].0, carry);
+            lower.limbs[i + j] = Limb(new_limb);
+            j += 1;
+        }
+        while j < LIMBS {
+            (carry, new_limb) =
+                muladdcarry(u, modulus.limbs[j].0, upper.limbs[i + j - LIMBS].0, carry);
+            upper.limbs[i + j - LIMBS] = Limb(new_limb);
+            j += 1;
+        }
+
+        (new_sum, meta_carry) = upper.limbs[i].adc(Limb(carry), meta_carry);
+        upper.limbs[i] = new_sum;
+
+        i += 1;
+    }
+
+    // Division is simply taking the upper half of the limbs
+    // Final reduction (at this point, the value is at most 2 * modulus,
+    // so `meta_carry` is either 0 or 1)
+
+    upper.sub_mod_with_carry(meta_carry, modulus, modulus)
+}