Int - inv_mod (#737)

Author: erik-3milabs

src/int.rs

@@ -20,6 +20,7 @@ mod cmp;
 mod div;
 mod encoding;
 mod from;
+mod inv_mod;
 mod mul;
 mod neg;
 mod resize;

src/int/inv_mod.rs

@@ -0,0 +1,108 @@
+//! Operations related to computing the inverse of an [`Int`] modulo a [`Uint`].
+
+use subtle::CtOption;
+
+use crate::modular::SafeGcdInverter;
+use crate::{ConstantTimeSelect, Int, InvMod, NonZero, Odd, PrecomputeInverter, Uint};
+
+impl<const LIMBS: usize, const UNSAT_LIMBS: usize> Int<LIMBS>
+where
+    Odd<Uint<LIMBS>>: PrecomputeInverter<Inverter = SafeGcdInverter<LIMBS, UNSAT_LIMBS>>,
+{
+    /// Computes the multiplicative inverse of `self` mod `modulus`, where `modulus` is odd.
+    pub fn inv_odd_mod(&self, modulus: &Odd<Uint<LIMBS>>) -> CtOption<Uint<LIMBS>> {
+        let (abs, sgn) = self.abs_sign();
+        let abs_inv = abs.inv_odd_mod(modulus).into();
+
+        // Note: when `self` is negative and modulus is non-zero, then
+        // self^{-1} % modulus = modulus - |self|^{-1} % modulus
+        CtOption::ct_select(
+            &abs_inv,
+            &abs_inv.map(|abs_inv| modulus.wrapping_sub(&abs_inv)),
+            sgn.into(),
+        )
+    }
+}
+
+impl<const LIMBS: usize> InvMod<NonZero<Uint<LIMBS>>> for Int<LIMBS>
+where
+    Uint<LIMBS>: InvMod<Output = Uint<LIMBS>>,
+{
+    type Output = Uint<LIMBS>;
+
+    fn inv_mod(&self, modulus: &NonZero<Uint<LIMBS>>) -> CtOption<Self::Output> {
+        let (abs, sgn) = self.abs_sign();
+        let abs_inv = abs.inv_mod(modulus);
+
+        // Note: when `self` is negative and modulus is non-zero, then
+        // self^{-1} % modulus = modulus - |self|^{-1} % modulus
+        CtOption::ct_select(
+            &abs_inv,
+            &abs_inv.map(|abs_inv| modulus.wrapping_sub(&abs_inv)),
+            sgn.into(),
+        )
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use crate::{InvMod, I1024, U1024};
+
+    #[test]
+    fn test_invert_odd() {
+        let a = I1024::from_be_hex(concat![
+            "FFFDDA166EAC4B985A4BAE6865C0BAE2510C4072939ADE2D05DB444E80D6ABB1",
+            "CB85BED4F9A48A5CAE1568E61DBCF2DB884EE3363063E5291211D934EA0B9C07",
+            "4338D107815CFD7716A5B75586DDD93136A6234F98D270627F5AB344157A3527",
+            "C7D13DDB214D0A87B19D2F33D07E3D1952EB145419B92989B4CF3CD47897767B"
+        ]);
+        let m = U1024::from_be_hex(concat![
+            "D509E7854ABDC81921F669F1DC6F61359523F3949803E58ED4EA8BC16483DC6F",
+            "37BFE27A9AC9EEA2969B357ABC5C0EE214BE16A7D4C58FC620D5B5A20AFF001A",
+            "D198D3155E5799DC4EA76652D64983A7E130B5EACEBAC768D28D589C36EC749C",
+            "558D0B64E37CD0775C0D0104AE7D98BA23C815185DD43CD8B16292FD94156767"
+        ])
+        .to_odd()
+        .unwrap();
+        let expected = U1024::from_be_hex(concat![
+            "24D3C45CFFAF0D5C7620A469C3DC2F3313DDE78A0987F0CEF7EABFF4A5407219",
+            "645BBF1E19580A3619798AAA545597FCA2496DAA2DF4685D313F98F52DD151C3",
+            "48BF956F72C8BDA32FC3F3E5F955226B8D9138C6E64AA568075BA2AEDBE58ED2",
+            "173B01FCA9E1905F9C74589FB3C36D55A4CBCB7FA86CC803BE979091D3F0C431"
+        ]);
+
+        let res = a.inv_odd_mod(&m).unwrap();
+        assert_eq!(res, expected);
+
+        // Even though it is less efficient, it still works
+        let res = a.inv_mod(&m.to_nz().unwrap()).unwrap();
+        assert_eq!(res, expected);
+    }
+
+    #[test]
+    fn test_invert_even() {
+        let a = I1024::from_be_hex(concat![
+            "FFFDDA166EAC4B985A4BAE6865C0BAE2510C4072939ADE2D05DB444E80D6ABB1",
+            "CB85BED4F9A48A5CAE1568E61DBCF2DB884EE3363063E5291211D934EA0B9C07",
+            "4338D107815CFD7716A5B75586DDD93136A6234F98D270627F5AB344157A3527",
+            "C7D13DDB214D0A87B19D2F33D07E3D1952EB145419B92989B4CF3CD47897767B",
+        ]);
+        let m = U1024::from_be_hex(concat![
+            "D509E7854ABDC81921F669F1DC6F61359523F3949803E58ED4EA8BC16483DC6F",
+            "37BFE27A9AC9EEA2969B357ABC5C0EE214BE16A7D4C58FC620D5B5A20AFF001A",
+            "D198D3155E5799DC4EA76652D64983A7E130B5EACEBAC768D28D589C36EC749C",
+            "558D0B64E37CD0775C0D0104AE7D98BA23C815185DD43CD8B16292FD94156000"
+        ])
+        .to_nz()
+        .unwrap();
+        let expected = U1024::from_be_hex(concat![
+            "B64AAE72443C49FD5BE587DDE82A265E8C1226D73E5AE3DC3081E6C5471EE917",
+            "5BC37EF8AE73B8D7F0365652BC335F9BC375EA303381B08D4A15E0CBBF92FDF4",
+            "D58AB97A48B1507553808DC84F9C6F656F39F81D9157AE2E1FD98C487D4D52F8",
+            "F9F1107F0F4064B074637B983CB6672DAD75067A02F0E455DBB6E2CE7D7ED8B3",
+        ]);
+
+        let res = a.inv_mod(&m).unwrap();
+        assert_eq!(res, expected);
+    }
+}