Support modular inverses.

src/cryptonum/gold_tests.rs

@@ -220,3 +220,16 @@ fn barrett_reduction_test()
         assert_eq!(&result, r);
     });
 }
+
+#[test]
+fn modular_inverse_test()
+{
+    run_test("tests/math/modinv.tests", 3, |scase| {
+        let case     = make_unsigned(scase);
+        let a        = case.get("x").unwrap();
+        let m        = case.get("y").unwrap();
+        let r        = case.get("z").unwrap();
+        let result   = a.modinv(m);
+        assert_eq!(r, &result);
+    });
+}

src/cryptonum/unsigned.rs

@@ -130,23 +130,18 @@ impl UCN {
     }
 
     pub fn modinv(&self, phi: &UCN) -> UCN {
-        let (_, mut x, _) = self.extended_euclidean(&phi);
-        let int_phi = SCN::from(phi.clone());
-        while x.is_negative() {
-            x = x + &int_phi;
-        }
-        x.into()
-    }
+        let x = SCN::from(self.clone());
+        let y = SCN::from(phi.clone());
+        let (d, a, _b) = x.egcd(y);
 
-    fn extended_euclidean(&self, b: &UCN) -> (SCN, SCN, SCN) {
-        let posinta = SCN::from(self.clone());
-        let posintb = SCN::from(b.clone());
-        let (d, x, y) = posinta.egcd(posintb);
-
-        if d.is_negative() {
-            (d.neg(), x.neg(), y.neg())
+        if d == SCN::from(1 as u8) {
+            if a.is_negative() {
+                (a + SCN::from(phi.clone())).value
             } else {
-            (d, x, y)
+                a.value
+            }
+        } else {
+            UCN::zero()
         }
     }
 
@@ -1294,5 +1289,9 @@ mod test {
             let fast = a.fastmodexp(&b, &cu);
             slow == fast
         }
+        fn modinv(a: UCN, b: UCN) -> bool {
+            let i = a.modinv(&b);
+            i.is_zero() || ( ((a * i) % b) == UCN::from(1 as u64) )
+        }
     }
 }

tests/math/GenerateMathTests.hs

@@ -137,7 +137,7 @@ main =
              in if m == 0
                    then (Nothing, (), g''')
                    else (Just res, (), g''')
-     _  <- runGenerator g9 "barrett" () $ \ g () ->
+     g10<- runGenerator g9 "barrett" () $ \ g () ->
              let (m, g')  = randomVal (>= 0) g
                  (v, g'') = randomVal (>= 0) g'
                  barrett  = barrett_u m
@@ -151,7 +151,7 @@ main =
                         in if me /= standard
                               then error "Barrett broken"
                               else (Just res, (), g'')
-     _  <- runGenerator g9 "fastmodexp" () $ \ g () ->
+     g11<- runGenerator g10 "fastmodexp" () $ \ g () ->
              let (a, g')   = randomVal (>= 0) g
                  (b, g'')  = randomVal (>= 0) g'
                  (m, g''') = randomVal (>= 0) g'
@@ -164,6 +164,10 @@ main =
                                    ("m", m), ("u", bu barrett),
                                    ("k", fromIntegral (bk barrett))]
                         in (Just res, (), g''')
+     _  <- runGenerator g11 "modinv" Map.empty $
+             buildBasicLimitingGenerator (>= 0) $ \ a b ->
+               let res == recipModInteger a b
+               if b == 0 then Nothing else Just (recipModInteger a b)
      return ()
 
 -- Implement Barrett reduction using incredibly simplistic implementations, to