More tests to ensure that inversion of zero is zero

tests/test_bigints.nim

@@ -441,7 +441,8 @@ proc mainModularInverse() =
 
       check: bool(r == expected)
 
-    test "0^-1 (mod 0) = 0 (need for tower of extension fields)":
+    test "0^-1 (mod any) = 0 (need for tower of extension fields)":
+      block:
         let a = BigInt[16].fromUint(0'u16)
         let M = BigInt[16].fromUint(2017'u16)
 
@@ -456,6 +457,20 @@ proc mainModularInverse() =
 
         check: bool(r == expected)
 
+      block:
+        let a = BigInt[381].fromUint(0'u16)
+        let M = BigInt[381].fromHex("0x1a0111ea397fe69a4b1ba7b6434bacd764774b84f38512bf6730d2a0f6b0f6241eabfffeb153ffffb9feffffffffaaab")
+
+        var mp1div2 = M
+        mp1div2.shiftRight(1)
+        discard mp1div2.add(Word 1)
+
+        let expected = BigInt[381].fromUint(0'u16)
+        var r {.noInit.}: BigInt[381]
+
+        r.invmod(a, M, mp1div2)
+
+        check: bool(r == expected)
 
 mainArith()
 mainNeg()

tests/test_finite_fields_powinv.nim

@@ -153,7 +153,20 @@ proc main() =
           computed == expected
 
   suite "Modular inversion over prime fields":
-    test "Specific test on Fp[BLS12_381]":
+    test "Specific tests on Fp[BLS12_381]":
+      block: # No inverse exist for 0 --> should return 0 for projective/jacobian to affine coordinate conversion
+        var r, x: Fp[BLS12_381]
+        x.setZero()
+        r.inv(x)
+        check: bool r.isZero()
+
+      block:
+        var r, x: Fp[BLS12_381]
+        x.setOne()
+        r.inv(x)
+        check: bool r.isOne()
+
+      block:
         var r, x: Fp[BLS12_381]
 
         # BN254 field modulus