Test modular exponentiation with BN254 and BLS12-381 moduli

constantine/io/io_bigints.nim

@@ -399,7 +399,7 @@ func nativeEndianToHex(bytes: openarray[byte], order: static[Endianness]): strin
 #
 # ############################################################
 
-func fromHex*(T: type BigInt, s: string): T =
+func fromHex*(T: type BigInt, s: string): T {.noInit.} =
   ## Convert a hex string to BigInt that can hold
   ## the specified number of bits
   ##

constantine/io/io_fields.nim

@@ -25,7 +25,7 @@ func fromUint*(dst: var Fq,
                src: SomeUnsignedInt) =
   ## Parse a regular unsigned integer
   ## and store it into a BigInt of size `bits`
-  let raw = (type dst.mres).fromRawUint(cast[array[sizeof(src), byte]](src), cpuEndian)
+  let raw {.noinit.} = (type dst.mres).fromRawUint(cast[array[sizeof(src), byte]](src), cpuEndian)
   dst.fromBig(raw)
 
 func exportRawUint*(dst: var openarray[byte],
@@ -52,3 +52,8 @@ func toHex*(f: Fq, order: static Endianness = bigEndian): string =
   ## CT:
   ##   - no leaks
   result = f.toBig().toHex(order)
+
+func fromHex*(dst: var Fq, s: string) {.raises: [ValueError].}=
+  ## Convert a hex string to a element of Fq
+  let raw {.noinit.} = fromHex(dst.mres.typeof, s)
+  dst.fromBig(raw)

tests/test_finite_fields_powinv.nim

@@ -92,4 +92,20 @@ proc main() =
           # Check equality when converting back to natural domain
           20'u64 == r
 
+    test "x^(p-2) mod p (modular inversion if p prime)":
+      var x: Fq[BLS12_381]
+
+      # BN254 field modulus
+      x.fromHex("0x30644e72e131a029b85045b68181585d97816a916871ca8d3c208c16d87cfd47")
+      # BLS12-381 prime - 2
+      let exponent = BigInt[381].fromHex("0x1a0111ea397fe69a4b1ba7b6434bacd764774b84f38512bf6730d2a0f6b0f6241eabfffeb153ffffb9feffffffffaaa9")
+
+      let expected = "0x0636759a0f3034fa47174b2c0334902f11e9915b7bd89c6a2b3082b109abbc9837da17201f6d8286fe6203caa1b9d4c8"
+
+      x.pow(exponent)
+      let computed = x.toHex()
+
+      check:
+        computed == expected
+
 main()