Merge pull request #18 from codex-storage/convert-from-32-bytes

Convert from 32 bytes

poseidon2/compress.nim

@@ -6,5 +6,5 @@ func compress*(a, b : F, key = zero) : F =
   var x = a
   var y = b
   var z = key
-  permInplace(x, y, z)
+  permInPlace(x, y, z)
   return x

poseidon2/io.nim

@@ -1,8 +1,9 @@
-import ./types
+import std/options
 import constantine/math/arithmetic
 import constantine/math/io/io_bigints
 import constantine/math/io/io_fields
 import constantine/math/config/curves
+import ./types
 
 export curves
 
@@ -14,6 +15,13 @@ func fromBytes*(_: type F, bytes: array[31, byte]): F =
   ## canonical little-endian big integer.
   F.fromOpenArray(bytes)
 
+func fromBytes*(_: type F, bytes: array[32, byte]): Option[F] =
+  ## Converts bytes into a field element. The byte array is interpreted as a
+  ## canonical little-endian big integer.
+  let big = B.unmarshal(bytes, littleEndian)
+  if bool(big < F.fieldMod()):
+    return some(F.fromBig(big))
+
 func toBytes*(element: F): array[32, byte] =
   ## Converts a field element into its canonical representation in little-endian
   ## byte order. Uses at most 254 bits, the remaining most-significant bits are

poseidon2/permutation.nim

@@ -2,7 +2,7 @@ import ./types
 import ./roundfun
 
 # the Poseidon2 permutation (mutable, in-place version)
-proc permInplace*(x, y, z : var F) =
+proc permInPlace*(x, y, z : var F) =
   linearLayer(x, y, z)
   for j in 0..3:
     externalRound(j, x, y, z)
@@ -14,5 +14,5 @@ proc permInplace*(x, y, z : var F) =
 # the Poseidon2 permutation
 func perm*(xyz: S) : S =
   var (x,y,z) = xyz
-  permInplace(x, y, z)
+  permInPlace(x, y, z)
   return (x,y,z)

poseidon2/types.nim

@@ -38,3 +38,6 @@ func arrayFromHex*[N](
   for i in low(inp)..high(inp):
     tmp[i] = hexToF(inp[i], endian)
   return tmp
+
+func `==`*(a, b: F): bool =
+  bool(arithmetic.`==`(a, b))

tests/poseidon2/testIo.nim

@@ -1,5 +1,6 @@
 import std/unittest
 import std/sequtils
+import std/options
 import constantine/math/io/io_bigints
 import constantine/math/io/io_fields
 import constantine/math/arithmetic
@@ -11,6 +12,9 @@ suite "conversion to/from bytes":
   func toArray(bytes: openArray[byte]): array[31, byte] =
     result[0..<bytes.len] = bytes[0..<bytes.len]
 
+  func toArray32(bytes: openArray[byte]): array[32, byte] =
+    result[0..<bytes.len] = bytes[0..<bytes.len]
+
   test "converts 31 little endian bytes into a field elements":
     let bytes = toArray toSeq 1'u8..31'u8
     let paddedTo32 = @bytes & @[0'u8] # most significant byte is not used
@@ -18,6 +22,18 @@ suite "conversion to/from bytes":
     let unmarshalled = F.fromBytes(bytes)
     check bool(unmarshalled == expected)
 
+  test "converts 32 little endian bytes into a field elements":
+    let bytes = toArray32(toSeq 1'u8..32'u8)
+    let expected = F.fromBig(B.unmarshal(bytes, littleEndian))
+    let unmarshalled = F.fromBytes(bytes).get()
+    check bool(unmarshalled == expected)
+
+  test "convert fails for 32 little endian bytes larger than the prime field":
+    let bytes = toArray32(toSeq((255'u8).repeat(32)))
+    let expected = F.fromBig(B.unmarshal(bytes, littleEndian))
+    let unmarshalled = F.fromBytes(bytes)
+    check not unmarshalled.isSome()
+
   test "converts every 31 bytes into a field element":
     let bytes = toSeq 1'u8..62'u8
     let expected1 = F.fromBytes(bytes[0..<31].toArray)
@@ -26,6 +42,13 @@ suite "conversion to/from bytes":
     check bool(elements[0] == expected1)
     check bool(elements[1] == expected2)
 
+  test "conversion preserves field element":
+    let
+      expected = toF(1234)
+      bytes = expected.toBytes()
+      actual = F.fromBytes(bytes).get()
+    check bool(expected == actual)
+
   test "conversion from bytes adds 0x1 as an end marker":
     let bytes = toSeq 1'u8..62'u8
     let marker = @[1'u8]

tests/poseidon2/testPermutation.nim

@@ -15,7 +15,7 @@ suite "permutation":
     var y: F = toF(1)
     var z: F = toF(2)
 
-    permInplace(x, y, z)
+    permInPlace(x, y, z)
 
     check toDecimal(x) == "21882471761025344482456282050943515707267606647948403374880378562101343146243"
     check toDecimal(y) == "09030699330013392132529464674294378792132780497765201297316864012141442630280"