Rework the getSignature of ecrecover

nimbus/vm/precompiles.nim

@@ -16,35 +16,38 @@ type
     paEcMul,
     paPairing = 8
 
-proc getSignature*(computation: BaseComputation): (array[32, byte], Signature) =
+proc getSignature(computation: BaseComputation): (array[32, byte], Signature) =
   # input is Hash, V, R, S
   template data: untyped = computation.msg.data
-  var bytes: array[65, byte]
+  var bytes: array[65, byte] # will hold R[32], S[32], V[1], in that order
   let maxPos = min(data.high, 127)
+
+  # if we don't have at minimum 64 bytes, there can be no valid V
   if maxPos >= 63:
-    # extract message hash
-    result[0][0..31] = data[0..31]
-    if maxPos >= 127:
+    let v = data[63]
+    # check if V[32] is 27 or 28
+    if not (v.int in 27..28):
+      raise newException(ValidationError, "Invalid V in getSignature")
+    for x in 32..<63:
+      if data[x] != 0:
+        raise newException(ValidationError, "Invalid V in getSignature")
+
+    bytes[64] = v - 27
+
+    # if there is more data for R and S, copy it. Else, defaulted zeroes are
+    # used for R and S
+    if maxPos >= 64:
       # Copy message data to buffer
-      # Note that we need to rearrange to R, S, V
-      bytes[0..63] = data[64..127]
-    elif maxPos >= 64:
       bytes[0..(maxPos-64)] = data[64..maxPos]
+
+    if recoverSignature(bytes, result[1]) != EthKeysStatus.Success:
+      raise newException(ValidationError, "Could not recover signature computation")
+
+    # extract message hash, only need to copy when there is a valid signature
+    result[0][0..31] = data[0..31]
   else:
     raise newException(ValidationError, "Invalid V in getSignature")
 
-  var VOK = true
-  let v = data[63]
-  for x in 32..<63:
-    if data[x] != 0: VOK = false
-  VOK = VOK and v.int in 27..28
-  if not VOK:
-    raise newException(ValidationError, "Invalid V in getSignature")
-  bytes[64] = v - 27
-
-  if recoverSignature(bytes, result[1]) != EthKeysStatus.Success:
-    raise newException(ValidationError, "Could not recover signature computation")
-
 proc getPoint[T: G1|G2](t: typedesc[T], data: openarray[byte]): Point[T] =
   when T is G1:
     const nextOffset = 32