fix modexp

nimbus/vm/interpreter/utils/utils_numeric.nim

@@ -54,21 +54,30 @@ func cleanMemRef*(x: UInt256): int {.inline.} =
     return high(int32) shr 2
   return x.toInt
 
-proc rangeToPadded*[T: StUint](x: openarray[byte], first, last: int): T =
+proc rangeToPadded*[T: StUint](x: openarray[byte], first, last: int, toLen = 0): T =
   ## Convert take a slice of a sequence of bytes interpret it as the big endian
   ## representation of an Uint256. Use padding for sequence shorter than 32 bytes
   ## including 0-length sequences.
+  const N = T.bits div 8
 
   let lo = max(0, first)
-  let hi = min(min(x.high, last), (lo+T.bits div 8)-1)
+  let hi = min(min(x.high, last), (lo+N)-1)
 
   if not(lo <= hi):
     return # 0
 
-  result = T.fromBytesBE(
-    x.toOpenArray(lo, hi),
-    allowPadding = true
-  )
+  if toLen > hi-lo+1:
+    var temp: array[N, byte]
+    temp[0..hi-lo] = x.toOpenArray(lo, hi)
+    result = T.fromBytesBE(
+      temp,
+      allowPadding = true
+    )
+  else:
+    result = T.fromBytesBE(
+      x.toOpenArray(lo, hi),
+      allowPadding = true
+    )
 
 # calculates the memory size required for a step
 func calcMemSize*(offset, length: int): int {.inline.} =

nimbus/vm/precompiles.nim

@@ -133,11 +133,10 @@ proc identity*(computation: BaseComputation) =
 proc modExpInternal(computation: BaseComputation, base_len, exp_len, mod_len: int, T: type StUint) =
   template rawMsg: untyped {.dirty.} =
     computation.msg.data
-
   let
-    base = rawMsg.rangeToPadded[:T](96, 95 + base_len)
-    exp = rawMsg.rangeToPadded[:T](96 + base_len, 95 + base_len + exp_len)
-    modulo = rawMsg.rangeToPadded[:T](96 + base_len + exp_len, 95 + base_len + exp_len + mod_len)
+    base = rawMsg.rangeToPadded[:T](96, 95 + base_len, base_len)
+    exp = rawMsg.rangeToPadded[:T](96 + base_len, 95 + base_len + exp_len, exp_len)
+    modulo = rawMsg.rangeToPadded[:T](96 + base_len + exp_len, 95 + base_len + exp_len + mod_len, mod_len)
 
   block: # Gas cost
     func gasModExp_f(x: Natural): int =
@@ -179,9 +178,9 @@ proc modExpInternal(computation: BaseComputation, base_len, exp_len, mod_len: in
     func zero(): static array[T.bits div 8, byte] = discard
     func one(): static array[T.bits div 8, byte] =
       when cpuEndian == bigEndian:
-        result[^1] = 1
-      else:
         result[0] = 1
+      else:
+        result[^1] = 1
 
     # Start with EVM special cases
     let output = if modulo <= 1:
@@ -216,7 +215,6 @@ proc modExp*(computation: BaseComputation) =
     mod_len = rawMsg.rangeToPadded[:Uint256](64, 95).safeInt
 
   let maxBytes = max(base_len, max(exp_len, mod_len))
-
   if maxBytes <= 32:
     computation.modExpInternal(base_len, exp_len, mod_len, UInt256)
   elif maxBytes <= 64: