New stdnse function 'fromhex' does hex decoding like bin.pack('H')

nselib/stdnse.lua

@@ -403,6 +403,30 @@ function tohex( s, options )
   return hex
 end
 
+---Decode a hexadecimal string to raw bytes
+--
+-- The string can contain any amount of whitespace and capital or lowercase
+-- hexadecimal digits. There must be an even number of hex digits, since it
+-- takes 2 hex digits to make a byte.
+--
+-- @param hex A string in hexadecimal representation
+-- @return A string of bytes or nil if string could not be decoded
+-- @return Error message if string could not be decoded
+function fromhex (hex)
+  local len = #hex
+  local out = {}
+  local i = 1
+  while i <= len do
+    local p, q, c1, c2 = find(hex, "^%s*(%x)%s*(%x)%s*", i)
+    if not p then
+      return nil, format("Invalid characters or odd number of hex digits at %d", i)
+    end
+    out[#out+1] = char(tonumber(c1..c2, 16))
+    i = q + 1
+  end
+  return concat(out)
+end
+
 ---Format a MAC address as colon-separated hex bytes.
 --@param mac The MAC address in binary, such as <code>host.mac_addr</code>
 --@return The MAC address in XX:XX:XX:XX:XX:XX format