----------------------------------------------------------------------------
  -- Modify version from https://github.com/keplerproject/md5
  ----------------------------------------------------------------------------
  
  local core = require "md5.core"
  
  ----------------------------------------------------------------------------
  -- @param k String with original message.
  -- @return String with the md5 hash value converted to hexadecimal digits
  
  function core.sumhexa (k)
  	k = core.sum(k)
  	return (string.gsub(k, ".", function (c)
  		   return string.format("%02x", string.byte(c))
  		 end))
  end
  
  local function get_ipad(c)
  	return string.char(c:byte() ~ 0x36)
  end
  
  local function get_opad(c)
  	return string.char(c:byte() ~ 0x5c)
  end
  
  function core.hmacmd5(data,key)
  	if #key>64 then
  		key=core.sum(key)
  		key=key:sub(1,16)
  	end
  	local ipad_s=key:gsub(".", get_ipad)..string.rep("6",64-#key)
  	local opad_s=key:gsub(".", get_opad)..string.rep("\\",64-#key)
  	local istr=core.sum(ipad_s..data)
  	local ostr=core.sumhexa(opad_s..istr)
  	return ostr
  end
  
  return core