diff --git a/lualib/md5.lua b/lualib/md5.lua
index ceaddb38..90e28739 100644
--- a/lualib/md5.lua
+++ b/lualib/md5.lua
@@ -15,19 +15,21 @@ function core.sumhexa (k)
 		 end))
 end
 
-function core.hmac_md5(data,key)
+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=string.sub(key,1,16)
-	end
-
-	local b=table.pack(string.byte(key,1,#key))
-	local ipad_s=""
-	local opad_s=""
-	for i=1,64 do
-		ipad_s=ipad_s..string.char((b[i] or 0)~0x36)
-		opad_s=opad_s..string.char((b[i] or 0)~0x5c)
+		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