How do I decode BCDs in Lua?

Question

In my Lua dissector, I have to decode BCDs. I see no readily available tools for this in Wireshark Lua. Can someone explain how I can do this?

For example, a BCD of 0x12345678 is sent on the wire as 0x21436587. I have to dissect it and show it as 12345678.

Accepted Answer

It depends on how you have the BCD sequence of bytes in Lua. For example do you literally have a Lua string of "21436587"? If so, then simply doing this will work:

-- assuming myString = "21436587"
local fixed = myString:gsub("(.)(.)", "%2%1")

That's just a shorthand for:

local fixed = string.gsub(myString, "(.)(.)", "%2%1")

But if the BCD is in the packet and your Lua script only has a Tvb object, for example from a dissector function being called, then one way is to turn it into a Lua string of hex ascii, since BCD is just nibble-reversed hex representation. I could be wrong, but I don't think Wireshark's API provides a quick accessor to Tvb to just get a hex ascii representation, so you have to go through ByteRange to get it. So for example, do this:

-- assuming 'buf' is the Tvb object passed into dissector
local tvb = buf(offset,length)  -- gets a TvbRange from your packet, starting at offset, for length bytes
local range = tvb:bytes()  -- converts TvbRange to ByteRange
local hexStr =  tostring(range)  -- turns ByteRange into hex representation string, e.g, "21:43:65:87"
local fixed = hexStr:gsub("(.)(.):?", "%2%1")  -- reverses the nibbles and removes the colons

That's the really long/verbose way, which can be shortened to something like this:

local fixed = tostring(buf(offset,length):bytes()):gsub("(.)(.):?", "%2%1")

Where 'offset' and 'length' are whatever numbers they need to be. Note I haven't tried the above snippets, but something like that should work.

Update: actually it looks like tostring metamethod of TvbRange gets the hex string, and without colons, so this should work:

local fixed = tostring(buf(offset,length)):gsub("(.)(.)", "%2%1")

Answer 2

I'm no LUA expert but either you have to fetch byte by byte and do the conversion or add something similar to tvb_bcd_dig_to_ep_str() to the LUA methods in wslua_tvb.c I suppose.