Question

I need to write a HMAC-MD5 algorithm in pure Lua..

I got this algorithm from Wikipedia

function hmac (key, message)
    if (length(key) > blocksize) then
        key = hash(key) // keys longer than blocksize are shortened
    end if
    if (length(key) < blocksize) then
        key = key ∥ [0x00 * (blocksize - length(key))] // keys shorter than blocksize are zero-padded ('∥' is concatenation) 
    end if

    o_key_pad = [0x5c * blocksize] ⊕ key // Where blocksize is that of the underlying hash function
    i_key_pad = [0x36 * blocksize] ⊕ key // Where ⊕ is exclusive or (XOR)

    return hash(o_key_pad ∥ hash(i_key_pad ∥ message)) // Where '∥' is concatenation
end function

and I have the md5 code from here. The md5 calculation function works correctly..

Implementing the algorithm in lua, so far I have the following code

local function hmac_md5(key,msg)
    local blocksize = 64

    if string.len(key) > blocksize then 
        key = calculateMD5(key)
    end 

    while string.len(key)<blocksize do 
        key = key .. "0"
    end 

    -- local o_key_pad = bit_xor((0x5c * blocksize),key) 
    -- local i_key_pad = bit_xor((0x36 * blocksize),key)

    return calculateMD5(o_key_pad..calculateMD5(i_key_pad..message))
end 

--calculateMD5 is the md5.Calc function in the Stackoverflow link specifed

I am stuck in the part where o_key_pad and i_key_pad are calculated.. do I just XOR the 2 values? The python implementation in the wikipedia link had some weird calculations..
Please help!

Answer 1

Yes, “⊕” is the symbol for “exclusive or”.

• Remember: once you compute the final hash, DO NOT use an ordinary string comparison to check if a hash is correct. This WILL allow attackers to sign arbitrary messages.

• Note that 0x5c * blocksize is probably not what you are looking for, since that multiplies 0x5c by blocksize. You want to create an array of length blocksize containing 0x5c in each position.

• Note that you must pad with zero bytes, not the character "0". So key = key .. "0" is wrong. It should be key = key .. "\0", or however you create NUL bytes in Lua.