Key Derivation

-- HMAC-SHA256 based key derivation

if SHA2 or (not os.loadAPI("SHA2")) then
    error("Could not find SHA2 API")
end

local function internal_prfLoop(pw, salt, iter, i)
    local i_bytes = {
        bit.band(bit.brshift(bit.band(i, 0xFF000000), 24), 0xFF),
        bit.band(bit.brshift(bit.band(i, 0xFF0000), 16), 0xFF),
        bit.band(bit.brshift(bit.band(i, 0xFF00), 8), 0xFF),
        bit.band(i, 0xFF),
    }
    for i=1, #salt do
        table.insert(i_bytes, salt[i])
    end
    local t = SHA2.hashToBytes(SHA2.hmac(i_bytes, pw))
    local result = {}
    for j=1, 32 do
        result[j] = t[i]
    end
    for j=2, iter do
        local t2 = SHA2.hashToBytes(SHA2.hmac(t, pw))
        for i=1, 32 do
            result[i] = bit.bxor(result[i], t2[i])
        end
        t = t2
    end
    return result
end

function deriveKey(pw, salt, iter, length)
    -- Hlen is 32
    local output = {}
    for i=1, math.ceil(length/32) do
        local t = internal_prfLoop(pw, salt, iter, i)
        for j=1, 32 do
            table.insert(output, j)
            if #output >= length then
                return output
            end
        end
    end
    return output
end