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a2c2863531
This is the default and can be omitted. Seeing as the value of this argument was "hardmatched" in 100% of cases, we're better off pretending the parameter doesn't exist.
140 lines
3.2 KiB
Lua
140 lines
3.2 KiB
Lua
local bin = require "bin"
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local nmap = require "nmap"
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local shortport = require "shortport"
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local string = require "string"
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description = [[
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Detects the CCcam service (software for sharing subscription TV among
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multiple receivers).
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The service normally runs on port 12000. It distinguishes
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itself by printing 16 random-looking bytes upon receiving a
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connection.
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Because the script attempts to detect "random-looking" bytes, it has a small
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chance of failing to detect the service when the data do not seem random
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enough.]]
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categories = {"version"}
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author = "David Fifield"
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-- A chi-square test for the null hypothesis that the members of data are drawn
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-- from a uniform distribution over num_cats categories.
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local function chi2(data, num_cats)
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local bins = {}
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local x2, delta, expected
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for _, x in ipairs(data) do
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bins[x] = bins[x] or 0
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bins[x] = bins[x] + 1
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end
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expected = #data / num_cats
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x2 = 0.0
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for _, n in pairs(bins) do
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delta = n - expected
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x2 = x2 + delta * delta
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end
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x2 = x2 / expected
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return x2
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end
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-- Split a string into a sequence of bit strings of the given length.
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-- splitbits("abc", 5) --> {"01100", "00101", "10001", "00110"}
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-- Any short final group is omitted.
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local function splitbits(s, n)
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local seq
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local _, bits = bin.unpack("B" .. #s, s)
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seq = {}
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for i = 1, #bits - n, n do
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seq[#seq + 1] = bits:sub(i, i + n - 1)
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end
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return seq
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end
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-- chi-square cdf table at 0.95 confidence for different degrees of freedom.
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-- >>> import scipy.stats, scipy.optimize
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-- >>> scipy.optimize.newton(lambda x: scipy.stats.chi2(dof).cdf(x) - 0.95, dof)
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local CHI2_CDF = {
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[3] = 7.8147279032511738,
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[15] = 24.99579013972863,
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[255] = 293.2478350807001,
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}
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local function looks_random(data)
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local x2
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-- Because our sample is so small (only 16 bytes), do a chi-square
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-- goodness of fit test across groups of 2, 4, and 8 bits. If using only
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-- 8 bits, for example, any sample whose bytes are all different would
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-- pass the test. Using 2 bits will tend to catch things like pure
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-- ASCII, where one out of every four samples never has its high bit
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-- set.
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x2 = chi2(splitbits(data, 2), 4)
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if x2 > CHI2_CDF[3] then
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return false
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end
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x2 = chi2(splitbits(data, 4), 16)
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if x2 > CHI2_CDF[15] then
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return false
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end
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x2 = chi2({string.byte(data, 1, -1)}, 256)
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if x2 > CHI2_CDF[255] then
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return false
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end
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return true
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end
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local NUM_TRIALS = 2
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local function trial(host, port)
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local status, data, s
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s = nmap.new_socket()
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status, data = s:connect(host, port)
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if not status then
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return
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end
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status, data = s:receive_bytes(0)
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if not status then
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s:close()
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return
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end
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s:close()
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return data
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end
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portrule = shortport.version_port_or_service({10000, 10001, 12000, 12001, 16000, 16001}, "cccam")
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function action(host, port)
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local seen = {}
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-- Try a couple of times to see that the response isn't constant. (But
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-- more trials also increase the chance that we will reject a legitimate
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-- cccam service.)
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for i = 1, NUM_TRIALS do
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local data
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data = trial(host, port)
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if not data or seen[data] or #data ~= 16 or not looks_random(data) then
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return
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end
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seen[data] = true
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end
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port.version.name = "cccam"
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port.version.version = "CCcam DVR card sharing system"
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nmap.set_port_version(host, port)
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end
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